LIGHT EMISSION FROM ACTIVE NITROGEN SYSTEMS

2 LIGHT EMISSION FROM ACTIVE NITROGEN SYSTEMS

I. The Molecular Spectrum of Nitrogen Lofthus has recently pointed out that "from the spectroscopist's point of view, nitrogen is probably the most interesting molecule, and no doubt it is the molecule which has been most extensively studied" {194). Consequently, more band systems are known for nitrogen than for any other molecule (22, 27). However, because of the relatively short lifetimes of the precursors, many of the bands are observed only in the region of excitation. All the significant emission in the Lewis-Rayleigh afterglow (mostly the first positive system of N2) comes from excited molecules which may be produced by the recombination of ground state nitrogen atoms, that is, from excited nitrogen molecules of less than 9.76 eV (225 kcal) energy content. This will be a main concern of the discussion that follows, but certain afterglows of short duration are also of interest and will be considered. These include selected band systems emitted by the molecular ion, N2+, and by nitrogen molecules of higher energy content, formed by recombination of excited nitrogen atoms. For a more complete treatment of the numerous molecular spectra of nitrogen, the reader is referred to the recent and comprehensive review by Lofthus (194). Most of the experimentally determined energy levels, and also many predicted energy levels, for the vast number of possible electronic states of the nitrogen molecule have been tabulated by Mulliken (195-197). Many of the features of the afterglows have been reviewed recently by Mannella (198). 13

14

2, Light Emission front Active Nitrogen Systems

The states and transitions of N 2 and N2+ that have been observed in various modifications of active nitrogen are illustrated in Fig. 1. The Roman numerals tabulate the systems in the order of increasing electronic energy of the emitting state. The Arabic numerals represent the highest vibrational 241

29

20 B% + ΙΘ 3ΠΕ \

16

NJ(X^)

N( f D)+N( f P) N(2D)+N(2D)

14

N(4S)+N(*P)

b' 4 Z

«J

26

10 20

β

Σ+

N(4S) + N(4S)

a TTç

HI 27

C'TT,

m

T8

N(4S)+N(2D)

C ΤΤυ

4-JL

H

riy £i

B3ïï>

'Δί,Ι?) 3ΠΓ

Α33 VΣ +;

m

N2(X

lq)

FIG. 1. States and transitions of N 2 and N2+ that have been observed in various modifications of active nitrogen. (I) The Vegard-Kaplan (forbidden) bands, A *ΣΗ+ —► X τΣ9+. (II) The first positive system of N 2 , B 877a -> A 3 i7 u + . (Ill) The " Y " bands of N 2 , B' 327u- -> B 3I7g . (IV) The Lyman-Birge-Hopfield system of N 2 , a ιΠ9 -* X ^Σβ+. (V) The second positive system of N 2 , C 3IJU -+ B 3ITg. (VI) The Goldstein-Kaplan system of N 2 , C 3 /7 u -> B 3 /7 f f . (VII) One of the Birge-Hopfield systems of N 2 , 6' 1ΓΜ+ -> X ^ + , (VIII) The first negative system of N 2 +, B 2Σ„+ -► X *27α+.

//. Electronic Energy Levels below 9.76 eV

15

energy level of each particular electronic level that has been detected, although not necessarily in nitrogen afterglows. The positions of the energy levels relative to the energy available by recombination of various (ground state or excited) nitrogen atoms may also be seen in the figure. It is apparent that the energy required for emission of these molecular systems is available from the recombination of ground state N(4S) atoms, except for the second positive system of N 2 , the Goldstein-Kaplan and Birge-Hopfield systems of N 2 , and the first negative system of N 2 + . It may also be seen that light emitted in the first positive system of N 2 , the main component of the Lewis-Rayleigh afterglow, can account for a maximum of only 3.6 eV (^84 kcal), that is, N2(B)V"=12 —► N 2 (A)„' =0 . This is only a fraction of the total energy (9.76 eV or 225 kcal) available from the recombination of ground state atoms. To discuss the light emission from various active nitrogen afterglows, it will be convenient to introduce each system (except for the well-known first positive) with a brief description of the transitions involved, as revealed by studies of the emission from discharges or from the upper atmosphere. The discovery of the emission in the corresponding active nitrogen afterglow, and its spectrum under such conditions, will then be considered. It seems desirable to describe many of the older investigations in some detail, so that contemporary reports of several of the emissions from active nitrogen may be properly related to studies of these same emissions made many years ago by such investigators as Kaplan, Hamada, and the Hermans. Also, many references to older work quoted in the current literature appear to be incomplete, or in error.

II. Emission from Molecular Species with Electronic Energy Levels below 9.76 eV 1. THE FIRST POSITIVE SYSTEM OF N 2 , B *Π9 -► A 3i7M+ (5000-25,000 A)* Selected bands of this system completely dominate the Lewis-Rayleigh afterglow (199), although other emissions make an appreciable contribu­ tion (200). Much of the earlier literature on the Lewis-Rayleigh afterglow has been summarized by Kneser (201), while some of the more recent observations have been reviewed by Mitra (32) and others (198, 202, 203). Only the recent developments will be discussed in detail here. The most apparent characteristic of the first positive system in the LewisRayleigh afterglow is a maximum intensity of radiation from the vicinity of * The figures in parentheses indicate the approximate maximum wavelength region over which the particular system is known to emit light {194).

16

2. Light Emission from Active Nitrogen Systems

the eleventh vibrational level of the B 3Π9 state, with the complete absence of any emission from vibrational levels higher than twelve.* This is in accord with the well-known predissociation in the first positive system of nitrogen at v' = 12 (29, 205). At this level, the molecule contains almost 225 kcal of energy in excess of the ground state, corresponding to the highest vibrational level of the B 3Π9 state that may be populated during recombina­ tion of 4S nitrogen atoms. A second, less-pronounced population peak at v' = 6 has long been recognized (55, 200). In 1928, Kichlu and Acharya reported emission from the afterglow in the near infrared region (7500-8900 Â) for exposure times of 42 hours (206). They later identified this emission as an extension of the first positive bands emitted in the green, yellow, and red regions, but without the selected band structure associated with the visible emission (207). The recent studies of Bayes and Kistiakowsky have confirmed the enhanced populations of the afterglow, in pure nitrogen at 4 torr and 300°K, at v = 11 and v' = 6 of the B zIJg state (208). Their observations also revealed enhanced populations in the infrared over the range from about v' = 3 to v' = 0, where a maximum is attained. At a pressure of 1 torr, the number of transitions per second from v' ~ 2 is comparable with the number of transitions per second from the v' = 11, 12 vibrational levels (209). About 12% of the total visible radiation is emitted in the transition Ν 2 (Β)„' =11 -> N2(A)V=7 (5820 Â) (75). The behavior of this peak under various external influences is similar to that of the (6, 3) peak and, indeed, appears to charac­ terize the overall intensity of the visible nitrogen afterglow. For this reason, the emission at 5820 Â has been used to monitor the visible emission of the first positive system in the Lewis-Rayleigh afterglow (75, 210). The first positive bands have been observed to constitute entirely a diffuse glow that is produced around an electron beam, of energy 10.5 eV, passed through nitrogen at low pressures ( ^ Ι Ο - 3 torr) (211). The bands were found to be limited by v' = 12 but, unlike the Lewis-Rayleigh afterglow, the diffuse glow showed well-represented emission from vibrational levels less than v' = 11 or 12. Since the low pressure precluded significant recombination of nitrogen atoms in the region of the glow, it was suggested that the B *IJg state was populated by fluorescence from the a xng level excited in the electron beam (211). Oscillations associated with the first positive system have recently been reported from the laser action, at numerous wavelengths in the near infrared (8600-12,400 Â), which may be induced by a pulsed electric discharge * It might be noted at this point that, as indicated in Fig. 1, the B zYIg state lies7.351 eV above the ground state (196), and probably dissociates into one 4S atom and one 2D atom (22,196, 204).

//. Electronic Energy Levels below 9.76 eV

17

(~40 keV) through nitrogen (272, 213). It was once again suggested that the singlet system of states was preferentially excited on electron impact. These states may then partially decay to the metastable a λΠ9 state and populate the B 3Π9 state upon collision with ground state molecules. A population inversion then exists in the B 3Π9 state until the A 327M+ level (first positive emission) is filled (272). Stimulated radiation in the first positive system at 7753 Â has also been reported with dc pulsed excitation (274), while visible first positive emission has been detected from (C0 2 + N 2 + He) laser mixtures exposed to the laser emission from a discharge (214a). The first positive bands are also emitted in the aurora and in the "airglow" of the earth's upper atmosphere (275). However, they are not responsible, as previously supposed, for the emission, under these conditions, in the region of 6560 Â; this is now known to be due to the OH radical (216). It is a point of interest, perhaps, that the first positive bands are expected to be more prominent in the Martian "airglow," since, in that environment, the nitrogen atom concentration is probably at least equal to the oxygen concentration (277). The short-lived "auroral" afterglow, produced by Kaplan under special conditions of excitation and operation (75), also emits the first positive bands (713), without selective enhancement of emission around v' = 11, 6, but with emission from levels of energy in excess of 9.76 eV (74, 114, 218-221). Tanaka and Jursa have recently observed emission in the first positive system, in a similar "auroral" afterglow, from levels as high as v' = 26 (222) ; such vibrational levels are considerably higher than any that have been recognized in the earth's auroral spectrum. Decay of this "auroral" afterglow was exponential in time, with a half-life of about 0.05 sec for the emission at 5906 Â (222, 223). This value may be somewhat high owing to the concomitant presence, to some extent at least, of the Lewis-Rayleigh afterglow (222). The first positive system has also been reported by Bryan, Holt, and Oldenberg to be emitted from the so-called "blue" and "red" afterglows of short duration (up to 5 msec after the discharge) that may be produced after a "weak" and "strong" discharge, respectively, through nitrogen (224). These afterglows, with spectra rather similar to those produced by Kaplan (75), were obtainable at pressures of 3 to 30 torr. The intensity distribution in the band system was characteristic of the discharge (e.g., emission from v' > 12), although a plot of (intensity) -1 / 2 against decay time was linear and of equal slope for both the "blue" and "red" afterglows. This indicates that these bands result from a second-order reaction in the decay zone. The first positive bands also appear as a dominant characteristic in the short-duration "pink" afterglow recently described by Beale and Broida (772).

18

2. Light Emission from Active Nitrogen Systems

This afterglow may be produced with very fast flow rates of pure nitrogen at pressures between 4 and 15 torr, by either an electrodeless (772, 210) or an internal electrode system (772). It is preceded and followed by Lewis-Rayleigh afterglow, and is of maximum intensity about 5 msec after the discharge. In the "pink" afterglow, the intensity of the (11,7) band is about 1/20 that in the discharge region, and some 20,000 times greater than that in the LewisRayleigh afterglow region. A slightly higher rotational "temperature" is also indicated for the "pink" glow. The usual characteristics of the first positive bands, as found in a discharge, are present in the "pink" glow, including transitions from levels of the B 3Π9 state above the predissociation limit (ν' = 12). However, the transitions from 21 < v' > 12 are increased more than 50% relative to lower levels emitted in the discharge region, while the highest vibrational bands in the blue-green region (Δν = 4, 5, 6) are extremely weak or absent. When the "pink" afterglow is "quenched" by the addition of 1% oxygen (772), the emission of the first positive bands is then quite similar to that observed in the experimentally stimulated "auroral" afterglow (222). The relative intensity differences between the "pink" and "auroral" afterglows may be a result of the vastly different temperatures of the nitrogen in the systems under observation (225). First positive afterglow emission may also result when small amounts of nitrogen are added upstream from a de Laval nozzle in which a high-frequency electric discharge generates a helium afterglow (226). The first positive system then shows a vibrational distribution similar to that found in a nitrogen discharge, but with low rotational temperature. The first positive emission did not result when the nitrogen was added to the helium afterglow downstream from the nozzle. Many studies have shown that the intensity and spectral distribution of the first positive system in the Lewis-Rayleigh afterglow are influenced by a number of factors. Some of these have been studied relatively little, for instance the application of a weak discharge to the afterglow region, which reduced the intensity greatly (227-229). Others have received extensive investigation, and are considered separately in what follows. The earlier studies of active nitrogen become particularly relevant to this aspect of the discussion. a. Dependence on the Temperature The overall intensity of the visible afterglow has been reported by Rayleigh to vary inversely with the absolute temperature, with a negative coefficient of —0.64 (57). Recent studies confirm that the intensity decreases with increasing temperature (230). Herzberg first reported selective enhancement of the bands with v' = 12 when the afterglow was cooled to near liquid air

//. Electronic Energy Levels below 9,76 eV

19

temperatures (231). More recent studies have confirmed a shift of the maximum intensity toward the violet under comparable conditions, with most of the radiation emanating from the twelfth vibrational level of the B 3Π9 state (210). As the temperature is reduced, the maximum of the (B ζΠβ)υ'=12,11Λ0 population curve has been shown to shift linearly to higher energies, accompanied by a narrowing of the population peak (208). A "dark" modification of active nitrogen may be produced by passing the glowing gas through a heated tube, by which the visible afterglow is greatly decreased in intensity (232). However, the intensity returns to its normal level downstream from the heated region. b. Dependence on the Pressure During his pioneer studies, Strutt reported that the characteristic yellow afterglow could be observed even at pressures of one atmosphere (227). Later, he (Rayleigh) found that the addition of molecular nitrogen in the afterglow region of a low-pressure system caused an increase in the instan­ taneous emission (57). More recently, high-resolution work has demonstrated that, at 300°K, the overall intensity of the visible portion of the first positive system goes through a maximum as the pressure is increased from 1 to 50 torr (210). The maximum occurred at higher pressures, but was decreased in magnitude as the flow rate of the gas was increased. (The highest intensity was observed in this system at a pressure of 9 torr and a flow of 9 cm3 sec -1 .) These studies indicated that an increase of pressure resulted in a small decrease in intensity of the low-level transitions [(9, 5), (8, 4), (7, 3), etc.] relative to the intensity of the (11, 7) band (210). This change could not be explained in terms of the simple, preassociation model then favored for the afterglow. However, Bayes and Kistiakowsky have shown that the spectral distribution within the first positive system is not significantly pressure dependent in the pressure range 1 to 10 torr (208). Other evidence also indicates that the intensity of the first positive bands, hence the luminous association reaction of atomic nitrogen, is essentially independent of pressure over the range 1 to 80 torr (233, 234,234a). However, more recent data show that the visible emission must be at least partly excited by a process that depends on pressure over the range 0.5 to 12 torr (235). Stanley was able to produce a nitrogen afterglow at pressures from a few torr up to one atmosphere with a high-voltage quartz capillary arc (69). He reported gradual changes in the appearance of the first positive system at 150°C as the pressure was increased. The characteristic intensity distri­ bution, with a maximum at v = ~\\, gave way to a spectrum with roughly equal populations in all the observed vibrational levels. Bands with lower v' values, many of which were not seen at all at lower pressures, became

20

2. Light Emission from Active Nitrogen Systems

quite bright at atmospheric pressure. It appeared that, at these higher pressures, sufficient collisions to induce considerable vibrational relaxation occurred during the radiative lifetime of the B state. The inefficiency of vibrational relaxation of N2(B ΖΠ9) by N2(X 1Σβ+), reflected in the data of Stanley, has been corroborated recently by the absence of significant vibra­ tional relaxation in the high-temperature first positive emission from a microwave discharge over the pressure range 1 to 8 torr (235a). First positive emission at atmospheric pressure has also been observed behind an arc discharge in streaming nitrogen (71). In contrast to the observations of Stanley, and within the 15% accuracy of his measurements, Noxon found no change in the relative intensity distribution of the first positive bands at 20°C in the pressure range 8 torr to one atmosphere (89). In particular, he noted the usual enhancement of the bands originating from v' = 11. However, the intensity of the first positive system relative to that of the Vegard-Kaplan bands decreased rapidly as the pressure was increased from 20 to 760 torr. In contrast to the behavior discussed above for pressures above 1 torr, a decrease of pressure below 1 torr has been found to decrease the intensity of the infrared bands (ν' < 6) relative to that of the visible bands (v' > 6), in the first positive system (236). The quenching of the infrared bands was apparently not due to the presence of an impurity. Below 1 torr, too, the intensity of overall emission of the first positive bands was found to increase, for a given N(4S) concentration (233). This suggested that light emission below ~l torr might involve significant surface recombination of atoms. The pressure dependence of the rotational profile of the (11, 7) first positive band, over the range 0.05 to 1.0 torr, has been ascribed to rotational relaxation in the B ΖΠ9 state (237). c. Dependence on the Decay Time The time interval over which the first positive system may be observed is strongly dependent on the surface conditions of the vessel and the pressure of the gas. For example, when an electrodeless high-frequency discharge was passed through nitrogen contained in a 22-liter Pyrex flask (not baked) the afterglow attained maximum duration (187 minutes) only after the flask had been sealed for about 1 year (238). The system was subjected to a discharge of about 0.1 sec duration at intervals of 0, 122, 324, and 563 days after the flask had been isolated. Rayleigh reported that extensive wall "poisoning," a coating of metaphosphoric acid, for example, allowed the afterglow to be observed up to 5 | hours after termination of the discharge at relatively low pressures (0.1 torr) (56). Afterglows of such long duration were observed only at low

//. Electronic Energy Levels below 9.76 eV

21

pressures (57); the lifetime was decreased markedly by lowering the temperature of the decay tube (239). Rayleigh found the decay of the afterglow in the poisoned system, over a 24 sec interval, to be bimolecular in the species responsible for the long life [a linear relation between (intensity) -1 / 2 and the decay time] (57). This observation was important in indicating that recombination of atoms might be a prelude to emission of the afterglow. Later experiments with highly purified nitrogen, in which accurate measurements were made over a 105-fold range of intensities, indicated that decay of the afterglow occurred in three stages after inter­ ruption of the discharge (240, 241): an initial second-order decay; a further second-order decay ; and finally, a logarithmic decay when the glow became very faint. The rates of decay during these different periods varied inde­ pendently of one another with temperature, pressure, and the concentration of oxygen as an impurity in the system. Anderson suggested that the two bimolecular processes involve different third-body catalysts in a homogeneous recombination of two different particles present in the afterglow (242). He reported a bimolecular rate constant equal to 5.6 x 10~10 Γ1/2 exp(—3000/Γ) for the first afterglow, and equal to 1.0 x 10 -14 Γ1/2 for the second afterglow (230, 243). Pillow and Rogers have also reported a change from a pre­ dominantly bimolecular mode of decay to a logarithmic (surface) decay for afterglow decay times up to 5 minutes (244). Young and Clark have shown that at a pressure of 1 torr, in a static system with the walls coated with phosphorus pentoxide, the early hyperbolic decay of the first positive bands, indicative of recombination of two active particles, changes to a later exponential decay due to wall losses (209). However, at this pressure they found no change, during a decay time of 140 sec, in the characteristic relative vibrational distribution within the B zIJg state (maxima at values of v of about 11,6, and 2). These results indicated that all levels of the B state are populated by a common excitation mechanism, inde­ pendently of any known metastable particles formed in the discharge or generated during the decay. Young extended these observations to show that, over time intervals up to 9 minutes, the infrared and visible portions of the first positive system also follow superimposable decay curves at pressures of 0.11, 1.32, and 8.8 torr (236). However, the intensity-time curves differed for each pressure. Noxon observed a greatly increased rate of decay of the first positive bands at atmospheric pressure (89). He attributed this to a rapid loss of the precursors of the afterglow, N(4S) atoms, by chemical reaction at high pressures with trace impurities of oxides of nitrogen.

2. Light Emission front Active Nitrogen Systems

22

d. Dependence on the Concentration of Ground State 4S Nitrogen Atoms Kistiakowsky and co-workers have recently demonstrated that the intensity of the visible Lewis-Rayleigh afterglow—in particular, of emission from around the 11th or 6th vibrational levels of the B3TJg state—is directly proportional to the square of the concentration of ground state nitrogen atoms (75, 200). Their results confirmed the relation suggested by earlier workers (245-248), and formulated by Rayleigh (57) and Stanley (69): I = C(T)n2p

where / = the intensity of the visible afterglow ; n = the concentration of "active particles of active nitrogen", i.e., N(4S); p = the pressure of inert gas (usually N 2 ); C(T) = a parameter dependent on temperature.

Since 1956, the intensity of the total visible afterglow, or of the charac­ teristic emission (5820 Â) from levels around v' = 11, has been used frequently (76, 89, 109, 249-253) to monitor the relative concentration of N(4S) in an active nitrogen stream. When both the intensity of the visible afterglow and the concentration of nitrogen atoms were measured at a particular level in a flow system, the linear relation between intensity of emission from v' = 11 and the square of the N(4S) concentration appeared to have a greater slope for the results obtained in a clean Pyrex vessel than in one poisoned with a trace of water vapor (254). The reverse appeared to be true for emission from v' = 6. Since the first experiments on active nitrogen, it has been apparent that, at least for pressures above 1 torr, the afterglow is produced only during homogeneous (volume) decay of the active species (56, 239). Light emission has never been reported as a result of heterogeneous recombination of nitrogen atoms on either clean or "poisoned" glass surfaces.* This is in accord with the conclusion that recombination of nitrogen atoms in the temperature range 55°C to 400°C, on glass surfaces poisoned with metaphosphoric acid, leads directly to nitrogen molecules in the ground state(255). To determine the fraction of gas phase recombination of N(4S) atoms that leads to emission in the first positive system, it is necessary to know the quantum yield of the afterglow. This remains somewhat in doubt, since present estimates of it have involved the use of calibrated light sources and geometrical factors to obtain an integrated light intensity. Attempts by Rayleigh to measure the absolute intensity of emission in the visible and photographic infrared regions of the spectrum led him to suggest * Although this concept has been invoked (233) as a possible explanation of the anoma­ lous behavior of first positive emission at pressures below 1 torr.

//. Electronic Energy Levels below 9.76 eV

23

that only about 0.1% of the active species present contribute even to the brightest afterglows attainable (57). Berkowitz, Chupka, and Kistiakowsky correlated the intensity of the visible first positive bands, measured with a calibrated photomultiplier, with the N(4S) concentration at low pressures, determined with a mass spectrometer (75). They deduced a value for Ärag of 7.2 x 1014 (X 10±2) cc2 mole- 2 sec"1 in the relation, / = £ ag [N( 4 S)] 2 [N2]. A value for &ag of 2.8 x 1014 cc2 mole -2 sec -1 was obtained by Wentink, Sullivan, and Wray for the pressure range 0.1 to 1.0 torr (251). Their intensity measurements were also in the visible range, but they determined the nitrogen atom concentration from the heat released by the active nitrogen to a platinum hot-wire detector system. (Such calorimetric methods for measuring the atomic nitrogen concentration may be subject to considerable error; cf. Chapter 3). The data also provided a value for the rate constant for homogeneous recombination of nitrogen atoms which agreed fairly well with other values for this constant that appeared almost simultaneously in the literature (249, 256). A combination of this quantity with the value for A:ag indicated that one photon is emitted in the visible first positive system for every 40 N(4S) atoms that collide with the formation of molecules in the several allowed states of N 2 (257). These authors suggested, on this basis, that about one out of every 20 stable molecules formed during the volume recombination of nitrogen atoms is in, or is quickly converted to, the Β*Π9 state. If the infrared first positive bands from low v', observed in the pure nitrogen afterglow (208), are included in the value for the afterglow intensity, it might be concluded that about one recombination in 10, at pressures of about 1 torr, eventually results in the emission of a first positive photon. Campbell and Thrush have deduced a value for& ag of 5 x 10 14 cc 2 mole -2 sec -1 over the pressure range 2 to 10 torr (234). Young and Black have recently obtained a value for £ ag equal to 6 x 1013 cc2 mole -2 sec -1 , although they concluded that the visible afterglow is excited by a process independent of pressure, over the range 1 to 12 torr, as well as the third-order process defined by this rate constant (235). For a pressure of 20 torr, Noxon's results indicated that about 70 nitrogen atoms (probably an upper limit) disappear for each photon emitted in the visible first positive band system (89). The production of emission during the gas phase decay of nitrogen atoms appeared to become less efficient as the pressure was increased above 20 torr. However, the apparent disappearance of a very large number of N(4S) atoms (about 1000) for emission of one visible first positive photon at atmospheric pressure was attributed to a loss of nitrogen atoms by chemical reaction (at this pressure) with trace impurities. In a recent study, Marshall and Kawcyn used electron spin resonance to measure the nitrogen atom concentration, and determined the intensity of

24

2. Light Emission front Active Nitrogen Systems

emission in the range 5400 to 9000 Â, at a pressure of 5 torr in a wall-poisoned system (43). They concluded that, within a factor of about 2, one photon was emitted for every 200 recombinations of nitrogen atoms. The results described above all indicate that only a fraction of the homogeneous recombination of nitrogen atoms leads to afterglow emission. However, when the N(4S) concentration was estimated chemically from the HCN yield from the ethylene reaction (Chapter 3) (255), it was found that one-half the rate constant determined for homogeneous decay at 25°C was comparable with the value of kag obtained by Berkowitz et al. (75). This led to the suggestion that all homogeneous decay at this temperature, and at the pressures used (0.3 to 4.0 torr) may result in afterglow emission. Similarly, Campbell and Thrush have recently concluded that about 50% of N(4S) recombination in the pressure range from 2 to 10 torr passes through the emitting B 3TJg state (234a). At higher pressures, this state appeared to be efficiently quenched by N2(X 1Σ9+). e. Dependence on Addition of Rare Gases In this section will be discussed only the effects of additives on the emission of the first positive system in the afterglow. Excitation of emission spectra of the added gas, or changes in the afterglow emission due to chemical reaction with the additive, will be discussed later. It was reported during the 1920's that addition of inert gases shifts the maximum intensity of emission in the first positive system from its normal position in the neighborhood of v' = 11 toward bands corresponding to lower vibrational levels (ν' = 9 and 10) (14, 257). Intensity shifts toward the red appear to result whether the rare gas (helium or argon) is added to the afterglow or mixed with the molecular nitrogen prior to the discharge (210, 258-260). When the inert gas concentration was increased at 300°K, at a total pressure of 6 torr, the intensity maximum was shifted slightly toward lower vibrational levels (210). Argon was found to be less efficient than helium ; for example, at a total pressure of about 6 torr the intensity of the (10, 6) band in a nitrogen-argon mixture did not become equal to that of the (11, 7) band until the mixture was 97% argon, while a similar effect was obtained with a helium-nitrogen mixture that was only 80% helium. With a (95% He + 5% N2) mixture, an increase ojf total pressure from 1 to 27 torr increased slightly the shift in the intensity maximum to lower vibrational levels, although emission from the 10th level remained the most intense throughout the pressure range. Similar intensity shifts occur in the emission spectrum of the blue NO afterglow when rare gases are added to the after­ glow mixture (261). In the presence of argon or helium, the "fully modified" spectrum of the

//. Electronic Energy Levels below 9.76 eV

25

red (Δ^ = 3) sequence of the (B ζΠβ -> A 3Ση+) system showed a maximum at about v = 8 at 300°K (208). Argon and helium were about 20 times less effective than N 2 in influencing the "fully modified" population of the B 3Π9 state. These two gases, of all the additives studied, shifted the maximum intensity of the "fully modified" emission to vibrational levels less than about 10. At 77°K, with a threefold excess of argon at a total pressure of 35 torr, the total emission originating from the B 3Π9 state showed a large maximum around υ = 0, with a much smaller maximum at v' = 12. Correction was made for emission other than that originating in the first positive system. The effect of a large excess of helium on the afterglow has been illustrated in a colored photograph (262). Argon and helium appear to be more effective than N 2 itself as a third body in promoting the homogeneous recombination of N(4S) atoms leading to afterglow emission. The value of /cag has been variously estimated to be doubled in the presence of excess argon (75), to be increased by only about 25% by Ar or He (234), to be increased by a factor of about 2.5 and 1.5 by He and Ar, respectively (262a), or to be increased by a factor of about 25 by Ar or He (235). In the presence of excess (99%) helium, approximately 75% of the atomic nitrogen recombinations appear to contribute, ultimately, a photon in the nitrogen first positive system (263). This corresponded to a rate constant of approximately 3 x 10~32 cc2 sec -1 for excitation of N2(B 3Π9) with He as the third body. Campbell and Thrush conclude that enhanced afterglow emission in the presence of He or Ar is not due to significant changes in the third-order rate constant for N(4S) recombination (234a). instead, they attribute it to a reduced number of collisions between N2(B 3Π9) and N2(X 1Σ9+), which they consider to be efficient for quenching the B state. On the other hand, the normal, almost exponential decay of the afterglow does not appear to be affected by moderate additions of He or Ar (264), although the lifetime of the afterglow may be extended by the addition of these inert gases to a flow system in "unpoisoned" Pyrex glass (265). f. Dependence on Addition of Trace Amounts of Gases Such as Oxygen Consideration will be given here only to the effects on afterglow emission of traces of additives, such that no noticeable quenching of the afterglow or chemical reaction of the additive occurs. The effects of larger concentra­ tions will be discussed later (Chapter 5). The effect of small amounts of 0 2 on the nitrogen afterglow has long been a matter of controversy. Von Mosengeil showed, quite early, that the afterglow could be produced in nitrogen completely free of oxygen (266), and Baker and Strutt contended that the best nitrogen afterglow was produced with nitrogen as free from 0 2 as possible (267). On the other hand, Tiede (268), Tiede

26

2. Light Emission front Active Nitrogen Systems

and Domcke {269), and Compte {270) insisted that an electrical discharge did not produce the afterglow in nitrogen that had been completely freed of oxygen before the discharge, although it did so upon introduction of a trace of 0 2 . These contrary observations were resolved only when Tiede and Domcke transported their apparatus from Germany to England to demonstrate the validity of their conclusion. Strutt later recognized the beneficial effect of traces of 0 2 in producing the afterglow (277). However, he obtained similar results with traces of such other diverse gases as CH 4 and H 2 , and he remained convinced that the activity of the active nitrogen was due solely to some form of nitrogen itself, probably nitrogen atoms {272). Subsequently, Berkowitz, Chupka, and Kistiakowsky were able to show that the ratio of the intensity of the visible afterglow to the square of the N(4S) concentration, determined mass spectrometrically, did not alter upon introduction of traces of 0 2 to the system {75). However, Anderson still felt that a minute amount of some impurity, probably H 2 , was required for the long-lived afterglow {242). From the effect of traces of impurities such as ammonia {231, 273), water vapor {243, 274), or nitrous oxide {275), it soon became apparent that they enhanced the afterglow mainly by inhibiting the wall destruction of a precursor to the afterglow (probably by forming an adsorbed layer on the wall), and that they did little to promote the formation of active nitrogen within the discharge {231, 273, 276). This view was strengthened by Rayleigh's report that he was able to observe the afterglow for more than 5 hours, after the discharge had been stopped, in a vessel with the wall efficiently "poisoned" against atom recombination by a layer of concentrated sulfuric or metaphosphoric acid {56). He later concluded that the increase in the intensity of the afterglow in the presence of traces of 0 2 , or many other impurities, was due primarily to an effect on the walls of the vessels and not to any phenomena in the gas phase {277). The beneficial effect of low concentrations of 0 2 on the intensity of the afterglow has been confirmed {278). On the other hand, it has also been demonstrated that addition of oxygen in amounts up to 0.5% can induce changes in the spectrum of the afterglow emission {279). McCormick and Anderson have revived the claim that the long-lived afterglow cannot be obtained from pure nitrogen discharged in a Pyrex system {230). They further reported that addition of small amounts of 0 2 does not induce first positive emission, although addition of 1.8% H 2 produces a copious, long-lived afterglow. They conclude that most of the hydrogen is adsorbed on the glass until about 1.8% addition, and that minute amounts in the gas phase then "catalyze" the long-lived afterglow {230, 243). However, they consider that this long-lived, "second" afterglow involves excitation by collision of vibrationally excited, ground state molecules. Their "first"

//. Electronic Energy Levels below 9.76 eV

27

bimolecular afterglow appeared to involve collision of N(4S) atoms complexed with H 2 0 molecules present in the gas phase in trace amounts (243). It has now been shown that trace impurities, such as H 2 0 (59), or 0 2 , NO, and SF 6 (60), may facilitate the dissociation process in a discharge. For example, the dissociation of oxygen, as it normally occurs in a microwave discharge, may be almost entirely due to the presence of nitrogen- or hydrogen-containing impurities, primarily N 2 and H 2 0 (59). (The effect of N 2 appeared to be due to some slight formation of NO.) Similarly, it has been reported that the amounts of hydrogen dissociated in microwave or electrodeless discharges are increased by trace amounts of 0 2 or N 2 , but not by H 2 0 (280). Since N 2 is dissociated less efficiently than 0 2 or H 2 in most discharges, the possible importance of impurities on the discharge process in nitrogen is also indicated. Perhaps this accounts for the observation that trace amounts of water vapor added to nitrogen before the discharge not only serve as an excellent "poison" against surface recombination of atoms downstream from the discharge, but enable a high reproducible concentration of nitrogen atoms to be produced (281, 282). By such additions, it was possible to study the effects of adding ammonia to active nitrogen (254, 283). These effects were obscured in an "unpoisoned" system by increases downstream in both the nitrogen atom concentration and the afterglow intensity when small amounts of ammonia were introduced (254,282). A trace of benzene vapor has also been reported to increase the lifetime of the Lewis-Rayleigh afterglow (53, 284, 285). 2. THE " Y " BANDS OF N 2 , B' 3ZU~ -+ B *Π9 (6000-10,800 A)

Emission of triplet transitions in the infrared have been observed from nitrogen discharges (286, 287), and also as four bands between 6897 and 8455 Â in the Lewis-Rayleigh afterglow (288). Kistiakowsky and Warneck have suggested that four similar bands that they have observed in the longlived afterglow, between 6934 and 10,434 Â, may be emitted by some " Y " state of N 2 (200). It appeared that these bands made a considerable contri­ bution to the spectrum of the active nitrogen afterglow in the infrared region (vf < 6 for the first positive system of N 2 ). It was deduced that the " Y " state is populated during a collision-induced radiationless transition from the 5Σβ+ state of N 2 , in competition with the transition that populates directly the high (ν' ~ 11) vibrational levels of the B BIJg state. Subsequent experiments soon indicated that the lower state of the new system terminated on the B 3TJg state, and that either of the two electronic states, 3ΔΜ and 3 Ση~, could account for the upper state of the observed bands (289, 290). Examination of the rotational structure of these infrared bands under large dispersion proved that the upper state of the transition is of the species *Ση~,

28

2. Light Emission front Active Nitrogen Systems

while its lower state is B 3Π9 (291, 292). The zero vibrational level of the emitting 327M~ state appeared to lie very close to the level (~8.7 eV) of such a state predicted theoretically by Mulliken (196), which would probably dissociate into 4S and 2P nitrogen atoms. Precise measurements of four band heads (8-2, 8-3, 4-0, and 5-1) have recently been reported from a discharge through labeled 15 N 2 wherein the "Y" emission is shifted from the intense, overlapping first positive emission (293). The observed frequencies were in excellent agreement with values calculated from the description of the "Y" bands given by Bayes and Kistiakowsky (208). It is of interest that two bands observed between 6578 and 6756 Â, in the twilight sky spectrum, may also result from emission in the "Y" system (294). Afterglow emission in the "Y" system resembles that in the first positive system in its dependence on environmental conditions (290). A decrease in temperature favors emission from the highest vibrational level, and the population at υ = 8 becomes overwhelming at the temperature of liquid nitrogen. Decrease of temperature and addition of inert gas (argon or krypton) each enhances the "Y" system relative to the first positive bands. This behavior has been useful for high-resolution studies on the " Y " system (289, 292). The extensive measurements of Bayes and Kistiakowsky on the effects of various parameters on the Lewis-Rayleigh afterglow have substantiated the hypothesis (290) that the "Y" bands are populated by a mechanism completely analogous to that responsible for populating the highest vibrational levels of the B 3TIg state (208). They have also indicated that part of the emission of the first positive bands from the vicinity of v' = 3 may be due to the cascade Y —► B 3TJg -> A 327Μ+. In the emission spectrum of a nitrogen discharge, Ogawa and Tanaka observed bands in the vacuum UV region (1600 to 2050 Â) which could be attributed to transitions from the ZEU~ state to the ground X ΧΣ9+ state of N 2 (295, 296). This highly forbidden system (violating both the Σ~ ·/> Σ+ rule and the spin selection rule AS = 0) has apparently never been detected during emission from the afterglow. However, Wilkinson observed this forbidden band system in absorption when he used a path of 3.4 meter-atmospheres, and confirmed that the upper state of this transition, which is also the upper state for the "Y" bands, was "Y ζΣη~" (297). He suggested that, in accordance with nomenclature recommended by Carroll, the upper state of both of these bands be designated as B' 3Σί1~. Relative band strengths and r centroids have recently been evaluated for the forbidden nitrogen system B' 3Συ/~ — X λΣ9+ (297α). 3. THE VEGARD-KAPLAN SYSTEM OF N 2 , Α*Σν+-+Χ

1

Σβ+ (2100-5000 A)

This intercombination system, an electric dipole transition, is forbidden in the sense that it can occur only by violating the approximate (298) spin

//. Electronic Energy Levels below 9.76 eV

29

selection rule Δ5 = 0. It is not ordinarily observed in either the discharge or afterglow regions, and has been detected only under rather special conditions. However, it is of considerable interest in any discussion of active nitrogen, since its upper state is continually populated by the principal emission in the long-lived Lewis-Rayleigh afterglow, that is, by the first positive system. Nitrogen molecules in both the upper state, A 3ΣΗ+, and the lower (ground) state, X 1Σ9+, of the transition, dissociate into two 4S nitrogen atoms (196, 204). Vegard was the first to detect this system, although his observations were limited to the luminescence of solid nitrogen (299). Kaplan later observed emission of 14 bands in the system from a "special" discharge (300) which had been reported to produce a short-lived nitrogen afterglow, different from the Lewis-Rayleigh afterglow, after continuous operation for several weeks in nitrogen containing initially a trace of oxygen (74,301). This different, green-colored afterglow appeared as a flash at pressures from 5 to 10 torr in the conditioned system. It showed emission of band systems characteristic of a discharge through nitrogen, although with a different intensity distribution (74). To this short-lived glow, Kaplan gave the name "auroral" afterglow (75), because its spectrum resembled that of the atmos­ pheric aurora (215, 302, 303), and of the "airglow" (215, 304). Similar bands were also observed from a mixture of argon and nitrogen bombarded by electrons (305), and from an "ozonizer-type" discharge through pure nitrogen at atmospheric pressure (88). Forty-five bands of the Vegard-Kaplan system have been detected in a discharge through xenon containing traces of nitrogen (306). Intense emission of 13 bands of the system was observed from a high-voltage discharge through pure nitrogen at atmospheric pres­ sure (307). Ultrahigh-vacuum purification techniques have now permitted observation of triplet splitting of the A 327M+ state from a 60 cps high-voltage discharge through pure nitrogen at a pressure of about 3 torr (308). Emission of the Vegard-Kaplan bands in a nitrogen afterglow was first reported by Kaplan from an "auroral" afterglow produced by mild excitation at pressures of about 0.01 torr (218). The observation was offered as proof for the existence of metastable A state molecules in the afterglow region. A few years later, he observed relatively strong emission of the bands from an "auroral" afterglow produced at a pressure of about 10 torr by a strong exciting discharge (309, 310). Still later, a similar afterglow was obtained at pressures of 50 and 100 torr (311). The Vegard-Kaplan bands were also found by Kaplan to constitute the most intense emission from the so-called "blue" afterglow (312). This differed from the "auroral" afterglow only in having no N 2 + bands, and was produced in nitrogen at a pressure of 10~2torr when the exciting current was "gradually diminished until a visible band barely managed to exist."

30

2. Light Emission front Active Nitrogen Systems

A short-duration afterglow, similar to Kaplan's "auroral" afterglow, may be produced more simply with a discharge through nitrogen cooled by liquid air, at pressures from 10 to 20 torr (113, 219, 220). This afterglow has also been reported to show strong emission (ν' = 0 to 5) of Vegard-Kaplan bands (313). A transition in the Vegard-Kaplan system has also been proposed to account for the band around 2000 Â in the UV emission spectrum of the "active nitrogen" afterglow emitted during pyrolysis of azides (180). Efforts to identify afterglow emission due to A state molecules were also made by Hamada (314). He reported emission of Vegard-Kaplan bands (with nearly equal intensity from v' = 0, 1, and 2) from short-lived "metastable nitrogen" produced by a discharge through nitrogen immersed in liquid air, at pressures ranging from a few torr to a few tens of torr. The intensity of the bands increased with increasing nitrogen pressure. Hamada considered the "metastable nitrogen" formed by this special discharge to have a fairly high content of metastable N2(A 3Σ„+) molecules, and to differ from the active nitrogen of the Lewis-Rayleigh afterglow, which he considered to derive its energy mainly from atoms. Bryan, Holt, and Oldenberg have reported emission of several VegardKaplan bands during the decay, 0.15 to 1.5 msec after the discharge, of their "blue" and "red" afterglows of short duration (224). These were produced in a static system at pressures between 3 and 30 torr, after "weak" and "strong" discharges, respectively. Comparatively strong Vegard-Kaplan emission has been observed from the afterglow produced when highly purified nitrogen is excited by a microwave (2900 Mc sec-1) in a baked (410°C) system (315, 316). The pressures used were 1.92 and 3.64 torr, corresponding to decay times, following the discharge, of 40 and 70 msec, respectively. With large con­ centrations of N2(A 327M+) molecules, they appeared to be rapidly destroyed in metastable-metastable collisions. It has recently been found by Tanaka and Jursa that an intense "auroral" afterglow may be excited by an uninterrupted ac, discharge through nitrogen at a pressure of about 100 torr, in a bulb immersed in liquid nitrogen (222). The afterglow was produced immediately when the discharge was started, whereas, according to these authors, the Kaplan technique sometimes required operation of the discharge for as much as three months before the desired afterglow was produced. They confirmed that the Vegard-Kaplan bands were the weakest system emitted in either the discharge or the "auroral" afterglow, but indicated that the intensity of these bands, relative to the second positive system of nitrogen (C 3 TJU -> B 3Πα), is much higher in the afterglow than in the discharge region. Noxon was the first to report the extensive and unequivocal emission of

//. Electronic Energy Levels below 9.76 eV

31

the Vegard-Kaplan system from a long-lived afterglow (89). A very weak emission of these bands from the Lewis-Rayleigh afterglow had been reported previously by Hamada (314). However, he used such short decay times (<0.1 sec) that it is questionable whether the bands he observed should be associated with the true Lewis-Rayleigh afterglow. In Noxon's studies, the afterglow was produced at high pressures, with an "ozonizer-type" discharge through pure nitrogen. In most respects, it resembled the Lewis-Rayleigh afterglow; for instance, there was no emission from species of energy content greater than 9.76 eV. Relative to emission of the first positive system, the Vegard-Kaplan bands increased continuously in intensity as the pressure was increased from 20 to 760 torr. Most significantly, no Vegard-Kaplan bands originating from v > 1 were observed in the afterglow under any conditions of operation. The intensity of emission from v' = 0 increased continuously between pressures of 20 and 760 torr; that from v = 1 reached a maximum intensity between 74 and 170 torr and then decreased, although the intensity at 760 torr was still greater than that at 20 torr. The ratio of the intensity of emission from v = 1 to that from v = 0 reached a maximum at a pressure of about 74 torr and then decreased rapidly with a further increase in pressure. When the afterglow region was cooled to liquid nitrogen temperature, at one atmosphere pressure, the Vegard-Kaplan bands from υ = 1 increased threefold in intensity relative to those from υ = 0. It was apparent that, at room temperature and at pressures of 80, 190, and 760 torr, the Vegard-Kaplan bands decay at a slower rate than do the first positive bands, and that the Vegard-Kaplan bands from v = 1 decay more rapidly than do those from υ = 0. In the high-pressure afterglows, the mean lifetime of the bands was about 1 sec. This indicated that N2(A 3Ση+) metastable molecules owe their presence in the afterglow partly to their survival from the discharge and partly to recombination of N(4S) atoms in the afterglow region. It was suggested that the absence of emission from levels with v' > 1 might be due to a more favorable mode of vibrational relaxation for N2(A) molecules in higher vibrational levels. Emission of Vegard-Kaplan bands has also been reported not only from the discharge itself, but from the afterglow produced by a high-pressure Tesla-type discharge through (0 2 + N 2 + Ar) mixtures (i/7)and apparently when molecular nitrogen is added downstream to discharged argon (87). With a specially constructed discharge, through extremely pure nitrogen, at a pressure of 8 torr, it became possible to estimate a transition probability of only 1.6 ± 0 . 4 x 10~6 for the Vegard-Kaplan system, relative to that for the first positive system (318). It has been correspondingly difficult to observe the Vegard-Kaplan transition in absorption, but this has been accomplished by various investigators within the past few years (41, 318~320). Approximate values for the cross sections for excitation of the Vegard-

32

2. Light Emission from Active Nitrogen Systems

Kaplan bands by electron impact have been derived from a recent kinetic study of emission in "uniform field" positive-column glow discharges (321). The A SZU+ state may be populated with an effective cross section of about 0.6 x 10 _16 cm 2 during inelastic collision with electrons of threshold energy 6.7 eV (322). 4. THE LYMAN-BIRGE-HOPFIELD SYSTEM OF N 2 , a λΠ9 -> X ΧΣ9+ (1200-2600 A) This system, although allowed as a magnetic dipole and quadrupole transition, is forbidden (g -/* g) as an electric dipole transition. Consequently, it is not a strong system and, although it has been extensively studied in discharges through nitrogen, it was not detected until quite recently in nitrogen afterglows. The a λΠ9 state dissociates into two 2 D nitrogen atoms (196, 323). The stronger part of this singlet band system occurs in the vacuum ultraviolet region. The system was first detected in emission by Lyman in a high-voltage transformer discharge through pure nitrogen at low pres­ sure (324). In 1928, Birge and Hopfield reported a large number of bands of this system, both in emission and in absorption, and made a detailed study of their vibrational structures (325). Early studies of the rotational structure of this system indicated that the transition was between the 1Π state and the ground state of molecular nitrogen (326). It was finally concluded that the transition was a λΠ9 — X ΧΣ9+, with the upper level exhibiting metastable character (327). The a xTIg state shows a predissociation quite similar to that found in the 3 B Π9 state. This was first reported to occur above v' = 9 (328), but it was later demonstrated that this weak predissociation, detected only at low pressures, occurs just above v' = 6 of the a ^ state (775, 286, 323, 329). The predissociation limit, between levels v' = 6 and 7, lies very close to the energy of 9.76 eV required to dissociate the ground state nitrogen molecule, and is completely analogous, therefore, to the limit between v' = 12 and 13 in the first positive system. Gaydon suggested that the strongly forbidden predissociation of the Lyman-Birge-Hopfield system might be due to a cross-over from the a λΠ9 state (between v = 6 and 7) to the 5Σ9+ state, which then dissociates into 4S nitrogen atoms (22). Accurate rotational and vibrational data for the Lyman-Birge-Hopfield bands were not obtained until 1956. The use of a 21-foot grating vacuum spectrograph then permitted Wilkinson and Houk to make high-resolution measurements on emission in the vacuum UV region of the spectrum (330). The band system has now been observed in auroral spectra with rocketborne spectrometers, (303, 331), and relative intensities have been measured for

//. Electronic Energy Levels below 9.76 eV

33

50 of its major bands excited in a high-voltage discharge through nitrogen (332). Extensive measurements on the bands in absorption (magnetic dipole or electric quadrupole interaction) were also made in about 1956 (333-335), and have been continued in recent years (336, 337, 337a). This singlet system of nitrogen and the Birge-Hopfield system, also singlet (p. 42), were the only bands of nitrogen observed in absorption until quite recently (41). This is probably because of the multiplicity change involved in exciting the ground state molecule directly to triplet levels by electron or photon impact (338). Emission of the Lyman-Birge-Hopfield bands from the active nitrogen afterglow was first reported by Tanaka and co-workers in 1957 and 1959 (339, 340). At pressures of 1 to 12torr, they found only these bands from low vibrational levels of the a ΧΠ0 state, in the vacuum UV region of the LewisRayleigh afterglow in pure nitrogen. The emission was extremely weak, however, and, unlike the first positive system, no clear enhancement of any particular band was observed. They suggested that the a λΠα state might be populated by a radiationless collision-induced transition from the 5Σ9+ state produced during recombination of N(4S) atoms, as proposed by Gaydon (22). They attributed the extreme weakness of the bands to the metastability of the a λΠ9 state, together with the highly forbidden nature of the radiationless transition involved. Bayes and Kistiakowsky similarly suggested that the a λΠ9 state (up to the sixth vibrational level) may be populated in the LewisRayleigh afterglow region from the 5Σ9+ state, through a collision-induced radiationless transition competitive with that which populates the B *IJg state (at v = 12, 11, 10) and the B' ZEU~ state (at v' = 8, 7, 6) (208). This mechanism has received support from Young, who found that the Lyman-Birge-Hopfield bands from υ' = 6, 5, 4 (emitting around 1300 Â) decayed with time in exactly the same way as the first positive bands (236). However, the ratio of the intensity of the ultraviolet emission to that of the visible first positive bands varied in an approximate inverse relation to the pressure over the range 0.1 to 6 torr. This suggests (236) that the υ = 6, 5, 4 vibrational levels of the a xTlg state were subject to relaxation to levels lower than the fourth on collision with ground state nitrogen molecules during the radiative lifetime of about 10~4 sec (341). The extension of spectroscopic observations on the short-lived "pink" afterglow (112) into the vaccum UV region has recently demonstrated emission of Lyman-Birge-Hopfield bands (342-344). The very weak emission, with v' < 4, showed an intensity distribution similar to that in the LewisRayleigh afterglow (342, 343). The weak intensity of the bands, the only metastable radiation observed in the "pink" afterglow (342), and their intensity distribution, suggested that the a λΠ9 state probably is not an important energy source in the short-lived "pink" afterglow. The Lyman-

34

2. Light Emission from Active Nitrogen Systems

Birge-Hopfield emission may be part of the Lewis-Rayleigh afterglow which always underlies the "pink" glow (342). The weak Lyman-Birge-Hopfield bands, as well as all other nitrogen emissions, were quenched when an ac electric field was applied to the "pink" afterglow region (342, 343).

III. Emission from Molecular Species with Electronic Energy Levels above 9.76 eV 1. THE SECOND POSITIVE SYSTEM OF N 2 , C 3IJU -+ B 3TJg (2800-5450 A) This system involves a fully allowed transition. Since the zero vibrational level of the upper state lies at 11.05 eV (194, 196), it contains considerably more energy than that available from the recombination of N(4S) atoms (9.76 eV). It is, therefore, not normally observed in emission from the Lewis-Rayleigh afterglow. It appears quite readily, however, in electrical discharges through air (55,345), pure nitrogen (346,347), or traces of nitrogen in an atmosphere of xenon (306), and in "ozonizer-type" discharges through pure nitrogen at atmospheric pressure (307), and in various types of discharges through argon-oxygen-nitrogen mixtures (348). Ultraviolet coherent light (laser action of about 20 x 10~9 sec pulse width), generated directly at room temperature in a pulsed, high-voltage (100-150 keV) nitrogen discharge, has been attributed to emission in the second positive system, resulting from an inversion in the triplet state of nitrogen (349, 350). The emission (30 lines identified) extended over the range 3000 to 4000 Â, with a maximum intensity at 3371 Â. Similar laser action in the second positive band system has been driven into saturation for discharge operation over the nitrogen pressure range 1 torr to > 10 torr (351). Direct electron impact excitation of the C 3IIU and B 3Π9 triplet states appears to overpopulate the C state for times of the order of its radiative lifetime (352). The molecular laser system can be made more efficient, at pressures about 6 torr, if it is constructed in the form of a coaxial transmission line, with the gas discharge tube divided into two independent sections (353). Nitrogen pulsed gas lasers of high peak output power in the UV are now possible,* while megawatt power levels might be available eventually from modified discharges (214a, 353a). The first systematic studies of this system, published in 1924, indicated a sharp cutoff of the rotational levels in the fourth vibrational level (354-356). Kaplan subsequently pointed out that no bands were observed in the second positive system with v' > 4 (205), and Herzberg suggested that the obser* For example, a 100 kilowatt unit, with emission at 3371 A (Model C102) may be ob­ tained from the Avco Everett Research Laboratory, Everett, Massachusetts.

///. Electronic Energy Levels above 9.76 eV

35

vations could be attributed to a predissociation in the C 3 /7 M state at v' > 4 (357). The C 3 /7 state dissociates into one 4S and one sp* 4P nitrogen atom (196). The occurrence of this predissociation at a limit of about 12.14 eV was confirmed in later experiments of Büttenbender and Herzberg (26). They suggested that it was due to a radiationless transition from the C 377M state to another state, probably 3IJU in type, the potential curve of which crosses the curve for the C 3ilu state below the asymptote. The predissociation in the C 3/7M state has, in fact, become the best known of the several predissociations in the spectrum of N 2 (22). More recently, Janin observed new branches in some bands of this system and discussed the various perturbations detected (307), while Carroll and Sayers reported five new bands in the green region of the spectrum (287). Carroll and Mulliken believe that the predissociation of Büttenbender and Herzberg is due to a 5IJU state and not a 3TJU state (358). A second predissociation in the second positive bands is interpreted as due, in all probability, to the C 3ilu state, the emitter of the Goldstein-Kaplan bands. The second positive system of N 2 has often been observed in emission from the earth's upper atmosphere (359), for example, in the spectrum of the "light of the night sky" (304), perhaps better referred to as the "airglow" (360), and from the auroras (215,302). It has recently been detected in auroral spectra taken by rocketborne spectrometers (303). Calculations have been made of the probability that the upper state of this system may be populated in the auroral regions through electronic excitation of the nitrogen molecule in its ground state (359), that is, N2(X *Σ+) + e — N2(C 3/7M) + e

or through radiative recombination of an electron with an ionized nitrogen molecule in its ground state (359, 361, 362), that is, N2+(X 227/) + e -> N2(C 377u) + hv

Recent work has confirmed that the C 3/7M level may be populated directly from the X λΣβ+ ground state by both electron excitation (321, 322,363-366b), and photon absorption in the region from 1075 to 1650 Λ (122, 333, 337a). Extensive calculations have been made of the vibrational intensity distribution theoretically predicted for the C STJU state (for all experimentally observed bands of the second positive system) during excitation of nitrogen in gaseous electrical discharges (361). These calculations have been found to agree well (up to v' = 4) with experimental data obtained for discharges through nitrogen at 0.2 torr and cooled with liquid nitrogen (367). Emission of the second positive system of N 2 has been induced by methods other than electrical discharges. The impact of 200 keV protons on gaseous nitrogen at higher pressures has been found to be effective (368). The results

36

2. Light Emission front Active Nitrogen Systems

indicated, however, that excitation of the C STJU state was a two-step process, probably because the direct excitation by proton impact fails to conserve spin, and electron excitation becomes involved. Second positive emission has also been reported from impact of 20-100 keV protons on nitrogen or air at pressures from 0.001 to 0.1 torr (368a). Emission has also been induced by bombardment of nitrogen or air between the pressures of 0.0001 and 1 torr, with ions (protons, deuterons, H 2 + ) of energies about 500 keV (369), or by soft X-rays between the pressures of 1 and 760 torr (370). The second positive system has also been observed during the high-frequency excitation of nitrogen or ammonia at low pressures, under conditions such that the first positive system of N 2 is not observed (95). Strong second positive emis­ sion has been detected in pure nitrogen bombarded with polonium-210 (143, 370a), or curium-244 (371) α-radiation at room temperature. It is also emitted from nitrogen subjected to fission fragment radiation (370a). Again, it may be excited preferentially, with eight band heads, by collisions of the second kind between metastable argon atoms containing 11.55 or 11.72 eV of energy and ground state nitrogen molecules, without excitation of nitrogen to any other light-emitting state (83, 87, 171, 372). Excitation to the v = 3 or 2 level of N2(C3/7W), by collision with Ar(3P0) or Ar(3P2), appeared to occur with cross sections of 3 x 10~ 15 and0.8 x 10- 15 cm 2 , respectively (373). The distribution of intensity of second positive emission followed the FranckCondon factors for excitation from the ground state molecule for the resonance reaction with Ar species (374). Schultz has derived a value of 4 x 10~16 cm2 for the excitation of second positive bands from metastable Ar* species formed during α-particle irradiation of (Ar + N2) mixtures (371). Argon or neon species, excited with soft X-rays, also appear to be capable of inducing second positive emission from trace amounts of nitrogen (87a). Finally, the thermal excitation of the C 3IJU state, corresponding to excitation by molecular collisions, has been achieved in a shock wave through nitrogenargon mixtures (375) or through air (100). Brusyanova, Kolesnikov, and Sobolev claim that their reported emission of second positive bands, from a constant-current arc discharge between cooled tungsten electrodes, in high-purity nitrogen at atmospheric pressure, represents thermal excitation of molecular nitrogen (376). Comparison of the second positive emission from discharges through air (55, 377) with that from discharges through pure nitrogen has indicated that, at a pressure of 1.2 torr, the second positive bands are much less sensitive to impurities (e.g., 0 2 ) than those of the first positive system (377, 378). Indeed, the second positive bands have been detected from discharges through a (4% N 2 + 96% 0 2 ) mixture over a pressure range of 1 to 5 torr (279). Measurements by Heath have indicated that, between pressure limits of 0.01 and 760 torr, quite comparable spectra of the second positive emission

///. Electronic Energy Levels above 9.76 eV

37

are obtained from comparatively weak discharges through either pure nitrogen or air (379). He concluded that molecules in the C 3IJU state suffer no significantly greater alteration of their vibrational distribution when they collide with the atoms and molecules present in discharges through air than when they collide with those in discharges through pure N 2 . However, vibrational relaxation appeared to become more important as the pressure was increased in a discharge through pure nitrogen, since, over the pressure range indicated, the bands from v' = 4 decreased in intensity by a factor of 5.5 relative to those from v' = 0. Similarly, Feast observed that the distribution of vibrational intensities in the second positive system was not the same for emission from a high-voltage arc at atmospheric pressure and from the positive column of an ordinary low-pressure discharge (380). He attributed the difference to different extents of vibrational deactivation by collision of molecules in the C 3TJU state at the two pressures. This view has been criticized, however, on the basis that the radiative lifetime of the C3IJU state is too short to permit significant vibrational relaxation by collisions with ground state nitrogen molecules, even at atmospheric pressure (69). This view, in turn, is at variance with Tyte's recent observations on the effect of helium on the intensity of the second positive system of nitrogen emitted from a cooled discharge (381). He found the relative population of higher vibrational levels to decrease quite rapidly with increasing pressure (above 1 to lOOtorr). He suggested that collisions may be responsible for considerable vibrational deactivation of the excited N2(C 3Πη) molecules under these conditions. Similar vibrational deactivation was indicated during the observations of Tawde and Desai on the effect of added argon on the second positive bands in a nitrogen discharge (382). Recent studies with a high-temperature microwave discharge have strongly suggested that considerable vibrational relaxation in the C 3TIU state may be induced by collisions with N2(X 1Σβ+) molecules at pressures in the range 1 to 8 torr (235a). Kenty has described strong emission of the second positive bands of N 2 in the "striking blue flame" that streams away from a flat tungsten electrode when a Tesla spark is passed through argon, in the pressure range from 100 to 600 torr, to which a trace of nitrogen has been added (92, 93). The nitrogen emission persisted for a measurable distance from the electrode, with production of an "afterglow." This suggested to Kenty that the emitting C 3IJU state might be produced by recombination of a metastable N(2D) atom and a normal N(4S) atom in the presence of a third body. More recently, the transition of an electron avalanche through nitrogen (or nitrogen plus traces of methane) into anode- and cathode-directed streamers has been investigated by Tholl (383), and Wagner has reported second positive emission induced by electron avalanche in a pulsed discharge gap (384).

38

2. Light Emission front Active Nitrogen Systems

The emitting state appeared to be quenched on practically every impact with added methane molecules during a mean lifetime of 36 x 10 -9 sec (384). Jennings and Linnett have observed several bands around 3290 Â in the reaction flames of hydrocarbons with active nitrogen (385,386). They tentatively assigned these to C N 2 . They correspond to bands (with v' < 4) reported by Pannetier and his group as originating in the second positive system of N 2 emitted from a variety of sources. These include low-pressure, high-voltage ac discharges through pure nitrogen, hydrazine, ammonia, and (N 2 + H2) mixtures (387), and through organic nitrogen-containing compounds such as dimethylamine, dimethylhydrazine, and acetonitrile (388). Emission of the second positive bands from a discharge through acetonitrile was considerably less intense than that from a discharge through the substituted amine or hydrazine (389). It is of particular interest that these workers also observed emission originating from the C 3/7M state when the following reactants were added to the Lewis-Rayleigh region of an active nitrogen stream (formed in the presence of a trace of water vapor): hydrazoic acid (390), atomic hydrogen (the emission disappeared if the reaction mixture was subjected to ultraviolet irradiation) (391,392), aliphatic amines (393,394), dimethylhydrazine (394), chlorinated or brominated hydrocarbons (394-397), phosphine (398), SnBr4 (399), and PC13 (400). The second positive bands were also observed upon introduction of H2S and H 2 0 into a region that probably corresponded to the "pink" afterglow (401, 402). In most cases, the emission of the second positive bands from the reaction flames appeared along with emission from an excited NH radical (3/7 -^ 3Σ) (394). Emission at 3444 Â from the reaction of active nitrogen with CH2C12 and CHC13 was first considered to be due to the (4, 5) band of the second positive system of N 2 (396). It was later proved, through rotational analysis, to arise from the (20, 16) band of the violet system of the CN radical (403). As previously noted, emission from the fifth vibrational level of the C 3IJU state has not been detected in discharges through nitrogen. On the other hand, the (5, 5) band of the second positive system has been reported in the emission from the reaction flame of atomic nitrogen with aliphatic amines, dimethylhydrazine, and chlorinated hydrocarbons, with the exception of pure CC14 (393-396). There is recent evidence, however, that the band at 3259 Â, emitted in active nitrogen-hydrocarbon flames, might be due to emission in the (^Π — ΧΔ) system of the NH radical, rather than the (5, 5) band of the second positive system (404). This suggestion throws some doubt on the report that extensive emission of the second positive system, which appeared to include that (3616.0 Â) of the band (5, 7) originating in the fifth vibrational level of the C 377M state (405), was excited by a discharge at 4200 volts through nitrogen, containing a trace of methane,

///. Electronic Energy Levels above 9J6eV

39

at a total pressure of 3.5 torr. When the voltage was increased to 9500 volts, and the pressure to 6 torr, the higher electronic levels of the nitrogen molecule were preferentially populated, and a new band at 2685.1 Â was disclosed. This was attributed to the transition (4, 0) of the second positive system (406). The second positive system of N 2 has often been observed from the various short-lived, high-energy afterglows of nitrogen. Weak emission of bands from v' < 4 is characteristic of Kaplan's "auroral" (15, 74, 218, 310, 311) and "blue" (312) afterglows, of the "orange" afterglow behind a hightension arc (221) or produced in a high-frequency discharge (58, 87), and of the "pink" afterglow described by Beale and Broida (87, 112). Herman (113), and Herman and Herman (220), observed the second positive bands in the "auroral" afterglows produced in cooled discharge tubes at pressures of about 0.1 torr. In these, bands from υ' = 3, 4 were of greater intensity than in the discharge itself. Second positive bands have also been reported in the short-duration "blue" and "red" afterglows of Bryan, Holt, and Oldenberg mentioned earlier (224). Here, emission from higher vibrational levels was more extensive and the rotational temperatures were higher, than in the bands emitted by the discharge. A plot of (intensity) -1 / 2 against time of decay was linear for the "blue," but not for the "red" afterglow. Second positive bands have also been reported from a short-duration (10~3 sec) afterglow of nitrogen emitted behind an arc discharge at atmos­ pheric pressure and about 1500°K(77), and from Hamada's "metastable nitrogen" described previously (314). They have been detected from free-jets of low-density, arc-heated nitrogen or argon-nitrogen plasmas (72a). They are also emitted from the short-lived ( ~ 1 0 - 6 sec) afterglow induced in low-temperature nitrogen by 10 keV electrons (407). It is possible that these bands are emitted also from the "nitrogen afterglow" produced during the pyrolysis of azides (179, 180). (The half-life of this afterglow may be as much as 40 sec at very low pressures.) The spatial separation of the "pink" afterglow obtained by Beale and Broida in a rapid flow system allowed them to make comparative observations on the emission of the second positive bands in this afterglow, and in the discharge, at a pressure of 6 torr (112). It appeared that, at maximum intensity, the (0, 0) band was only about 1/300 as bright in the "pink" afterglow as in the discharge. This may be compared with the corresponding factor of 1 /20 for the first positive system of N 2 , or the first negative system of N 2 + . However, the same second positive band was at least 35 times brighter in the "pink" afterglow than in the Lewis-Rayleigh afterglow that preceded and followed it, under the conditions of operation used. Upper vibrational levels, in the range v' < 4 of the second positive system, were somewhat less pronounced in the "pink" afterglow than in the discharge region. In a sufficiently fast flow system, where the "pink" afterglow appears

40

2. Light Emission from Active Nitrogen Systems

to be a "pointed flame," the absence of the (1, 1) and (2, 2) bands of the second positive system has been confirmed as an abnormality in the vibrational distribution (404). Young has recently reported that the application of an ac electric field to the "pink" afterglow quenches the weak emission of the second positive bands (342). His results suggested that the observed spectra in the "pink" afterglow region may be the result of dissociative recombination of N3+ (342, 408). Fairchild, Prag, and Clark have observed similar quenching of the second positive bands, which they found to be prominent in the ultraviolet spectrum of the "pink" afterglow in their system at a pressure of 5.3 torr (343) It is perhaps relevant to Young's suggestion that maxima in the ion density and electron temperature, 5 to 10 msec after the discharge, coincide with the maximum intensity in the "pink" afterglow (409). The second positive bands of N 2 are very intense in the "auroral" afterglow produced by the simplified technique of Tanaka and Jursa described previously (222). The possible presence of weak bands originating in v' = 5 of the C 3/7M state was indicated. These correspond to an energy of 12.195 eV, slightly above the predissociation limit of 12.14 eV to produce N(4S) plus N(2D) atoms. The second positive band emitted at 3371 Â was found to have a half-life of 0.02 sec at a pressure of 100 torr. The "auroral" afterglow produced by Brömer and Frette, with a radiofrequency discharge, showed an initial decrease in intensity of the second positive bands with time of decay (410). This was followed, at pressures greater than 1.8 torr, by an increase to a maximum value. Emission of the second positive bands in the Lewis-Rayleigh afterglow was reported by Ruark et al. over the nitrogen pressure range 0.01 to 0.2 torr when a trace of mercury was present (411, 412). Their emission from the afterglow of "pure" nitrogen was first reported by Hamada (314). An ordinary discharge at low pressure was used, rather than the discharge at liquid air temperature by which he produced his short-lived afterglow of "metastable" nitrogen. The weak emission was passed through a rotating sector disk, by which it was possible to record, with long exposure, the afterglow that developed between 0.01 and 0.1 sec after interruption of the discharge. With such short decay times, it is possible that the observed second positive emission was really due to the presence of some short-lived afterglow, such as the "pink," in the region of observation. The emission of second positive bands in the Lewis-Rayleigh afterglow, under special conditions, seems to have been established beyond doubt by Tanaka, LeBlanc, and Jursa (413). In agreement with all previous investiga­ tions, the bands were not observed at room temperature, but they did appear, with intensity almost as strong as the first positive bands, when the glow was cooled with liquid nitrogen. The intensity distribution within the second

///. Electronic Energy Levels above 9.76 eV

41

positive system was then considerably different from that emitted in an ordinary discharge through nitrogen. In particular, it showed abnormally strong emission from the highest vibrational level, v' = 4 (cf. enhanced emission from v' = 12 in the first positive system under comparable conditions). It was suggested that, at liquid nitrogen temperature, the wall conditions became favorable for preventing decay of metastable atoms. The C snu state at v' = 4 may then be populated, even in the presence of quite small concentrations of N( 2 D) atoms, by a process of inverse homo­ geneous predissociation during recombination of 4S with 2 D nitrogen atoms. Harteck and co-workers have reported that second positive emission may be induced in the Lewis-Rayleigh afterglow upon addition of copper metal to the gas stream (414, 415). The blue emission extends about 2 mm down­ stream from the metal. It was attributed to a "surface-catalyzed excitation," which must involve at least three ground state atoms, or possibly two excited molecules. 2. THE GOLDSTEIN-KAPLAN SYSTEM OF N 2 , C 3IJU -> B 3Π9 (2850-5100 A) This system is usually very weak or absent in ordinary discharges through nitrogen. The upper state, C 3IJU (416), dissociates into one 4S atom and one s2p23s 4P atom (196). The system was first detected by Goldstein in 1905, from an induction-coil discharge in nitrogen at liquid air temperature (417), and subsequently studied by Kaplan (418) and by Hamada (346). The latter's investigation showed that the bands were emitted at ordinary temperatures only in very pure nitrogen. They were enhanced, relative to the second positive bands, by an increase of pressure. Their intensity was markedly increased at low temperatures, and especially at the temperature of liquid air. They also appear to be emitted in the "light of the night sky" (346, 419), from the aurora (303, 346), from nitrogen that has been mildly excited with an "ozonizer-type" discharge (91,307) or with a Tesla coil (91), from nitrogen excited by 50 keV electrons (366b), and from discharges through argon-oxygen-nitrogen mixtures (348). Hamada reported 13 bands of the Goldstein-Kaplan system in the shortduration afterglow (observations 0.01 to 0.1 sec after termination of the discharge) from a 3.5 kV dc discharge through nitrogen at liquid air temperature, and pressures of 1 to 30 torr (314). The v = 1 progression was very feeble compared with the v' = 0 progression, but both increased in intensity with increasing nitrogen pressure. However, recent measurements on emission from a discharge through labeled 15N2 indicate that the Goldstein-Kaplan bands originate only from the v = 0 level in the upper

42

2. Light Emission front Active Nitrogen Systems

state (293). It was therefore suggested that the bands observed by Hamada do not belong to this system. Some emission of the Goldstein-Kaplan system has been observed in the short-duration "auroral" afterglow of Kaplan ( ^ 1 0 torr) (310), in the "auroral" glow of Herman and Herman (0.01 to 0.1 torr) (220), and in the short-duration "blue" and "red" afterglows of Bryan, Holt, and Oldenberg (3 to 30 torr) (224), all of which have been described earlier. The bands were also emitted in the intense "auroral" afterglow of Tanaka and Jursa (222). They showed that the intensity, relative to other bands of N 2 such as the second positive, was much greater in the afterglow region than in the exciting discharge. No emission in the Goldstein-Kaplan system from v' > 1 has yet been observed. Three bands, in the region 4450 to 5112 Â, have been observed from active nitrogen condensed at liquid helium temperature (420, 421). These were first attributed to a transition from the quintet state of N 2 to the A 32,w+ state (44), but Oldenberg has suggested that they correspond to transitions in the Goldstein-Kaplan system (422). 3. THE BIRGE-HOPFIELD SYSTEMS OF N 2 , b 1TIU , AND b' 1Συ+ -> X χΣβ+ (930-1650 A) These singlet systems were first observed by Birge and Hopfield in the emission of an ordinary discharge through pure nitrogen at low pres­ sures (325). Partial resolution of the rotational structure of some absorption bands of the (b — X) system indicates that the upper state is of the xnu type, subject to perturbations (423). More precise rotational analyses have recently been reported from an electrodeless discharge through inert gas—nitrogen mixtures (424). Afterglow emission from the b state (vf = 0) appears to have been detected (one report) only in the "pink" nitrogen afterglow (343). The Birge-Hopfield system b' XZU+ —► X τΣβ+ is very prominent in the vacuum UV spectrum of nitrogen. The b' 1Σιι+ level dissociates into one 3 P ion of N+ and one 3 P ion of N - , and is 12.85 eV above the ground state (196). The first rotational analysis of the emission from the b' state was made by Watson and Koontz (328), and Setlow later confirmed that the upper state of the transition was b' 1Συ+ (425). Thirty-seven bands of the system have recently been observed in emission (330). A rotational perturbation occurs in the v = 1 level of b' (330, 425), as well as a breaking-off in the υ = 4 level (330). Nevertheless, emission has been observed from levels up to v = 6, and the predissociation in the v' = 4 level appears to be weak, with an observed limit about 1.1 eV above the dissociation limit of N(4S) plus N( 2 D). The predissociation could be the result of a radiationless

///. Electronic Energy Levels above 9.76 eV

43

transition from the b' 1ZU+ state into the C ZTIU state above its dissociation limit (12.36 eV) (330). Rotational analyses of the emission from an electrodeless discharge through nitrogen—inert gas mixtures suggested that five bands, previously assigned (330) to the V 1Συ+(ν' = 5) —► X 1Σ9+ transition in the region 973-1060 Â, may be part of another transition (mf —* X *Σβ+) (45,424). In 1962, during observations on the "pink" afterglow of nitrogen in the vacuum UV region, Young detected very weak emission of the b' system with υ' < 2 (342). The radiation orginated from levels of the b' 1Σν+ state 12.94 eV above the N 2 ground state, and extended to the cutoff of the lithium fluoride observation window. As with all other nitrogen emission, the emission of these bands was strongly quenched upon application of an ac electric field to the region of the short-duration "pink" afterglow. Results similar to those of Young were later reported by Fairchild, Prag, and Clark, for emission from the v' = 0 level of the b' 1Σιι+ state in the "pink" after­ glow (343). 4. THE FIRST NEGATIVE SYSTEM OF N2+ , B 2Ση+ - * X 2Σ9+ (2900-5900 A) This system, involving a transition to the ground state of the molecular ion, N2+, is one of the most prominent band systems of nitrogen. It is observed in the negative column of a discharge through nitrogen or air, and in various other sources of excited nitrogen. A rotational analysis of these bands was first made by Fassbender in 1924(426). With a discharge through helium containing a trace of nitrogen, Douglas was able to make observations on the first negative system of N2+ in the absence of the second positive bands of N 2 , by which it is generally overlapped and partially obscured (35). He detected bands with higher vibrational quantum numbers than any previously observed, and showed that the B 227w+ state dissociates at a limit of 8.7 eV above the ground state of the molecular ion, N 2 + (X 2Σ9+). A number of studies of the rotational perturbations in the B 2ΣΗ+ state gave indications of an effect induced by an unknown state of the type 2TIU . A state fitting the description of this postulated level was detected by Meinel in 1950 from studies of infrared auroral spectra (427). He suggested a transition A 2IJU -> X 2Σ9+ (ground state of N 2 + ), in which the A state corresponds to v' = 0 at 16.8 eV, that is, considerably below the B 2Σ1ί+ state. Tyte has observed differences in emission from various vibrational levels of the B 227M+ state when a discharge is passed through helium containing small amounts of nitrogen (428). These indicate a very complex process for excitation of the first negative system under such conditions. Indeed, contrary to theoretical expectations, variations were observed, especially at low current densities, in the ratio of the intensities of two bands from a common upper level. The low gas kinetic temperature of such a discharge, which limits the

44

2. Light Emission front Active Nitrogen Systems

rotational development of the bands, has recently permitted him to observe some new bands that originate in the tenth and eleventh vibrational levels of the upper state and which are overlapped in the usual sources (429). A number of bands of the first negative system not yet discovered (a gap in the Deslandres table) would be weakly degraded, and also coincide with strong bands of this system, or of the second positive system of N 2 . Tyte has pointed out that the more or less continuous background due to these missing bands (in the region 3075-5058 Â) may be a cause of error in intensity measurements on the first negative and second positive systems. The first negative bands of N2+ are an important component of emission from the earth's upper atmosphere. This system was observed in the twilight aurora in 1933 (430), and bands have been positively identified in the spectrum of both the auroras and the "airglow" (215). Observations of the (0, 0) and (0, 1) bands during the "Great Aurora of 1958" gave a measure of the rotational temperatures involved (302). Observations in the 4652 to 4709 Â region indicated the presence of high vibrational excitation (ν' = 2, 1) in the B 2Σιι+ state of N2+ (431). The latter observation favored some excitation of the first negative bands through absorption of sunlight by N 2 + (X 2Σ9+), as had been suggested previously by Oldenberg (432) and by Bates (359). The B 227M+ state of N 2 + can also be populated (431) in auroras during collision of N2(X χΣβ+) with energetic secondary electrons [relative intensities of the various vibrational bands have been calculated by Bates (359)], by ionization induced by high-speed protons, or by the charge exchange reaction, H+ + N 2 (X ^ + ) — N 2 +(B 2Ση+) + H

Observations on the zenith intensities of the 3914 Â band of this system have suggested (a) that the N2+ ions in the twilight airglow are probably located at 550 to 620 km, and produced there by extreme ultraviolet solar radiation (433); and (b) that there is probably a bimodal excitation of the N2+ first negative system in the auroras (434). A photoelectric "temperature photometer," based on intensity measurements of the (0, 0) band of the N 2 + first negative system at 3914 Â, has been used to obtain a temperature profile for the atmosphere between 95 and 170 km (435, 436). A temperature reading from the aurora may be obtained in about 1 sec with the device, and a gradient of 6°K km - 1 was observed over the region studied. Emission of a N 2 + band at 4278 Â has permitted rocket optical observation of daytime auroras (437). Cross sections for excitation of the B 2Συ+ state of N2+ during inelastic collisions with electrons have been measured by an optical method (438) and, more recently, by a cross-beam technique employing a fast neutral beam

///. Electronic Energy Levels above 9.76 eV

45

of around 2.7 keV energy and interaction energies from near threshold to 500 eV (439). At an electron energy of 200 eV, the electron excitation cross section of the (0, 0) band of the N2+ first negative system was five times that of the (0, 0) band of the second positive system of N 2 (440). Maximum cross section for excitation to the zero vibrational level was about 12 times that for excitation to the first vibrational level (441). Sheridan and Oldenberg have found that, for electrons of about 10 keV energy, secondary electrons are 5 to 10 times more efficient than primary electrons for exciting the first negative bands (442). Excitation by monoenergetic electron beams, in the energy range 19 to 300 eV, to produce 3914 Â emission, appeared to be by single electron impact (366b, 442a). However, Culp and Stair reported that the rotational temperature depended upon electron energy for energies below 100 eV. Muntz has described a method for measuring the rotational and vibrational temperatures, and the molecular concentrations, in a nonradiating stream of nitrogen at low density (443). Emission from the first negative system of N 2 + was excited by passing a narrow beam of high-energy electrons through the flow. Similar techniques for measurement of rotational and vibrational temperatures have been employed by Marrone (444) and by Schmeltekopf et al. (131). Bombardment of nitrogen or air at pressures in the neighborhood of a few hundred torr, with 50 keV electrons, has also been reported to produce intense first negative emission (366, 366a, 445). Similar emission has been observed from electron-irradiated (N 2 + 0 2 ) mixtures in the pressure range 1-10 torr for N 2 concentrations in excess of 66% (446). The efficiencies for excitation of first negative emission, by collisions of energetic electrons with N 2 , have recently been examined theoretically (447, 448). Davidson and O'Neil have measured a fluorescent efficiency of 0.013% for production of 3914 Â radiation by 10 keV electrons in nitrogen at 22 torr (449). The corresponding value in air was 0.0067%. There has been some indication that proton impact (200 keV) on nitrogen, which strongly excites the N 2 + first negative bands (368), may be more effective than electron impact in exciting the υ = 0 level of the B 2Ση+ state (438). At high energies, the ionizing reaction, H + + N 2 -> H + + N 2 + +e~, is the dominant feature (368), whereas in the energy range from 1.5 to 4.5 keV the dominant feature is charge exchange (450). It has been calculated that, with protons of 5-130 keV energy, about 15% of the N 2 + molecules are formed in the excited B 227M+ state (451). Absolute cross sections for the production of first negative emission have been measured as a function of the energy of the incident H+, D+, He+, Ne+, and H2+ (368a, 369, 451-453). The distribution of the rotational line intensities at 3914 Â and 4278 Â indicated a deviation from the Boltzmann distribution for excitation by a mixed 30 keV proton and H atom beam (454). First negative bands are

46

2. Light Emission from Active Nitrogen Systems

also excited when nitrogen is bombarded with 1 MeV protons (455), with polonium-210 alpha particles (80, 143, 370a), or with soft X-rays (370). Clouston and Gaydon found that the first negative bands of N 2 + could be excited by molecular collisions at the high temperatures produced by a shock-wave through nitrogen-argon and air-argon mixtures (375). Bands of this system may also be excited in shock-heated air at temperatures from 4000°K to 9000°K (100). Behind shock waves in xenon containing small amounts of nitrogen, emission from the v' = 0 level appeared before the v' = 1 emission in the first negative system of N 2 + (456). During the approach to equilibrium, the emission intensity was found to overshoot its equilibrium value. These results indicated a mechanism for the nonequilibrium production of N 2 + (B 2Σν+), and suggested that the cross section for ionization of N2(X λΣ9+) is much greater than that for subsequent electronic excitation of the ion produced. [The appearance potential for N 2 + (X 2Σ9+) under electron impact has been measured as 15.5 eV (42)]. First negative emission has also been detected from shocked nitrogen that contained N(4S) atoms as a result of a previous (pulsed) discharge (109). The ionization cross sections for nitrogen molecules, on impact with N 2 and 0 2 molecules over a range from 20 to 1000 eV, have recently been measured by a "molecular beam" technique (457,458). Ionization was observed a few volts above the threshold for all combinations of interactions, and the values obtained ranged from 10 -20 to 5 x 10~16 cm2. First negative emission may also be excited (charge transfer) during bombardment of nitrogen with molecular nitrogen ions (459,460), and with Li+ ions of energy 1 to 3 keV (455, 461). The Rydberg series (660-730 Â) of the B 2Σ^ state of N2+, with a convergence limit at v' = 0, was first detected by Hopfield in 1930(462). Tt has recently been extended to m = 21, which demonstrates further that the B 2Σ„+ level may be populated during absorption in the 600-1000 Â region (140, 463). The fluorescence (N 2 + first negative emission) induced in nitrogen by UV radiation has suggested similar preionization (122, 464-466). The output beam of a giant-pulse ruby laser has been reported to produce line spectra (4000-5000 Â) characteristic of singly ionized molecules (presumably first negative emission) in nitrogen at pressures of 1 and 15 atmospheres (467). An apparently valid extrapolation of discharge theory for microwave frequencies to the optical frequency range suggested that multiple photon absorption processes are probably not important for nitrogen. Heath has studied the emission of the first negative bands of N 2 + , excited by mild discharges in nitrogen, in air, and in nitric oxide (379). Bands were observed from vibrational levels up to v = 12 of the B 227Μ+ state. At 0.01 torr, the ratio of the intensity of the first negative bands to that of the

///. Electronic Energy Levels above 9.76 eV

47

second positive bands of N 2 was greater for discharges through NO than it was for discharges through air. or nitrogen, in which the ratios were com­ parable. In the pressure range 1.0 to 10 torr, however, the presence of even traces of 0 2 or NO decreased this ratio significantly. At atmospheric pressure, Heath observed no suppression of the first negative bands in the discharge through air, relative to that through pure nitrogen. This is contrary to earlier observations by Feast (380). For discharges through pure nitrogen, Heath found the intensity of the first negative bands, relative to that of the second positive bands of N 2 , to show a minimum between pressures of 0.1 and 1.0 torr. At pressures less than 0.1 torr, the negative bands appeared with a high degree of vibrational excitation but with a low rotational temperature. The reverse was true in discharges at atmospheric pressure. He suggested that, in discharges, two separate mechanisms may be responsible for the production of the first negative bands of N 2 + [populating N 2 + (B 2^M+)] and the second positive bands of N 2 [populating N2(C STIU)]. Weak emission of N2+ first negative bands, due to nitrogen impurity, has been observed in a short-duration visible afterglow produced by a highfrequency electric discharge through helium. The glow, which was separated from the discharge by a de Laval nozzle, increased in brightness with increase of pressure in the range 0.4 to 20 torr (226). The addition of 1 part of nitrogen to 103 of helium before the discharge replaced the helium glow with the N 2 + glow. Collins and Robertson observed similar first negative emission when molecular nitrogen was added to a helium afterglow (468). They attributed the selective excitation to collisions of the second kind with the active helium particles, He(2 3S) and He2+. Charge-transfer excitation of the first negative bands was suggested as a "titration" for the He 2 + species (469). The distribution of intensity of first negative emission followed the FranckCondon factor for excitation from the ground state molecules by either (374) He(2 3S) + N 2 (X 'Σ^)

-* N 2 + (B 227M+) + e~ + He (Penning reaction)

or Hea+(227„+) + N 2 (X ιΣ9+) -> Ν2+(Β 2Ση+) + He 2 (charge-transfer reaction)

Similar first negative emission has now been observed by other workers following N 2 addition to activated helium (87, 470). Excited xenon or neon species produced by soft X-ray bombardment also excite first negative emission from trace nitrogen impurities (87a). Pannetier and co-workers have found that traces of oxygen or hydrogen completely suppress the first negative bands of N 2 + that are normally observed in a discharge through pure nitrogen at a pressure of 1 torr (388). They also observed that this high-energy system was emitted from the reactions of

48

2, Light Emission front Active Nitrogen Systems

active nitrogen with brominated and chlorinated hydrocarbons introduced into the Lewis-Rayleigh afterglow (397,471). Light emission due to transition from the N 2 + (B 2Συ/+) state had not previously been observed in such reactions. The bands (0, 1), (1,2), (0,0), (2,0), (3, 1), and (4,2) (in the region 3293 to 4278 Â) were particularly intense in the reaction with 1,2-dibromopropane. The authors suggested that the formation of this excited nitrogen molecular ion might be favored by the ease with which the Br2 molecule is ionized in flames. First negative emission was also detected from the corresponding reactions with phosphine (in 75% argon) (398) and with PC13 (400). Strong and extensive emission of the first negative system, with transitions corresponding to v' values of up to 18, has also been reported from a high-voltage (4200-9500 volts) electric discharge through a low-pressure (3.5 to 6.0 torr) mixture of nitrogen containing a trace of methane (405, 406). Rotational constants of the band between v' = 5 and v" = 3 have been obtained from a discharge through nitrogen-helium mixtures at low pres­ sures (472). Emission of the first negative bands of N2+, which requires a minimum of almost 19 eV excitation from ground state N 2 , is usually a dominant characteristic of the short-duration afterglows of nitrogen. Although con­ spicuously absent from Kaplan's short-lived "blue" afterglow (312), these bands are strongly emitted, at pressures from 10 to 100 torr, in his green 4 'auroral" afterglow, after continuous operation of a discharge has removed the last traces of hydrogen lines from the spectrum (75, 74,310,311). They are also observed in the "auroral" afterglow produced by Herman with a pulsed discharge between electrodes (113, 219, 313), and in the afterglow produced by Kunkel with a rf glow discharge through pure nitrogen at pressures from 0.5 to 10 torr (114). The first negative emission of N 2 + , described by Kunkel, persisted visibly for about 0.1 sec after the discharge. It was completely suppressed by the addition of about 0.1% 0 2 to the nitrogen stream. Emission of the N2+ first negative system was also observed in the shortduration (10~4 sec) "orange" afterglow, which may be obtained with a high-tension arc (221), or a high-frequency discharge (58), through nitrogen. The "blue" and "red" afterglows of Bryan, Holt, and Oldenberg have also been found to emit the (0, 1) and (0, 0) bands of the first negative system, the latter at decay times up to 1.5 msec after a discharge through nitrogen in a static system (224). The first negative emissions from these afterglows were weak. However, over the pressure range 3 to 30 torr, they were stronger, relative to the second positive bands of N 2 , than those obtained from the discharge itself. The first negative system (v' = 0) has also been detected in a short-duration (10~3 sec) afterglow behind an arc discharge in streaming nitrogen at atmospheric pressure and a temperature around 1500°K (71). It is

///. Electronic Energy Levels above 9.76 eV

49

strongly emitted by an "intermediate" afterglow that may be observed under special conditions of high flow rate with an electrodeless discharge {210). First negative emission has also been reported from free-jets of low-density, arc-heated nitrogen and argon-nitrogen plasmas (72a). The N 2 + emission has been used to monitor development and structure of laminar jets produced at the throat of a "plasma torch" (473). First negative bands are also strongly emitted from the short-duration (~10~ 6 sec) afterglow produced by bombardment of low-temperature gaseous nitrogen with a 10 keV electron beam (407). Temperatures as low as 16°K were obtained in this system by means of an expanded nitrogen flow. In the "auroral" afterglow, the N 2 + first negative bands were observed by Brömer to decay more rapidly than the N 2 bands that are also emitted in this afterglow (223). This behavior was confirmed by Tanaka and Jursa (222). They also showed that the first negative bands were the strongest of all systems detected in either the exciting discharge or the "auroral" after­ glow produced by their simplified excitation technique. They found the decay to follow an exponential curve, with a half-life of 0.01 sec for the band at 3914 Â. In the "auroral" afterglow produced in pure nitrogen, at 1.8torr with a rf discharge, the first negative system (as well as the first and second positive systems) increases in intensity to a maximum after an initial rapid decay with time (410). The short duration "pink" afterglow emits strongly in the visible and near UV regions of the first negative system (112). Innes and Oldenberg consider this afterglow to yield "the same spectrum as the green auroral afterglow excepting relative intensities" (225). However, Tanaka and Jursa believe the two afterglows to differ in spectra as well as in color (222). Beale and Broida found that, when the "pink" afterglow had attained maximum intensity, at about 5 msec after the discharge, the (0, 0) band of the first negative system of N 2 + was about 1/20 as bright as in the discharge itself (772). There was a small decrease in population of higher vibrational levels of this system in the "pink" afterglow relative to the discharge. However, the afterglow showed emission from the N 2 + (B 2Συ/+) level up to v' = 6. It was completely quenched when 0.1% 0 2 was added before the discharge, and partially quenched by the similar addition of 3% argon or helium. It was not affected by strong magnetic fields but the addition of 1% 0 2 between the discharge and the afterglow removed practically all emission in the first negative system. More recent studies have indicated a maximum in the ionization density and in the electron temperature in those regions of the "pink" afterglow where emission of the first negative bands of N 2 + is a maximum (409). This suggests a connection between mechanisms of ioni­ zation and mechanisms of atom formation in the early stages of the afterglow. Young has reported that the application of an ac (~60 kc) electric field

50

2. Light Emission front Active Nitrogen Systems

to the energetic "pink" afterglow strongly quenched the emission of the first negative bands of N 2 + in this region (342). He has suggested that excitation of these bands might be the result of dissociative recombination of N3+ with N(4S) atoms. Further comparative studies of the first negative bands of N 2 + and the first and second positive bands of N 2 in the "pink" afterglow, over a large range of pressures, in the absence and presence of an ac electric field, have indicated weak emission from levels as high as v' = 15 (22.7 eV) of the B 2i7M+ state (343, 408). It has been demonstrated that, relative to the emissions due to N 2 molecules, the first negative bands of N 2 + in the "pink" afterglow are considerably quenched when small amounts of carbon-containing reactants are intro­ duced (404). No such differential effect was observed with other com­ bustibles, such as H2S (401), H 2 0 (402), or C12S (474). It was suggested that the quenching of N 2 + (B 2Σί1+) may be due partly to a process that leads to formation of the CN radical. The long-lived Lewis-Rayleigh afterglow does not appear to emit the first negative system of N 2 + , although Strutt obtained evidence, during his early studies (16), for the presence of ions in this afterglow. He (Rayleigh) demonstrated later that the number of ion pairs generated in the afterglow decays at a faster rate than the number of photons emitted (475). Many ionizing effects in this afterglow may be due, however, to free electrons (discussed later), rather than positive ions, in the stream (23).

IV. Emission from Atomic Nitrogen This section will be mainly concerned with the forbidden emission from the low-lying metastable 2 D and 2P states of atomic nitrogen (ground 2pz configuration), which may be populated during mild excitation (arc spectra of Ni lines). Some mention will also be made of the allowed Nx emission from the much higher-energy (10.6 eV above the ground state) 3s 2P state of atomic nitrogen. Emission of such arc lines is characteristic of a cathode glow, or "erste Kathodenschicht" (476). No attention will be given to the N n lines emitted by the higher-energy, ionic form of the nitrogen atom (spark spectra). Light emission due to a transition originating from the lower atomic levels, although forbidden by electric dipole selection rules, may give rise to the doublet corresponding to (2D -► 4S) at 5200 Â and ( 2 P-> 4 S) at 3466 Â. However, both the 2 D state [2.38 eV above N(4S)] (477) and the 2P state [3.57 eV above N(4S)] (477) possess extremely long theoretical radiative lifetimes of about 26 hours (478) and 12 sec (479), respectively, toward these transitions. Hence, these lines are observed in emission only under special conditions that may favor persistence of these metastable atoms long enough

IV. Emission from Atomic Nitrogen

51

to permit some radiation. Although radiation from the 2P state requires prior excitation to a higher energy level than that for emission from N( 2 D), the shorter radiative lifetime of the former state obviously enhances the probability of observing the line at 3466 Â. This is particularly applicable in laboratory systems where deactivating collisions are important. In fact, as Stewart has pointed out (90), the total Einstein A coefficient of only about 3.5 x 10~5 sec- 1 for the 2 D -> 4S transition (480) makes it doubtful that it can be observed at all in spontaneous emission (5200 Â) from gaseous laboratory sources. The (2P —► 2D) multiplet (group 2/?3 configuration) of Νχ may produce emission at 10,400 Â that is theoretically 15 times stronger than that at 3466 Â corresponding to the transition from the upper doublet state to the 4S ground state (479). However, it is difficult to detect this multiplet since it occurs in the region where the first positive bands of N 2 are emitted (90), and it does not seem to have been reported in gaseous active nitrogen systems. This forbidden doublet has been resolved for the first time in laboratory spectra, during recent studies of emission from nitrogen excited by 50 keV electrons at pressures around 600 torr (366, 481). Radiation from the 3s 2P state of atomic nitrogen (at 10.6 eV) to the z 4 2p S ground state of the nitrogen atom is forbidden. However, an allowed transition from this state to the metastable 2/?3 2 D or 2/?3 2P states produces emission of the Nx lines at 1495 Â and 1745 Â, respectively (482). Emission of the highly forbidden lines of atomic nitrogen, originating from the low metastable levels, has been observed from the earth's upper atmosphere, a region where deactivating collisions occur only infrequently. The doublet at 5200 Â (corresponding to the 2 D 3 / 2 -> 4 S 3/2 and 2 D 5 / 2 -* 4 S 3/2 transitions of atomic nitrogen) has been observed in the planetary nebulas (483). The auroras have shown emission of the 5200 Â lines of atomic nitrogen from high altitudes (484, 485), and that of the 3466 Â lines (2P —► 4S) over an extended range in height (486). However, the (2P —► 4S) nitrogen line at 3466 Â is conspicuously absent from the spectrum of the "airglow." Some evidence had been obtained for the occasional appearance of the nebular transition at 5200 Â as a weak line during twilight and early night (487). It has been established only recently, however, that the Ni line due to the transition (2D -> 4S) is definitely emitted (with a maximum at 95 km) in the night-sky spectrum (488). Intense emission of the forbidden (2P —► 4S) doublet at 3466 Â has been observed from a special "ozonizer-type" discharge through pure nitrogen at atmospheric pressure (307) and from a discharge in an atmosphere of xenon containing a small amount of nitrogen (306). Since the forbidden N Ï lines are observed only in very pure nitrogen in low-pressure discharges (300), it has been concluded that impurities, but not nitrogen atoms or molecules, deactivate the metastable nitrogen atoms responsible for emission of these lines (24).

52

2. Light Emission from Active Nitrogen Systems

Emission of N Ï lines has recently been reported from a plasma produced by a mechanically constricted arc through argon containing some nitrogen {488a, b) and from nitrogen subjected to bombardment by protons of energy 20 to 100 keV {368a). The behavior of 32 nitrogen lines under the influence of an inhomogeneous static electric field has been described, and the observed term shifts compared with calculated values {488c). Bay and Steiner obtained the optical spectrum of atomic nitrogen as early as 1929, by applying a second rf electrodeless discharge to the Lewis-Rayleigh afterglow {489). However, observation of the forbidden atomic nitrogen lines has been limited, until quite recently, to the energetic, short-duration afterglows of nitrogen. The first laboratory source of the doublet lines at 3466 Â (2P -> 4S) was the "auroral" afterglow of Kaplan {15, 74, 490) at a pressure of about 10 torr {309, 310, 491, 492). The elusive nebular transition (2D -> 4S) was also reported in this afterglow on one occasion that nitrogen was contained, at similar pressures, in very small bulbs {492). At about 100 torr, both the Kaplan "auroral" afterglow {311), and that produced by Tanaka and Jursa with their simplified technique {222), emit the Nx line at 3466 Â (forbidden 2P —► 4S transition). However, the latter investigators did not detect any evidence of the forbidden 2 D —► 4S transition (5200 Â). Various Ni lines in the 1000-2000 Â and 6000-12000 Â regions have been reported from free-jets of low-density, arc-heated nitrogen plasmas {72a-c). Young's recent observations in the vacuum UV region of the "pink" afterglow have indicated strong emission, at 6.8 torr pressure, of the atomic nitrogen lines at 1495 À (3s 2P -+ 2/?3 2D) and at 1745 À (3s 2P -> 2/?3 2P) (342). This radiation, which orginates in the highly energetic (10.6 eV) 3s 2P state of atomic nitrogen, was strongly quenched when an ac electric field was applied to the afterglow (342, 343). As late as 1954, no success attended a careful search for emission at 3466 Â and 5200 Â, due to transition from the 2P and 2 D metastable nitrogen atoms, from the long-lived Lewis-Rayleigh afterglow, even at pressures up to one atmosphere (69). However, shortly thereafter, Stewart reported that a heavy Tesla spark in nitrogen at 2 to 10 torr produced a Lewis-Rayleigh afterglow with an isolated line at 3466.5 Â, which he attributed to the unresolved doublet (2P -> 4S) (90). Weak emission of this line has also been detected in the low-pressure Lewis-Rayleigh afterglow at liquid nitrogen temper­ ature (413). The presence of both metastable 2 D and 2P nitrogen atoms in the low-pressure Lewis-Rayleigh afterglow, in concentrations about 1/500 that of N(4S), has been confirmed by optical absorption studies in the vacuum UV region (absorption at 1493 Â and 1743 Â, respectively) (339, 340, 493). In 1962, Noxon reported extensively on the emission of the forbidden nitrogen doublet (2P -> 4S) at 3466 Â in the long-lived nitrogen afterglow, produced at pressures of 20 to 760 torr, by an "ozonizer-type" discharge

V. Emission front Condensed Active Nitrogen

53

through pure nitrogen (89). The absolute intensity passed through a maximum at 170 torr as the pressure was increased. Noxon used the transition probability calculated by Garstang (479) to demonstrate that the fraction of the nitrogen atoms in the metastable 2P state rose rapidly with pressure. At one atmosphere, the concentrations of N(2P) atoms and ground state N(4S) atoms in the long-lived afterglow were almost equal. Emission of the forbidden Ni line at 3466 Â indicated a relatively more rapid decay of N(2P) than of N(4S) atoms over the entire pressure range studied. The predominance of the forbidden emission in the high-pressure afterglow suggested that the metastable N(2P) atoms can survive a very large number of collisions (at least 109) with N2(X 1Σ9+). The reduction in relative intensity at lower pressures probably is a consequence of a more rapid diffusion of N(2P) to the walls. It appeared that the majority of the N(2P) atoms in the afterglow region simply survived from the discharge and lived, on the average, for almost 1 sec in the afterglow at one atmosphere. No trace of the forbidden atomic nitrogen doublet at 5200 Â (2D —► 4S) was observed under any of the operating conditions employed. Herman and Herman have reported the discovery of a short-duration afterglow associated with metastable nitrogen atoms produced in the positive column of a dc discharge through extremely pure nitrogen mixed with xenon (494). The emission, with a short-wavelength limit of 4920 Â, appears to be due to a two-body atom recombination, for which the authors suggest N(4S) + N(2D) -> N(4S) + N(4S) + hv

The difference in wavelength, compared with that for the forbidden (2D -> 4S) transition (5200 Â), might be a result of transition from the shallow minima of the unstable potential energy curves involved. On the other hand, the emission might be due to a transition in a XeN molecule (495). The afterglow produced by a high-pressure Tesla-type discharge through (0 2 + N 2 + Ar) gas mixtures has been reported to be a good source of the forbidden atomic lines of both nitrogen and oxygen (317). V. Emission from Condensed Active Nitrogen or Activated Solid Nitrogen Condensation of an active nitrogen stream at liquid helium temperature (4.5°K), or the production of active nitrogen in the solid state, may result in a local high concentration of species that are present only to a very small extent in the gas phase. This concentration effect, in combination with the perturbing effect of a matrix on the forbidden emission, has made it possible to observe intense emissions at low temperatures, corresponding to transitions that are usually not prominent in gaseous active nitrogen.

54

2. Light Emission from Active Nitrogen Systems

The behavior of active nitrogen at very low temperatures has been reviewed elsewhere (202, 496-499), and discussion of the matter here will be limited mainly, though not entirely, to the more recent observations at low temper­ atures. Moreover, although the low-temperature afterglows may show emission due to the presence of species other than nitrogen (421, 500, 501) (e.g., 0 2 , NO, NH, etc.), attention will be given here only to emissions due to nitrogen itself. As early as 1924, Vegard reported the emission of a green glow when solid nitrogen was bombarded with electrons and "canal" rays from an electrical discharge (502). In the same year, McLennan and Shrum described the emission of very intense lines (5556, 5617, and 5654 Â) when nitrogen vapor at — 252°C was irradiated by electrons of high or low speeds (503). On the other hand, the phosphorescence of nitrogen induced at the temperature of liquid hydrogen consisted, in the visible region, of a single intense radiation at 5231 Â. Both these investigations revealed a brilliant phosphorescence of nitrogen which persisted at the low temperatures used, for several minutes after the excitation ceased (502, 503). A bright blue flash was emitted when the temperature was gradually increased to 35.5°K, above which no further phosphorescence was observed. Some understanding of the results of these early workers is made possible by the demonstration that many free radicals may be stabilized by isolating them in a rigid matrix (504). Such stabilization of free radicals was first suggested by Lewis and Lipkin (504), and was subsequently studied by a number of workers. Broida and Pellam described a strong yellow-green glow that was emitted during condensation, at 4.2°K, of the products from a 2450 Mc sec -1 electrodeless discharge through nitrogen at pressures from 0.1 to 3 torr (420). In this early experiment, the flow of nitrogen from the discharge region was maintained simply by condensation in a tube cooled with liquid helium. With the discharge operating, the walls of the collecting chamber, to which the cooled tube was attached, also emitted a strong green glow, but this glow was different from that in the cooled tube. After several minutes operation of the discharge, the cold surface showed brilliant flashes of blue, characterized by the random occurrence of local bright spots. When the discharge was extinguished and the flow of nitrogen stopped, the green glow on the surface persisted for more than 2 minutes although it decreased continuously in intensity, and no longer emitted the bright flashes. The glow reappeared on warming to about 10°K, but above 25°K it was replaced by a less intense blue-green glow, which terminated in a bright blue flash when the temperature reached about 35°K. The spectra of these glows between 3100 and 9000 Â, studied in a flow system maintained by a pump, indicated (505) that the radiation was the same as that obtained during

V, Emission front Condensed Active Nitrogen

55

electron bombardment of solid nitrogen (502, 503). The glows also suggested that a considerable number of nitrogen atoms, including metastable, lowlying excited atoms (506), were transferred from the discharge to the con­ densation region (a time interval of 10 -4 to 10 -3 sec). They then caused emission of radiation from upper states whose half-lives were long compared with the time spent in the afterglow region (44, 507, 508). Five so-called a-lines, in the wavelength range 5214 to 5240 Â, and with half-lives of 10 to 20 sec, were emitted from the long-lived blue-green afterglow produced at temperatures between 28°K and 35°K. These were attributed to the metastable 2 D state of atomic nitrogen (44). It was suggested that these lines differ in wavelength from the highly forbidden (2D -+ 4S) doublet (5200 Â) in gaseous nitrogen because the "forbidden" transition, with its 20 sec half-life (509), is perturbed by the solid matrix when the N( 2 D) atoms are in a particular configuration relative to a neighboring N 2 molecule (44, 421). Similar green phosphorescence has suggested the presence of N( 2 D) atoms in the solid products (4°K and 20°K) from the UV photolysis of H N 3 , F N 3 , C1N 3 , and BrN3 suspended in N 2 and Ar matrices (189, 190). The (2D —► 4S) line, emitted at 5229 Â from condensed atomic nitrogen, has been found to decay exponentially, with a half-life of approximately 30 sec, independently of both the temperature over which the glow was emitted and the manner in which it was initiated (570). By using the (2D —► 4S) transition to follow the behavior of the system, Broida and Peyron were able to show that atomic nitrogen could be trans­ ported by evaporation from one cooled surface to another situated 40 cm away, in a system under high vacuum (577). Similar evaporation of a very small fraction of the nitrogen atoms trapped in a solid matrix was sub­ sequently shown to occur by mass spectrometric observations (572). A blue emission, in the 3572 to 6390 Â region of the low-temperature afterglow (the "A bands"), was originally believed to result from a transition between the 5 Σ9+ and A 327M+ states of molecular nitrogen (44, 421). However, this was later discredited when assignment of the "A bands" to the quintet state was shown to be incorrect (507, 573). The Vegard-Kaplan bands of molecular nitrogen are also emitted from the low-temperature afterglow, and are particularly enhanced when the nitrogen is diluted with 80 to 95 % argon (574). These bands have been attributed to recombination of 4S nitrogen atoms, previously deposited in the solid in a manner so as to populate the A 3ZU+ state (514-516). The products condensed at 4.2°K, from nitrogen that has been subjected to a discharge, have also been shown to have an absorption spectrum consisting of two very weak bands near 3400 Â (577). These disappeared at temperatures above 35°K. Weak emission at 3478 Â, from the solid condensed from active nitrogen,

56

2. Light Emission front Active Nitrogen Systems

has been attributed to the metastable 2P state of atomic nitrogen, that is, to the atomic lines (2P—► 4S) (507). Strong emission near 10,475 Â has also been reported, and ascribed to the (2P -> 2D) transition between the low-lying metastable states of atomic nitrogen (576). The presence of N(2P) atoms in active nitrogen condensed on carbon monoxide at 4°K was also indicated by emission attributable to excited NCO radicals (capable of emitting light), formed by reaction with CO at the solid surface (575). It has been suggested that some weak "satellite" lines, associated with the ( 2 D-> 4 S) and the (2P—*2D) emissions, are due to double transitions. These involve electronic transitions in nitrogen atoms simultaneously with vibrational transitions in neighboring N 2 molecules (507, 575, 516, 519). Such double transitions were, in fact, first proposed by Vegard many years ago to explain the bands corresponding to the transition N2(A ζΣη+) —* N2(X 1Σ9+), which he had observed to be emitted from solid nitrogen irradiated with electrons (299). It should be noted that the very weakly bound triatomic molecule N 2 -N, which would constitute the emitting species in such double transitions (575, 516, 520), is not to be confused with the N 3 radical, which has been proposed to explain certain infrared obser­ vations to be discussed shortly. When nitrogen was condensed at 4.2°K, the solid activated by irradiation with y-rays, and the product warmed, a green glow was emitted, similar to that obtained when activation preceded the condensation and subsequent warming processes (527). The optimum radiative yield was estimated to be about 0.2 atoms per 100 eV of energy absorbed. Schoen and Rebbert have found that solid nitrogen may also be excited into emission by ac and dc discharges (522). They observed both the (2D -> 4S) atomic nitrogen lines, and the Vegard-Kaplan molecular bands. They concluded that excitation in the solid state is well suited for the study of species having lifetimes in the vapor phase too short to permit stabilization by a deposition technique. Hörl reported that an increase in energy flux caused a change in color, from "bright green to a more yellow-white", of the bright glow obtained when solid nitrogen was bombarded at 4°K with electrons of medium energy (3 to 5 keV) (523). The main features of the spectra were very similar to those obtained from condensed products from a discharge, including the emission due to N( 2 D -> 4S) (44). Hörl drew attention, however, to fundamental differences between the two systems. In the discharge experiments, only species having relatively long lifetimes can reach the cold surface (not considering energy transfer in the solid). On the other hand, bombardment of the solid may, in principle, produce many excited states that can radiate. In practice, it appeared that, in the bombardment experiments, the excited states capable of radiative transitions were limited, probably by recombina-

V, Emission front Condensed Active Nitrogen

57

tions during the warm-up process, to the same states that radiate from the solid obtained in the discharge experiments. The production of nitrogen atoms in solid N 2 by electron bombardment (12-20 keV) was confirmed by isotopic exchange measurements (524). Both the G value for atom formation and glow intensity were lower at 20°K than at 4.2°K. It was later demonstrated that the appearance of the nitrogen atom lines excited by electron bombardment may change markedly as the rate of deposition of the solid is altered (525). The thermoluminescence of solid nitrogen, after electron bombardment at 4.2°K, has been extensively investigated by Brocklehurst and Pimentel (526). With solid nitrogen that had been annealed at 20°K before it was bombarded, and warmed after bombardment, they observed peaks in the intensity of the glow at 10°K, 14.5°K, and 19°K. Most of their results agreed reasonably well with those obtained by Hemstreet and Hamilton (570) for solid nitrogen condensed after a discharge. However, in contrast not only to Hemstreet and Hamilton, but also to Edwards (498), they interpreted the afterglow to be due to diffusion and recombination of N(4S) atoms, rather than to storage of 2 D excited nitrogen atoms in special lattice sites. It was suggested that the energy of recombination, transferred through the lattice, may excite another 4 S nitrogen atom which subsequently radiates. On the basis of this mechanism, which requires three activation enthalpies (526), the emissions detected from solidified active nitrogen do not necessarily give evidence for the transport of excited nitrogen atoms from the discharge through the intervening afterglow region. It is of interest to note that the luminescence from cubic SiC at 6°K has been associated with a four-particle nitrogen-exciton complex, formed by interaction with a neutral nitrogen impurity (527). Studies at very low temperatures have given some information about the elusive N 3 radical, which has been postulated (16) as a possible reactive species in active nitrogen. The solid condensed at 4.2°K, from nitrogen subjected to a glow discharge, has been found by Milligan, Brown, and Pimentel to absorb in the infrared at 2150 cm - 1 (528). They attributed this to the asymmetric stretch of a linear N 3 radical. The N 3 radical is considered to be different from a loosely bound N2-N complex (516, 520), and cannot be composed of three equivalent nitrogen atoms (521). Absorption bands at 962 and 737 cm - 1 were tentatively assigned to nitrogen polymers of more than three atoms (528). From their results, the authors suggested that nitrogen atoms are not likely to remain as such in active nitrogen condensed at low temperatures. The activation energy retarding the reaction, N + N 2 = N 3 , would probably be too small to permit both nitrogen atoms and N 3 to be present. Pimentel and co-workers have therefore preferred to attribute the various glows observed under such condition to the presence

58

2. Light Emission front Active Nitrogen Systems

of N 3 (528). However, their absorption results have not been confirmed by other investigators (529). Nor does it seem possible that a linear inverted 2Π9 ground state of N 3 should result from interaction of a ground state 4S nitrogen atom with a ground state N2(X 1Σβ+) molecule (530). Further experiments have shown that the behavior attributed to N 3 is probably due to N 2 H 4 , since frozen products from a discharge through HN 3 were found to have spectral characteristics similar to the products condensed from the glow discharge through nitrogen (531). On the other hand, the infrared spectra from the photolysis of hydrazoic acid (190, 532) or from F N 3 , C1N 3 , or BrN3 (189) in solid nitrogen at 20°K, the electronic absorption spectrum in the photodecomposition of HN 3 trapped in krypton and xenon at 4.2°K (533), and the electron spin resonance spectrum of nitrogen subjected to y-irradiation at 4.2°K (534), all give some suggestion that the N 3 radical may be produced under these conditions. It also appears to be produced in argon matrices at 14°K in a reaction between fluorine atoms and H N 3 , and to yield nitrogen atoms by photodecomposition upon irradiation at 2720 Â (192a). Finally, when sodium azide containing F-centers, induced by irradiation with X-rays at — 196°C, was suddenly warmed from liquid nitrogen temperature to room temperature, a faint blue glow was observed (187). This has been assumed to be due to trapped nitrogen atoms, possibly produced as a result of energy absorption by the N3~ ion (187, 535). There have been numerous reports that colored materials are deposited, or that light is emitted, when active nitrogen, or products of its reactions, are condensed at temperatures higher than 35°K. Most of these phenomena appear to involve species containing atoms other than nitrogen and do not come within the scope of the present discussion. However, emissions from condensates around liquid nitrogen temperature (—196°C), especially those which seem to be due to the NH radical, are rather easily confused with emissions due to activated nitrogen and perhaps merit a brief description. In 1930, Lavin and Bates first reported a greenish glow from the products condensed at — 180°C from a discharge through ammonia (536). The glow, which could be obtained by trapping as far as 1.5 meter from the discharge, did not appear to arise from either atomic hydrogen or active nitrogen. It was suggested that it might be due to NH or NH 2 radicals (536). These observations were confirmed by Lunt and Mills a few years later (537). They also observed a weaker blue glow, when the products of a discharge through (N 2 + H2) mixtures were condensed at — 180°C, but not when NH 3 was added to glowing active nitrogen at low temperatures (—180°C or — 80°C). They suggested that the green glow from the products of the ammonia decomposition derived from excited NH 2 radicals, produced during the decomposition of a hydrazine molecule which, in turn, was the

V. Emission front Condensed Active Nitrogen

59

product of a reaction between two ΝΗ(ΧΔ) radicals and an H2 molecule in the ground state. Broida and Bass have also confirmed that a feeble blue-green glow is emitted by the products of a low-pressure discharge through ammonia, when these are condensed at 77°K (577). They found the spectrum of the glow to consist mainly of a continuum from about 5900 Â to well below 4000 Â, with a maximum around 4900 Â. In 1951, Rice and Freamo reported the condensation of a blue, para­ magnetic solid when the products of the thermal decomposition of hydrazoic acid were passed through a trap cooled in liquid nitrogen (538). The color appeared to be due to effective stabilization, on the cold surface, of the imine radical, NH, which has a half-life of 9 x 10~4 sec in the gas phase. In further experiments, colored deposits at liquid nitrogen temperature were obtained following an electric discharge through hydrazoic acid (539) and dimethylamine (540). The latter presumably yielded a CH 3 NHCH 2 radical, which was then stabilized at the low temperature. The thermal and photochemical decompositions of tetramethyltetrazene led to the deposition of a paramagnetic violet solid at a liquid nitrogen-cooled surface (547). This changed irreversibly to a white solid when warmed to about — 160°C, and it appeared that this solid might contain trapped dimethylamino radicals. Identification of the blue deposits is by no means unambiguous. In 1959, Rice and Ingalls reported that the absorption spectrum of the solid obtained at — 196°C, following the thermal decomposition of H N 3 , did give some indication of the presence of NH radicals (542). Nevertheless, they concluded that the constitution of the "blue material" was still not proven after some 10 years of work in their laboratory. The presence of ammonia in the reaction products, when the blue material from HN 3 is warmed above 77°K (542, 543), makes it doubtful that NH radicals from the decomposition of HN 3 are simply condensed on the cold surface. However, it is possible that most of the imine radicals react with HN 3 in the gas phase to yield NH 3 before the cold trap. The "blue material" may result when only a small fraction of the NH radicals reach the cold trap, where they form a colored polymer, (NH) n (544). It has been suggested that electrons might be trapped in the blue material, in a manner analogous to F-centers (545). The presence of trapped NH radicals, as well as NH 2 radicals, has been established in the products that are condensed on solid rare gases at 4.2°K after a discharge has been passed through mixtures of them with NH 3 or N 2 H 4 (546, 547). The presence of both radicals has also been noted following the photodecomposition of hydrazoic acid trapped in krypton or xenon at 4.2°K (533). The NH radical alone was detected when ammonia, contained in solid argon at 4.2°K, was irradiated with light of wavelengths less than

60

2. Light Emission front Active Nitrogen Systems

1550 Â (548). When the solid was warmed, the trapped NH radical appeared to be stable up to at least 36°K. Jacox and Milligan have obtained evidence, from infrared studies of reactions at low temperatures, that the NH radical, trapped in solid argon, may undergo matrix deactivation to a ground triplet state before it participates in reactions (549). Pannetier, Guenebaut, and Hajal have reported an intense greenish-blue emission from the condensed (78°K) products of the reactions between atomic hydrogen and hydrazoic acid or hydrazine (550). Under similar conditions, an intense, white "postluminescence" was observed from the products of the reactions between active nitrogen and primary, secondary, or tertiary amines (551). On the other hand, a blue solid was trapped in abundance from the reaction of active nitrogen with hydrazoic acid, although not from the corresponding reaction with hydrogen atoms (390). It appeared to be analogous to the "blue material" of Rice and Freamo (538). Postluminescences at liquid nitrogen temperature were also observed following reaction of active nitrogen, or an activated mixture of (N 2 + H2), with simple organic compounds such as acetylene, methyl chloride, methyl cyanide, dimethylamine, ethylamine, and dimethylhydrazine (552). The postluminescences consisted of the first positive system of N 2 , together with a continuum from 3800 to 6000 Â, and the violet system of CN (the latter probably as an impurity). They appeared to be associated with reactions between active nitrogen and substances containing any two of the atoms C, N, or H, or with reactions between atomic hydrogen and molecules containing a nitrogen atom. The reaction of HN 3 with atomic nitrogen was exceptional, in that it did not give a postluminescence (390, 552). Catalytic recombination of nitrogen atoms by the reactant, to yield a free radical collision complex, was postulated to explain both emission in the gas phase and the appearance, under suitable flow conditions, of a postluminescence downstream at a cold surface (552). A blue light has also been observed from a trap cooled in liquid nitrogen, following the gas phase reaction between active nitrogen and atomic hydrogen (392). It was suggested that the continuous emission might be due to the action of active nitrogen on an NH 3 species, metastable at 77°K, produced by the reaction of NH radicals with molecular hydrogen. More recently, the intense blue postluminescence obtained at 77°K downstream from a discharge through N 2 , N 2 H 4 , NH 3 , or the mixture (N 2 + H2), has been attributed to reaction of atomic hydrogen with nitrogen atoms or excited nitrogen molecules, to form complexes such as [N 2 (B 3 iJ ? ) — H]* and [N 2 (C 3 /7J — H]* (387). These may decompose to give NH radicals in different electronic states capable of emitting light.

VI. Summary of Light-Emitting Systems

61

VI. Summary of Light-Emitting Systems of Active Nitrogen According to Tanaka and Jursa, four afterglows of nitrogen are known at present, all of which differ in color and spectra (222): (1) the Lewis-Rayleigh afterglow (straw-yellow); (2) the "auroral" afterglow (green) (75); (3) the "blue" afterglow (312); and (4) the "pink" afterglow (112). Excellent colored photographs have recently been presented of the Lewis-Rayleigh afterglow, and of the short-lived "pink" afterglow produced by an electrodeless discharge in both the presence and absence of added helium (262). Also displayed is the "air afterglow" produced in the Lewis-Rayleigh afterglow region when 0 2 or NO was added. The Lewis-Rayleigh afterglow stands apart, in having a lifetime of the order of seconds, compared with lifetimes of milliseconds for the other glows. This best known of the afterglows may be produced by various types of discharges through molecular nitrogen at relatively low pressures (~1 to 20 torr). At these pressures the visible afterglow is completely dominated by the first positive system of N 2 , with emission from the B 3 /J 3 state enhanced for vibrational levels around v = 11, 6, and 2, and completely absent for vibrational levels greater than 12. Emission in the infrared region is largely due to the "Y" bands of N 2 (200, 208). These emissions may be attributed to recombination of ground state N(4S) atoms. Some extremely weak emission of the (2P —>- 4S) doublet may also occur in the Lewis-Rayleigh afterglow at pressures less than 10 torr (90), as well as very weak emission of the Lyman-Birge-Hopfield bands in the vacuum ultraviolet region (339, 340). At pressures above 20 torr the Lewis-Rayleigh afterglow shows strong emission of the Vegard-Kaplan bands (relative to the first positive system, which decreases rapidly in intensity as the pressure is increased), as well as emission of the atomic nitrogen doublet (2P —► 4S) (89). Emission of the second positive bands of N 2 from the Lewis-Rayleigh afterglow may also be detected when the decay region is cooled with liquid nitrogen (413). The "auroral" afterglow of Kaplan, produced under certain conditions by a discharge through very pure nitrogen, shows emission of all the band systems of nitrogen normally observed in a discharge (75). These include the first positive system of N 2 , with enhancement of the high vibrational levels above υ = 12, the second positive system, the Goldstein-Kaplan and Vegard-Kaplan systems of N 2 , the first negative system of N2+ (strongly emitted), and the forbidden atomic nitrogen line (2P —► 4S) at 3466 Â (222). Kaplan's short-lived "blue" afterglow is produced only at low pressures (10 _2 torr) and emits a spectrum which, although quite similar to that of

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2. Light Emission from Active Nitrogen Systems

the "auroral" afterglow, differs from it mainly in lacking the first negative bands of N2+ (312). A very short-lived afterglow is produced with a pulsed electrodeless discharge through nitrogen containing a trace of oxygen, in a static system at 0.017 torr (553,554). Its rate of decay was found to be decreased by 2.9 zb 0.5% when a magnetic field of 850 gauss was applied. This suggested that the energy released in ionic recombination contributes appreciably to the excitation of this afterglow. The short-duration ( ~ 2 x 10~3sec) "pink" afterglow may be produced in pure nitrogen at pressures between 4 and 15 torr, and occurs approx­ imately 5 msec after the discharge in & fast flow system (112). In the visible and near ultraviolet regions it is characterized by strong emission of the N2+ first negative bands and N 2 first positive bands (with v' > 12), and by weak emission of the N 2 second positive bands. It may also show, in the vacuum ultraviolet, weak emission of the Lyman-Birge-Hopfield bands, one of the Birge-Hopfield systems, and the atomic nitrogen lines (3s 2P —> 2/?3 2D) at 1495 À and (3s 2P -+ 2ps 2P) at \745 A (342). A recent report indicates that the D 2Π9 —► A 2IJU system of N2+ may also be emitted by the "pink" afterglow (555). This system was discovered by Janin and d'Incan in 1958 (556), and Franck-Condon factors have been tabulated by Nicholls (557) and by Halmann and Laulicht (558). A potential energy curve for the N2+(D 2Π9) molecule has been calculated by Guérin-Bartholin (559). It is possible, perhaps, to distinguish more afterglows than those indicated by Tanaka and Jursa, although there are marked resemblances between them. For example, Bryan, Holt, and Oldenberg reported a short-lived afterglow which showed emission of the N 2 + first negative bands, together with the Goldstein-Kaplan bands, the first and second positive bands, and the Vegard-Kaplan bands of N 2 (224). It was therefore quite similar to the "auroral" afterglow of Kaplan (15), although it occurred within much shorter time intervals after the discharge. It also greatly resembled the "pink" afterglow which, in fact, resembles the "auroral" afterglow of Kaplan (15), except that it lacks emission from the highest vibrational levels of the first positive system. The afterglow emitted by Hamada's "metastable nitrogen," which he considered to contain a relatively high concentration of N2(A 3Σ„+) molecules, also resembles the "auroral" afterglow of Kaplan in its emission of the Vegard-Kaplan bands (314). There is some resemblance, too, between the "auroral" and "pink" afterglows and the "striking blue flame" which streams away from a tungsten electrode, with emission of the second positive system of N 2 (92, 93), although the last may have a lifetime of seconds. Finally, in the predominence of the N2+ first negative system, the short-duration afterglows that have been described

VI. Summary of Light-Emitting Systems

63

by Herman (775, 279), Kunkel (774), Stanley (227), and Schulze (77) all resemble the "pink" afterglow. Oldenberg has recently suggested that the diffuse glow which surrounds an electron beam through nitrogen at low pressure (10~3 torr), and emits the first positive system (288), should also be interpreted as a short-lived afterglow (560). It is limited (288) by v' = 12, but emission from levels below this shows no particular enhancement around ^' = 11 (cf. first positive system in the pressure range 1 to 20 torr). Condensation of active nitrogen at liquid helium temperature, or activation of nitrogen at this temperature, may produce emissions not usually dominant in the gas phase, particularly the Vegard-Kaplan bands (299, 514) and the atomic lines due to the transitions (2P —► 4S) and (2D —>■ 4S) of the nitrogen atom (44, 501). When active nitrogen is passed through traps at temperatures above — 196°C, or when the products from the decomposition of Ncontaining compounds are trapped at these temperatures, colored substances (542) and "postluminescence" (387) may be produced. However, these phenomena may be due to species, perhaps impurities, containing atoms other than nitrogen. Arrays of Franck-Condon factors to high vibrational quantum numbers, including isotope effects, have been published for the following band systems discussed in this chapter: the first positive (558, 561, 562, 562a), the VegardKaplan (561, 562, 562a), the Lyman-Birge-Hopfield (558, 561, 562), and the second positive systems of N2 (558, 561, 562), and the first negative system of N 2 + (558, 561, 563). Oscillator strengths (/-values) have been recently calculated from experimental data for the first positive (98a, 563a), the Lyman-Birge-Hopfield (337a), and the second positive systems of N 2 (337a, 563a), and the first negative systems of N 2 + (563a). Empirical rules for molecular spectroscopy of diatomic molecules in general have been discussed in detail by Müller and Bräuer (564). Improved formulas have been presented for calculation of the vibrational transition probabilities for the nitrogen first positive (565, 566) and second positive (565) systems. Golden has used a "smeared" line model of individual vibrational bands to calculate approxi­ mate spectral absorption coefficients for most of the electronic transitions of molecular nitrogen considered in this chapter (566a). The fluorescent efficiencies for electron (50 keV) excitation of the first positive, second positive, and Goldstein-Kaplan systems of N 2 , and of the first negative system of N2+, have been measured by Davidson and O'Neil in nitrogen and in air at 600 torr (449, 481). The total fluorescent efficiencies were (0.14 ±0.02)% for nitrogen and (6.7 ± 1.0) X 10~3 % for air. For excitation by 50 keV electrons, Brocklehurst and Downing have measured G values from 0.025 to 0.035, over the pressure range 20 to 325 torr, for excitation of second positive emission (366b). They suggested that the

64

2. Light Emission front Active Nitrogen Systems

B *I7g state might show a G value close to unity at the higher pressures, but that collisional quenching greatly reduces the population of this state before first positive emission can occur. Sets of electron excitation and ionization cross sections for N 2 , as functions of incident electron energy, have been calculated from experimental data using extrapolations of Bethe's theory of generalized oscillator strengths (567). Cross sections have been calculated for various types of electronic excitations in the first Born approximation for incident electron energies from 50 to 500 eV (568). It has been demonstrated that a discharge with a firing voltage of 6 kV through nitrogen, in the pressure range 18 to 22 torr, may provide a fast (i.e., submicrosecond) flash lamp, with a 1000 Â wide output centered about 3600 Â (569). Five anti-Stokes lines have been observed from nitrogen compressed to 500 kg cm - 2 by means of laser irradiation (570). Wilson has reported operation of a pulsed nitrogen laser in a supersonic flow (577). The known band-head wavelengths of the N 2 molecule and N2+ ion have also been summarized (572). So, too, have the absorption coefficients of molecular nitrogen in the 1000-580 Â wavelength region (140, 573, 574). These were obtained with a photoelectric scanning technique, with an instrumental bandwidth of 0.5 Â, and a continuum as a background light source.