Luminescent charge transfer of metastable and ground state C+, N+, ions with N2 molecules

Chemical PhysicS 28 (1978) 97,112

:

0 North-Holland Publishing Company

LUMINESCENTCHARGETRANSFEROFMETASTABLE --~;CRO~D~TATEC+,N+,O~IONSWITHN,MOLEC~L~ Ch. O’ITINGER and J. SIMONIS Max-Hanck-Institut fiir SttCbtuzgsforschung, 3400 Giittingen. FRG Received 22 August 1977

Light emission from charge transfer collisions between C’, N+, and O*ions and N2 molecules is observed in abeamtarget gas experiment. Projectile ion beams with different concentrations of metastable ions are used in order to explore charge transfer from ground state and metastable ions separately. The excitation cross sections of the Nz(B *EG) state and the relative populations of the vibrational levels u’= 0 - 4 are derived for ground state and m&stable primary ions at 1000 e&b. Cross sectionsfor ground state C*, @, and O’ions range from 4 to 8 X lo-l8 cm*. leading to similar, broad vibrational distriiutio~ of &z(B). Cross sections for metastable C’and 0’ ions are substantially larger, up to 4 X 10-” cm* for O’
1. Introduction In charge transfer collisions between atomic ions and molecules the product molecular ion is sometimes left in an electronically excited, radiating state. Such processes lend themselves to a spectroscopic study of the reaction mechanism. With sufficient resolution, the population distribution over the internal states of the product particles can be measured. In particular, the problem of vibrational excitation in molecular ions produced by charge transfer has been studied a number of times [i-6]. For high collision energies, upwards from several keV, the observed distributionsP(u’) over the vibrational levels v’ of the eiectronicaIly excited molecular ion are cbrrectly described by Franck-Condon (FC) transitiotis from the molecular ground state to the excited ion state [1,2]. At lower collision energies, deviations from this high-energy limit can occur. The question then arises whether the vibrational distributions can still be described as FC transitions, taking into account a distortion of the molecular ground state in the electric field of the approaching projectile ion [7]. As an alternative to FC transitions energy resonance effebts have been held responsible for the vibra-

tional population at low collision energies [3] _In this model the vibrational level with the smallest energy deficit relative to the recombination energy of the projectile ion should be predominantly populated. The importance of energy resonance and FC overlap is also discussed by Marx [8]and, for thermal energy charge transfer, by Laudenslager et al. [9]. A favorite target particle in these investigations has been the N2 molecule. It is easily excited into the q(B22:) state by impact of a variety of projectile ions, and the emission spectrum of I$(B) is one of the best studied molecular spectra. The energy transfer characteristics of N, and I$ govern many important processes in the atmosphere, plasmas, and shock waves. A recent example is the observation of laser em&ion from N;(B) produced by charge transfer from Hef [lo]. Previous studies of N;(B) resulting from charge transfer with various projectile ions have shown that it is the velocity of the ions that governs the resulting vibrational distribution, regardless of the chemical identity of the projectile [l]. Below IO8 cm/s, with decreasing velocity, an increasing population of the levels u’= 1,2 and 3 relative to u’= 0 was observed, which means an increasingly stronger deviation from a FC

98

Ch. Otdnger. J. SimonisjCha~e

transfer of CT NIand 0’ ions with Nz molecuks

distribution in this case. This deviation was rationalized by Lipeles [7] in terms cf a distortion of the N2 molecule due to polarisation by the incoming ion. We decided to undertake an optical study of 4(B) produced by charge transfer from C+, N’ and 0” ions. These ions all have low lying electronically excited states which are met&able [l I], see table 1. Some of these metastable states have recombination energies that are close to the excitation of s(B); possibly giving rise to a very different behaviour from ground state ions. Particular emphasis was placed in this work on a comparison of the charge transfer reactions of metastable and ground state projectile ions. Luminescence experiments reported in this work cover the wavelength range of the Nz fast negative system, 2900-4750 A. Possible emissions from excited C, N, 0 atoms, particularly those in the vacuum UV, were not studied, although in the case of C+ + N, impact strong CI lines at 19318, and 2478 A were observed. No emissions from N;(C) (second negative system, 1800-2200 A) or from q(D) (Janin-dIncan system, 2000-3100 A) were seen with any of the projectiles C+, l@, O+. $(A) emission (Meinel bands, 6100-9500 A) falls outside the wavelength range covered in this work; excitation of N;(A) is known to occur in 0” + N, collisions [ 121, although the $(A --;, X) transitions appeared weaker than ?$(B + X). Additional experiments in our laboratory on He+, Ne+, Ar+ + N2 collisions [ 131 gave extremely weak %(A) emission. The collision energy used in these experiments was 1000 eI&t, corresponding to velocities of 12.6, 11.7, Table 1 Energy balance for ground state and metastabIe projectile ions Projectile

State

EXCitdiOll

energy WI

c+

Energy deficit relative to N;(B, d = 0)

leV1

2P 4P

11.26 (= IF’)

7.48

16.59

2.15

N+

3P ‘D ‘S

14.55 (= IP) 16.45 18.60

4.19 2.29 0.14

0+

4s 2D

13.62 (= IP)

5.12

16.94 18.64

1.80 0.10

2?

10.9 X 106 cm/s for C!+,N* and O”, respectively_ Thus the effect of the collision velocity should be practically the same in the three cases. In the present work we determined the vibrational population of N;(B) as well as the cross sections for excitation of this state with ground state and metastable C+, N’ and O+ ions separately.

2. Experimental

The apparatus used consisted basically of an ion source, an ion mass separator, a collision chamber and an optical spectrometer with associated detection electronics. The ions were produced from CO and from N2 in a plasma type ion source (Colutron, model 100). Typical operating parameters were: pressure 2 X10W2 to 10-l torr, discharge current 300 mA at voltages varying between 20 and 80 V, filament current 15 A DC at 10 V voltage (with a 0.5 mm diameter tungsten coil as a directly heated cathode). The anode is a sheet of tantalum, located about 7 mm from the cathode. Ions are extracted through a 0.5 mm diameter hole in the center of the anode. A conical extraction electrode, positioned opposite the anode at a distance of 4 mm, was held at -1000 V with respect to the anode. The total ion current passing through the 1 mm diameter hole in the extraction electrode amounts to about 7 a. Mass separation is performed with a 60’ 5 cm radius magnetic sector field. The magnet is mounted inside the vacuum chamber and is energized by a current of up to 100 A. The coil consists of 2 X 24 turns of copper tubing and is internally water cooled. The maximum field attainable is 7 KG which allows ions of mass up to 60 to be transmitted at 1000 eV_ The pole face distance is 7 mm. This distance limits the ion current accepted by the magnet to 2 PA, out of which 50% is focused into the collision chamber. The entrance slit of the collision chamber is 0.4 X 10 mm2. Located in the Image plane of the magnetic field, it defmes the mass resolution, in this case m/Am = 100. The collision chamber is a fairly gas-tight cell allowing a pressure differential against the surrounding vacuurn chamber of a factor of two hundred. The collision region proper is 24 mm long and shielded by stainless steel in order to keep it field free. The collision chamber is maintained at 120°C so as to avoid surface charges.

Ch. Ottinger, J. SimonisjCharge transfer of C+, NQandO’ions with N2 molecdes

99

Liquid nitrogen cooled surfaces are arranged 10 cm from the collision region in order to reduce the residual gas pressure in the collision cell. The target gas is supplied through a needle valve and passes the cold surfaces on its way to the collision region. The static target gas pressure, ranging between 1 X lOA and 2 X 1O-2 torr, is measured in a sidearm attached close to the collision region. For this measurement, a capacitance manometer (Datametrics Barocel, model 1173) is used, which is on its reference side connected to a vacuum of better than 5 X 10m6 torr. The ions that have traversed the collision chamber are monitored in two ways. Firstly, the current II on the rear plate closing the collision chamber is recorded. In addition, those ions are detected which leave the collision chamber through a 1 X 14 mm2 slit in the rear plate (current 1,). I, is a measure of the total current entering the collision chamber, whileI consists of ions traversing the chamber centrally. Since light is collected from tbIs central region, 1, is representative of those ions which generate the observed light signal. Energy analysis can be performed on the ions emerging from the collision chamber exit slit by focusing them onto the entrance aperture of a 90” 8 cm radius electrostatic sector analyser. It has an energy resolution of E/AE = 200and served to establish the energy spread A&?of the primary ions. AE was found to increase from a minimum of 0.7 eV up to 2.0 eV fwhm with decreasing ion source pressure. Typical C+, N+, Of ion currents at energies of 1000 eV were I1 = 5 X IO-* A and 12=1.5X10-8A. The kinetic energy of the ions in the collision chamber could be varied from 1000 eV down to about 2 eV by decelerating them with a simple immersion lens in front of the collision chamber. It consisted of a cylinder, 14 mm diameter, 24 mm long at low potential, followed by a cylinder, 14 mm diameter, 6 mm long at high potential. The collision chamber entrance slit was mounted on the rear end of the second cylinder. Although this system does not produce an equally well

sion from the collision region Was observed at right angles to the ion beam and in a direction parallel to the collision chamber slits. The light passes through a quartz window, located 33 mm from the ion beam, and is focused by a spherical mirror (radius of curvature 50 cm, 11 cm diameter, -Al+ MgFZ coated) onto the entrance slit of a grating spectrometer (McPherson, model 218, f = 30 cm, f/5.3 aperture). With a grating of 2400 l/mm, the dispersion is 13.3 A/mm. The slit widths mostly used were 300 1-1and 750 p, corresponding to spectral resolutions of4 A and 10 a fwbm, respectively. The spectrometer was equipped with a stepping motor titb a step size corresponding to 0.025 A. The light was detected by photon counting, using a cooled (-40°C) EMI 6256 S photomultiplier, followed by an emitter follower, a fast 16 X amplifier, discriminator and 1024 channel scaler. At 1400 V multiplier voltage and a discriminator threshold of iO0 mV, the dark current was 0.5 counts/s. Spectra were obtained by scanning repetetively over a preselected wavelength range, over 2000 a at a rate of 2.5 a/s or over 200 A at 0.5 A/s. Accumulation of the data for 2 up to 100 successive scans was necessary to achieve a sufficient signal to noise ratio. No data were taken during the rapid (25 a/s) return of the stepping motor after each scan. The spectral response of the overall optical system was calibrated using an Osram Wi 17/G tungsten standard lamp. The vacuum system consisted of four separately pumped chambers, one each for ion source, mass selector, collision cell and energy analyser. Four Edwards mercury diffusion pumps were used having a speed of 240 l/s each gbove the liquid nitrogen cold trap? Pressures in the four chambers were 2 X 10m5, 2 X 10m6, 5 X 1O-5 and 5 X 10-C torr under typical operating conditions.

collimated beam at all energies, the (energy dependent)

3.1. Charge exchange spec:ra

beam spread inside the collision chamber is of no consequence in the present work. Typically, the ion currents 1,. and 1, dropped by factors of two and four, respectively, as the ion energy was lowered from 1000 to 50 eV_ -The optical system* was arranged such that emis*The optical and electronicdetectionsystemsweredesigned by Brandt [14].

3. Results

Fig. 1 shows a set of $(B-X) spectra which were excited by impact of C+, fl, and O+ ions on N, at an energy of 1000 eVlab_ The 2900-4750 a wavelengtb range includes all of the Important bands of this system (fig. 1, left-hand side). The spectral resolution used in these survey spectia was 10 A fwhm. As can be seen, individual bands are hard to resolve, since

loo

transfer of C: NT and O+ions with

Ch. Ottinger, J. Simonis/Charge

1.

a

s

-

I

“1

‘1’

1

m

h,

’

N2 molecides

1

N2* ld neg. system Av=O IlliTl

Cll

Ava-1 CII

N2 2” pas. system

Fig. 1. Emission spectra from C’, N* and O’colLisions on N2 at 1000 eV lab taken with 10 A fwhm resolution, pN2 = 10 X lo-’ torr. The ins& taken at 4 A fwhm show viimtionaUy resolved spectra. The observed light is mostly due to the Nz@ -+ X) system. With O*, emissions from u’= 0 are much stronger than with C* and fl, indicating a predominant population of u’= 0 in the O+case.

bands’with the same Au = u’- u” are not widely separated. The Av= -1 sequence, consisting of the bands

(0, I), (1,2), (2,3), etc., is partially resolved. This se-

quence was measured with increased resolution (4 A

C7r.Ottinger, J. SimontikThargetransfer of C*. N+and 0’ ions with N2 molecules

fwhm), as shown in the three insets in fig. 1 on the righi. Here the five bands (0;l) through (4,5) are completely resolved. The sharp &peakscontain most of the P branch rotational lines, while the R branches can just be discerned between the P branch heads. The areas of the peaks reflect the relative populations of the vibrational levels u’= O.througb 4 of N;(B), as is analysed in detail below. It can be seen that, under the conditions of fig. 1, the vibrational populations produced by charge transfer from Cc and N’ on the one hand and from 0” on the other hand are grossly diiferent. The 0” excited spectrum contains much more emission from u’= 0 relative to the higher vibrational levels than do the spectra excited by Cf and N’. In order to ascertain that the emission shown in fig. 1 is due only to primary charge transfer processes, the dependence of the light intensity on the N2 target gas pressure was measured. The results are shown in fig. 2. In all cases the intensity increased linearly with increasing target gas pressure (for the deviations above 5 X 1tY3 torr in the 0” case, see below). This confirms that the light emission is due to a bimolecular process. A correction for the ion beam attenuation by the target gas has been applied in fig. 2. At 20 X 10m3 torr the current 1, of ions leaving the collision cell is attenuated by typically 50%. The light yield plotted in fig. 2 was therefore normalized to the “effective ion current”. leff at each pressure (see appendix A). As can be seen in fig. 2, the linear pressure dependence of the light yield for 0’ collisions only holds up to 5 X lo3 torr. Above +&ispressure the correction for the measured beam attenuation is insufficient to T produce a straight line pressure plot. This can be understood if one assumes that the 0” beam contains (at least) two components, of which one is predominantly responsible for the light production and is, at the same time, more strongly attenuated than the other, “inactive” component. Turner et al. [15] also found a two-component type of attenuation for 0” beams in N2 gas. They ascribed this to considerable (30%) admixtures of metastable 0” ions in the beam, which could be varied by using different ionizing electron energies in the source. In the case of C+ beams Lao et al. [16] found that the fraction of metastable C+ ions also depended on the ion source pressure. We applied both these techniques simultaneously nr an attempt to minimize the metastable content of our

101

I

Nz+(B-X

1

Pressure

[10S3Tor~

Fig. 2. Dependence of the light yield Z/I,, (Z = light intensity, Ieff = ion beam intensity in the cell) on the NZ pressure in the collision cell, measured at 4278 A (bandpass 10 -4 fwhm). The non-linear increase with 0’ above 5 X 10-j torr is caused by the contribution of me_JastableO+ions which are more strongly attenuated than ground state O+ions. Open and fill circles refer to measurements with different ion source conditions, which result in 14% and 3% metastable fractions in the 0’ beam, resljectively. The crosses are calculated with the results of appendix B.

0” beam. Running the ion source at the rather high pressure of 10-l torr and at the unusually low voltage of 26 V, the measurements of the $(B-X) light yield as a function of N, pressure were repeated. We found, after nom&ration to the effective ion current, a much more linear relationship (dots in fig. 2). This confirms the above interpretation for the non-linear pressure dependence found with a “normal” O+ beam. 22. Metastable projectile ions Luminescent charge transfer of the metastable Cc, N+, 0” ions was studied by measuring the N$(B-X)

102

ch. Ottinger, J. SirnonL&harge transfer of C: NCan(! O* ions with N2 molecules -.

Fig. 3. Dependence of the light yield Z/I (Z = light intensity, I = ion beam intensity) on the cathode-to-anode voltage in the ion source. With deczasing voltage, the me&stable fractions in the ion beams are reduced. In the case of N+, the change of the light yield at high voItages is due to Nz+ contamination in the beam.

light yield as a function of the ion source voltage, see fig. 3. The ion source was operated at 10-l torr in all three cases. The’cathode-to-anode voltage U* was varied between 20 and 80 V. The cathode heater voltage was connected in such a way that it did not contribute to the maximum electron energy in the source, equal to e VA. At UA = 20 V, the ion current was smaller bg about two orders of magnitude than at U, = 80 V . Fig. 3 shows that the light intensity, normalized to the ion current entering the collision cell, depends very strongly on VA in the case of Of f N,, but much less so for C+ and N+‘.The increase of the light yield in the e case above 40 V can be explained by the onset of Np production. N$+ is transmitted together with N’ by the mass selector into the collision ceil, where charge * The fact that ions are recorded even at voltages VA below the appearance potentials (22.4 eV for C*and 24.7 eV for O+ from CO, 24.3 eV for N’from Na) demonstrates that some ions are produced by secondary ionization processes in our plasma source. The possibility of these ions orig+ating from the region between the anode and the conical extraction electrode was eliminated by tuning the magnet off the projectile mass peaks, which ma& the ion and light signals disappear.

transfer can give’G(B). In the Cf and particularly_Ot cases, ihe dependerke of the light-yield upon U, also indicates a change of beam cpmposition. Here it must be the metastable content which decreases with decreasing U’. The dramatic variation of the light yield in the 0” case suggests that the 0” metastable ions have a much larger cross section for luminescent charge transfer with N2 than do ground state 0” ions. Turner et al. [15J developed a method by which the fraction of metastable O+ ions can be determined. It is based on the fmding that a beam of metastable-Of ions is much more strongly attenuated by N2 than a beam of ground state ions. The greater attenuation probably arises from the larger charge transfer cross sections for metastable O+. Lao et al. [ 16 ] applied the attenuation method to beams of Ci ions and Vujovic et aI. 117] to p ions. We performed attenuation studies for our Cf, p, and Ot beams in order to explore their composition as a function of the ion source conditions. The ion beam intensity I2 was recorded as a function of N2 pressure in the collision cell for three sets ofion source parameters: l)prs=lX1O-ltorr, U*=26V; 2)pIs=1X10+0rr, UA=80V(forN+:UA=40V 3 j pIs =2 X lo-* torr, I{so as to prevent formation of NF). The results are given in fig. 4 on semilogarithmic plots. With C+ and Ot the attenuation is only approximately exponential. Particularly large deviations occur in the Of case. Attenuation curves of this type can be described as superpositions of (at least) two exponentials with exponents U’and OLand relative weightsfand (1 - j), respectively, I@)/IO = (1-J)

exp(-oP) + fexe(-&) -

0)

If the attenuation is much greater for a given component, for example cr’> & then, at high pressure, a purely exponential decay (1 -J) exp(+rp) is approached. Fig. 4 shows that, in fact, all curves do follow a purely exponential decay in thehigh pressure regime. T&esteeper fall-off of the curves atlow pressures is due to the metastable fraction, f,of the beam whichis ‘attenuated more stronglyJean be determined by extrapolating the linear high-pressure portions of the curves to p = 0. The results obtained with the O+ beam are, for the three sets of i.on source conditions:fil) =3%,fi2)= 14%, andf13)=37%, with absoluteuncertainties of S%in each case. The remainder, (l--a, ofthe beam consists ofground state O+ ions.

Cii. Ortinger, L Simdnis/C%atge transferof CT N+ and O* ions wirt N2 ntolecules

, 5

,

I

15

10 Pressure

I

20

25

[lO”Torr]

Fig. 4. Attenuation of 1000 eV C’. N+,and O’ion beams in N2 for three different ion source conditions (see text). Extrapolation of the high pressure slopes to zero pressure yields the fractions of metastable ions in the beam. Curves (2a) and (3a) represent the difference between the measured points and ihe straight line. extrapolation and slow the decay of the metastable O* components alone. The dotted ewes are calculated using the results for 0’ obtained in this work.

Plotting the difference between the measured attenuation and the extrapolated portion of the highpressure slopes, one obtaines the attenuation of the metastable fraction f. This is shown in the lower part of fig_ 4 for the O+ case, curves (2a) and (3a). The difference points lie approximately on straight lines which have, however, different slopes with ion source conditions (2) and (3). This indicates that the two-component model of eq. (1) is only approximate. Actually

LO3

both the O+(2D) and O+(2P) metastable ions (see table 1) contribute to the low-pressure decay, which explains the curvature and different slopes of curves (2a) and (3a) in fig. 4. We conclude that the two metastabie O* species are attenuated differently and that the composition of the total metastable fraction f is different for ion source conditions (2) and (3). The accuracy is not sufficient to further resolve curves (2a) and (3a) into two exponential components each. The detaikd O+ beam composition is derived below from additional data. For C+ ions, only one metastable state, (4P), is known, and we find only smah fractions fin our beams: fll) = O%,f12) = 3%, andfi3) = 6% (-+3%). These curves disagree with [ 161 in that the difference between the attenuation coefficients for C’(2P) and C’(4P) found here is much less pronounced. Also the percentage of metastables found in our work is much smaller, which might be explained by the different ion sources used. In the case of N+, the attenuation is exponential, regardless of the ion source conditions. This means either that the beam does not contain appreciable fractions of the metastable N+(lD) or N’(lS) ions. or else that they do not show up in this experiment because the attenuation coefficients in N2 gas are similar for both ground state and metastable ions. In order to investigate quantitativeIy the influence of metastable O+ ions on the excitation of N;(B), we utilized the differential attenuation of the composite O+ beam to sort the various O+ species. Using ion source conditions (2), the ion beam was first attenuated in a separate chamber while the tight emission was observed from charge transfer in the collision cell. The attenuation was accomplished by admitting N2 into the mass selector chamber located between ion source chamber and collision ceh. The pressure PM3 in the mass S&Ctor chamber was raised up to about 5 X lo-” torr, while in the collision cell the N2 pressure remained fmed at 5 X 1O-3 torr. The intensity of the (0,O) band was norrnaliied to the attenuated ion current entering the cohision cell at each setting of pMs. The result is plotted in fig. 5 as a function of the ion beam attenuation in the mass selector chamber. For comparison, data for C+ and Nf impact are included in fig. 5. Note that fig. 5 is completely analogous to fig. 3, except that for these experiments the metastable fraction of the beam is diminiihed by the attenuating gas, instead of by reducing the ion source voltage.

ch. Ottinger, J. SimonisfCharge transfer of C: N+ and 0’ ions with iI5 m_ol~ctdes

104

I

a,

I

b,

I

I

,-

(0.01Eland

11 0

1

1

1

,

It is instructiire to compare the O* curve of fig. 5 shoivn in fig. 4. The-decay curve (2) has become exponential when the-attenuation reaches 50%. At the.iorresponcling point (III, = 0.5) in fig. 5, however, the normalized Of light yield has not at glI become constant, asit would for a pyre beam. Obviously, the O+ beam con&ins light-producing metastable ions far beyond the 50% attenuation point which do not show up in fig. 4 because of their low concentration. Combining the data of figs. 4 and 5, the O+ beam can be analysed in terms of attenuation coefficients, c+, relative cross sections for luminescent charge transfer, oi, and fractional abundances, -f;:,for ground state (i = 0) and the two, metastable 0 Ions (i = 1,2), respectively_ The calculations (see appendix B) yield:

with the O+ attenuation

N2+(f3-Xl

1

*

I

I

I

0.2 0.2 0.6 0.8 1.0 Attenuated ion intensity III. Fig. 5. Dependence of the light yield Z/I (2 = light intensity, I= ion beam intensity) on the ion beam attenuation before the collision cell. The metastable fraction in the beam is reduced by faltering the beam in N2 gas. For O*, the circles are experimental and the crosses are calculated points using the results for the 0+ Seam composition given in the text. With NC, the light intensity falls off proportionally to the ion current itself, as it should for a homogeneous beam composition, so that the normalized light yield plotted in fig. 5 is constant. With Ct, the attenuation produces an 11% drop in the normalized light yield, which is caused by removal of the 3% metastable fraction of the beam. A similar, though somewhat larger (17%) drop was noted in fig. 3. With O+, a steep drop of the normalized light intensity occurs as the metastables are removed. By the present technique the norrn&zed light yield curve can be followed further down than in fig. 3 into the region where it starts to level off. This occurs at an ion beam attenuation of I&, = 0.1, at which point the urmormalized light intensity has dropped to only 0.007 of its original value*.

* The N;(B) light measured with the attenuated beam is deli&eIy due to ions and not to fast neutral0 atoms generatedby chargeexchange with the attenuating gas. This was verifiedbjr raisingthe collision cell to a potential such as to prevent the ions from entering into it. Under these conditions no light signal above the dark current level was detected.

cyo:QI:(r*=

1:9:3,

fJ~:o~:u~=

1:9:300,

and, for ion source conditions (2), fo : fl : r; = 0.86 I 0.09 : 0.05 . The accuracy of these values is estimated

to be about

30%.

We presume that the metastable no. 1 may be attributed to O”(2D) and no. 2 to O+@) ions. The charge transfer with O+(*P) to the l$(B, u’= 0) level has an energy deficit of only 0.10 eV (table 1) and is thus a near-resonant process [18]. This appears as a plausible reason for the large cross section o2 = 300 uo. 3.3. Vibrationalexciration

In the following section we will focus our attention on the vibrational distribution of the N;(B) ions resulting from charge transfer with C!‘, N+, and 0’. Qualitative differences were already demonstrated in. fig. 1. For 0” we know from the analysis of appendix B the relative contributions to the $(B, u’= 0) lig@ emission from reactions of ground state, first and second metastable O+ with N2_ These are in the ratios OgOif;:, which are about 5% : 5% : 90% using the fi&d Ui giv& above (for the ion so&e conditions (2), applicable to fig. 1). Therefore the unusually ‘intense u’= 0 emission in the 0: 4 N, spectrum of fig. 1 is mainly due to the second Oi metastable. The relative effickncy of the met&able . . ions__in-the ekcitation of vibrational

Ch Ottinger, J. Simonir/Charge transfer of C: N* and 0’ ions with N2 motecutes

&+( B-X) C+ + N2

10.5

AV =-1 Sequence N++ N2

0’ + Nz

AIAI

I

Fig. 6. Vibrationally resolved spectra for different concentrations of me&table ions in the beams, at 1000 6&b energy. In (a) the metastable fraction is enhanced, in (b) it is reduced by use of ion source conditions (3) and (I), respectively (see text). In (c) the O+beam is faltered before the collision cell, leaving a metastable fraction of less than 0.2% in the beam. This small fraction still accounts for 33% of the (0, 1) band emission (portion of the peak above the arrow). With this correction, spectrum (c) represents the vibrational excitation produced by ground state O*ions, while (a) is almost exclusively due to m&stable O*ions. For C* only a small effect, for N’no influence of me&stable ions is found.

levels other than v’= 0 can be obtained from spectral measurements using ian beams with controlled concentrations of metastable ions. Spectra analogous to the insets of fig. 1 were taken using ion source conditions (3) and (l), i.e. with the maximum and minimum obtainable me&stable fractions. The results are shown in figs. 6a and 6b, respectively. It is evident that the metastable ions have a large effect on the spectral shape in the O+ case; they produce predominantly a population of low vibrational levels, culminating at u’= 0, but also discernible in the u’= l/u’ = 2 peak height ratio. The C!+spectra in fig. 6 exhibit a similar dependence on the metastable content, but the effect is Iess pronounced. No effect is seen in the N’ case. Thus a possible metastable N’ fraction, which was not detectable in the ion beam attenuation method, does not affect the N;(B) vibra-

tional distribution

either.aince,

in addition,

the (0,O)

band light yield was independent of the ion source voltage (fig. 3), we find no evidence whatsoever for the participation of metastable N+ ions. Through the use of ion source conditions (l), fig. 6b, the met&able Of concentration was reduced to 3%. Even lower metastable concentrations could be achieved by the filtering technique described in the previous section. Using the & and 9 give above, one finds that attenuating the total ion beam in the mass selector

chamber to I/IO = 0.X5 reduces the metastable ion concentration to as low as 0.17%. At the same time the filtering technique gives a good beam intensity. Cenerating the beam under ion source conditions (2), in spite of the somewhat bigher initial metastable concentration thereby produced, and then filtering it gives a total ion intensity fifteen times as large as using ion

CR.Ottinger,J. SimonisjChatgetransferof CT N’and 0”ions withN2 molecules

106

source conditions (1) without filtering. An O+ ion beam which was faltered in this way was used to excite N;(B) in the collision cell, giving the spectrum shown in fig. 6c. Compared to fig. 6b, the (0,l) peak has diminished considerably. However, COIrections for the residual O.i7% metastable ions still must be made, because of their very large cross section, u2 = 300 uo_ From the known beam composition f2 = 0.0017, fi = 0 (since the metastable component no. 1 has been completely faltered out of the beam at the attenuation to 15%), and fO = 1 - fi z 1 one

fmds a fraction off&/(j&, +f.o$ = 33% which has to be subtracted from the (0,l) peak in fig. 6c in order to remove the effects of metastable ions. The corrected peak height is marked by an arrow in the spectrum. Corrections to the other bands in fig, 6c can be assessed with the help of fig. 6a. Here 95% (=$&/Z&for ion source conditions (3), see appendix B) of the (0,l) peak originates from the metastable ions no. 2 (another 3% from metastable no. 1 and 2% from ground state 0”). The coctribution of metastable ions to the other peaks is not known, since a detailed analysis as in the previous section was not done. In any case, these peaks are all weak, on the order of l/10 relative to the (0,l) band. The maximum correction to the corresponding peaks in fig. 6c can therefore only be about l/l0 of the 33% correction to the (0,l) band, and is neglected_ We are now in a position to describe spectra as they would be produced by pure ground state OS and pure met&able Of, respectively. Fig. 6c, with the (0, 1) band corrected as indicated, represents the spectrum produced by ground state 0’. Fig. 6a approximates the spectrum produced by metastable Of. The ground state contribution to the (0,l) band in fig. 6a is only 2% and it is still small for the (1,2) band, but takes a

larger share of the weak (2,3), ($14) and (4,5) bands. We will now analyse theSe s(B) spectra in terms of relative vibrational populations. They are derived from the Au = -1 sequence as follows. The recorded light intensity Z,,,. withhi a band (u’, II”) is [19]: Z,,,;Uz,a: P(ti)?$un,R~qu,,V,,Dh .

flu’) is the population of the level u’, &,,v,, is the wavelength of thk band (v’, u”),R, is the average electronic transition moment, d&h is constant in this case [20], Q,, u,, is the Franck-Condon factor for the transition (IJ’:d’), and D, is the relative spectral response of the optical system. The band intensities Z,,,,. in the Au = -1 sequence, for u’= O-4, were determined as the sum of the accumulated counts in a number of memory channels comprising the half width of the P branch profile. The conversion from the measured count rate 2 to light intensity in erg/s [ 191 is included in the calibration of D,. We use the Franck-Condon factors computed by Nicholls [21]. The relative populations found from (2) were normalized in such a way that $=oP(u’) = 1_ The results are listed in table 2. The vibrational populations produced by ground state ion impact were obtained for C+ and N’ from the spectra in fig. 6b, and for O+ from fig. 6c with the (0, 1) peak corrected as described above. The populations due to metastable C+ and O+ projectile ions are extracted from the spectra in fig. 6a and in the insets of fig. 1 as well, correcting each band intensity for that fraction which is ascribed to excitation by ground state ions. These fractional light intensities were calculated using tbe concentrations 1 -f13) and 1 -f12), respectively, obtained from fig. 4 in the previous section, and the light yields for ground state ions derived from the spectra in figs. 6b and 6c, respectively. It is not possible to derive separate vibrational distributions for 0+(2D) and 0”(2P) ions.

Table 2 Measuredvibraticmal population ofN:(B),Ehb=1OOOeV Projectile

c+

(2)

State

u’=O

0’= 1

u'= 2

u’= 3

u’= 4

2P 4P

0.31 0.44

0.25 0.24

0.20 0.15

0.15 0.10

0.09 0.07

N+

Jp.

0.28

0.21

0.20

~0.15

0.10

0'

4s 4 2IpP

0.25 0.78

0.23 0.12

0.20 0.05

0.17 0.03

0.15 0.02

Ch. Oth%,gW, J. SimonisfChmge transfer The population given in table 2 refers to a~rnixture of the two me&table O+ components in the ratio of their concentrations fi in the ion beam, i.e. of about 3 : 2.

3.4. Cross sections Absolute cross sections were found by calibrating the apparatus using data on electron impact excitation of N,. The emission cross section of the Nz fust negative (0,O) band in the case of electron impact has been determined by several groups [22-241 with good agreement. We chose the result of Ajello [24] who gives 18.6 X lo-l8 cm2 for electrons with an energy of 115 eV. Replacing the ion source by an electron source, we focused a 115 eV electron beam into the collision cell and related the q(B-X) light yield to the emission produced by ion impact*. The accuracy of the absolute emission cross sections arrived at in this way was checked by comparison with other absolute cross sections given in the literature for s(B) excitation, namely by proton [25] and He+ [26] impact at 1000 eVIab_ Using these .projectiles, we found [13] the (0,O) band emission cross sections 11.6 X lo-l8 cm2 and 1.83 X IO-I8 cmz, respectively, as compared to the literature values of 8.65 X 1O-18 cm2 and 1.9 X iO-l* cm2. On this basis, we feel that the accuracy of the absolute cross sections determined in this work is better than 50%. The cross sections obtained with Ct, @, 0” are given in table 3. For the $(B) state no transitions from higher states or transitions to lower states other than the ion ground state are known. Therefore, the excitation cross sections for each vibrational level of N$(B) can be deduced from the emission cross sections. First, the cross section o(0) for excitation of the u’= 0 level was calculated from uo,o using the emission branching ratio [24] : 0,“,,40,“” = 0.7228 , bo,o = +$,qO > ,o I cv” h-3 hence

(3)

o(o) = uo,,/0.7228. * With N+ and @the intensity of the (0,O) band was measured directly. In the case of C*, where a C II line contaminates +he (0,O) band (cf. fig. l), the (0,l) band intensity was measured and related to the (0,O) band by means of the Franck-Condon factors.

of CT Nf and O+ions with N2 moIec&s

Table 3 Cross sections (in lo-l8

Projectile

State

C‘

*P 4p

107

cm*)

a) %I,0 0.92 13.4

N+

sp

1.46

0+

45 *D *P

0.73 6.6 220

0:)

cl

*CT

4.7 45 1

240

8.0

210

1yd) 400 d)

390 d

a) Emission cross section of the N:(B) (0,O) band, measured at 1000 eVLab b, Excitation cross section of the N;(B) state, measured at 1000 eVfab ‘) Total charge transfer cross section at 900 eV,ab (from Custafsson and Lindholm [27]). d, Based upon the same vibrational distribution for 0c(2D/2P) in table 2. With O’(*D), the actual distribution is probably somewhat broader, maybe similar to that resulting from C’(4P) impact, which would yield ch = 23 X 1 O-r8 cm*. With O+(*P), the distribution may be even steeper, presumably like an undistorted Franck-Condon distribution, which would yield oB = 340 X lo-t8 cm*. e, Strongly depending on the concentration of metastable ions inthebeam [27].

The electronic transition moment is known to be con-

stant [20] and therefore does not appear in (3). The excitation cross sections for other u’levels were found from the vibrational populations P(u’) given in table 2 u(u’) = u(O)P(u’)[P(u’ = 0) .

(4)

levels u’> 4 were unobservable in this work due to their low emission intensity_ Their contribution to the total B state excitation cross section ffB was estimated from the vibrational distribution to range from 6 = 30% for 0’ ground state ions down to S = 2% for O+ metastable ions. For uu we thus obtain: The

4 OB = (1 + &) uFo

&)

*

(5)

OR is listed in table 3 for C+, N+, and O+ Impact at 1000 eV. For comparison table 3 also shows the cross sections for total charge transfer at 900 eV, taken from a compilation of Gustafsson and Lindhohn [27]. In most cases onIy a fraction of a few percent of charge transfer processes leads to production of Nz ions in the ex-

108

Ch. Ottinger, J. SimonisjCkrge transferof CT N+amiO+ ions with N2 moleczdes

cited B state. Only for the metastable 0+(2P)is this fi-action likeJy to be larger. Rutherford and Vroom [28] h&e measured the charge transfer cross section for the other 0” component 2D up to Elab= 500eV. They find an almost energy-independent value of about 30 A2. If we assume that the total charge transfer cross sections for O”cD) and O+(‘P) are in the ratio of the attenuation coefficients derived above, i.e. 9 : 3, then the total charge transfer cross section with O+(*P) ions is about 10 j-12. Thus, as much as l/3 of the 0*(2P) charge transfer at 1000 eV leads to the production of $ ions in the B state. Since it has not positively been shown that all of the remainder of 2/3 of the total charge transfer cross section leads to ground state s, it is even possible that the 0”(2P) f N2 system produces a population inversion between the B and X states of $_ Efficient population of electronically excited states is one of the prerequisites for laser operation in the W.

sections for the excitation of N;(B). Therefore, the impact velocity is by no means the only factor affecting the vibrational excitation; the cross sectidn has to be considered as well. In comparing different projectiles, small cross sections in general seem to correlate with broad vibrational distributions*. The broad distributions observed previously were explained by Lipeles [7] in terms of Fran&-Condoti (FC) transitions from a perturbed ground state N2 molecule. The approaching ion was supposed to polarize the N2 molecule with a resulting extension of the internuclear distance up to the moment of electron transfer. In the N2 case, FC transitions from an unperturbed N2 to s(B) would result in a 90% population of u’= 0, the remainder leading to u’= 1. With an extension of the N-N distance by a few hundredths of an & however, the population, by FC transition to s(B), of the higher u’levels can be markedly increased. Thus one can try to correlate the broad u’distribution with strong N2 perturbations. Table 4 shows calculated u’-distributions for several assumed bond extensions

4. Discussion The results of the previous section are summarized, as follows: (a) The ions Ci, N’, and 0’ in their ground states behave rather similarly. At Elab = 1000 eV the cross sections for cizrge transfer into the s(B) state are all on the order of magnitude of some 1O-18 cm2 (table 3). The observed vibrational populations of the product ion state show a rather broad distribution over the vibrational levels u’(table 2). (b) With me&stable C+ and 0” ions the excitation cross sections are one to two orders of magnitude larger than with ground state ions. The U’= 0 level of I$(B) is favored and the vibrational distriiution drops steeply with increasing u’. No effect of metastable Nf ions could be found. Broad vibrational distributions of N;(B) were found previously to result from charge transfer between N2 and vario’rlsatomic ions [l]. The principal factor determining the distribution appeared to be the velocity of the projectile ion, decreasing velocity leading to increased vibrational excitation. In the present experiments, ground state and me&table ions, although having exactly the same impact velocity, were fcugd to give radically different vibrational populations. At the same time metastable ions had much larger cross

*The inverserelationship between the size of the cross section and the G(B) vibrational excitation sometimes appears to hold aIs6 as a function of impact velocity, for a given projectile. This was found in charge transfer experiments with rare gas ions [13,29], down to avelocity of 5 X lo6 cm/s. At stiU lower velocities, the vibrational excitation and the size of the crow section were no longer correlated. Low ve‘locity experiments (2 X lo6 cm/s, SO eVhb) with C’, N’, O+ [13] also showed no correlation between the cross section and the vibrational distriiution. The dktriiutions all resembled the ones given in tbis work at 1000 eV, although the cross sections were smalier by factors of 20,10,3, respectively. Table 4 Calculated vibrational distriiution of N;(B) from FranckCondon transitions be [A] a)

v’= 0

v’= 1

v’= 2

d=3

v’=4

0.00 0.02 0.04 0.06 0.08 0.10

0.891 0.641 0.384 0.189 0.077 0.028

- 0.107 0.312 0.425

o.bo2 0.045 0.165

0.385

0.299

0.259 0.140

0.345 0.293

0.002 0.025 0.109 0.235 0.328

0.001 0.018 0.084 0.211

a) Assumed shift of the N2 groundstate potential curve from its equilibrium valuer, = 1.10 A.

Ox. OthIger. f. Simonisfcharge

transfer of C*, N+and O+ ions witi n;, m&c&s

Ar, of Nst The N2 ground state curve was shifted outwards by the indicated amounts, and FC transitions to the unperturbed N;(B)-curve were calculated using a program by Zare [30]. It can be seen that all calculated distributions peak far too sharply compared to the broad distributions observed with ground state C+, N+, O+. A fit possibly could be achieved by averaging the calculated vibrational populations over a range of Are between 0.04 and 0.1 A. However, in order to stretch the N-N bond to this extent, the incoming ion, according to the Lipeles polarization model, has to get extremely close (0.9 to 1.4 A) to the N, molecule. At such distances the polarization potential certainly breaks down. The difficulty is aggravated in a dynamical picture. Simple timedependent calculations [13] show that at 1000 eV there is not sufficient time for the internuclear distance to readjust while the ion is approaching. Therefore, we conclude that the broad u’distributions of s(B) observed with ground state Ci, Nf, and O+ projectiles cannot be explained by the simple polarization model. Additional, stronger perturbations of the target molecule will certainly have to be taken into account. Whatever the mechanism, small cross sections are expected for these strong interactions. This is in agreement with the observation. A much simpler situation seems to pertain to the case of the metastable projectile ions, particularly Of. The measured cross sections for N;(B) production are large. This indicates electron transfer over much larger distances from the N, molecule than with ground state ions. Consequently, the target molecule will be perturbed very little, and the transition to q(B) will approximate a FC transition from an unmodified N, ground state potential curve. The corresponding calcldated vibrational distribution is given in table 4, for Are = 0, and is seen to come close to the distribution observed for metastable Of. The case of metastable C+ is intermediate between the three ground state ions and metastable Of, both as regards the size of the cross section and the width of the vibrational distribution. A situation similar to metastable O* has been found in studies of luminescent charge transfer between He; and some diatomic molecules by Lever&al et al. [6,3 I] and, at thermal energies, by Piper et al. [32]. Here, too, the cross sections were large, and the resultant ions were produced predominantly with an unperturbed FC type vibrational distribution. Thus the cor-

109

respondence of large cross sections and FC transitions appears to be a general rule [9]. The large cross section for metastable O+ is a result of the close match between the excitation energies of O+(2P) and Ng(B, II: = 0), see table 1. At the same time predominant oopulation of this level is favored by FC transitions from unperturbed N,. It appears that it is the coincidence at u’= 0 of two selection principles, energy resonance and FC overlap, which leads to such overwhelming preference for excitation of N$, u’= 0) in this case. In the case of N+, zable 1 shows that the energy balance of charge transfer is almost the same as that for O+ ions, especially for the upper metastable states N’(lS) and 0+(2P). From this point of view, we should have found a similarly striking influence of metastable N+ ions, even with a very small metastable fraction in our Ni beam. Since it seems unlikely that the Nt beam should contain no metastable ions at all under the various ion source conditions, we conclude that metastable N’ ions behave very differently from metastable 0” ions. The reason for this’is the conservation of the total electron spin of the colliding particles [33]. The nearresonant charge transfer of 0+(2P) into the O(3P) ground state is “spin-allowed”: 0’(2P)+N2(1Z~)+0(3P)+N;(B2Z~)-0.10eV,(6) while the analogous transition with N’(lS) ions is “spin-forbidde;l”: N+(1S)+NZ(1C~)*N(4S)+N~(B2X~)-0.14eV.(7) For metastable N+ ions, spin conservation requires tran&tions into N doublet states, e.g. into the metastable N(*D) state. This state lies 238 eV above the ground state N(4S). Thus, spin-allowed charge transfer with metastable N+ ions is not close to energy resonance and, therefore, does not have a large cross section. This is probably the reason for the lack of any contribution of metastable N? ions to N;(B) excitation in our experiments.

5. Conclusion The size of the cross section for low energy luminescent charge transfer is an important factor determining the product vibrational population. Large cross sections lead to a distribution that is essentially de-

110

&‘kOttinger.J. SimonidChargetransferof C”, N’ and 0‘ ions with N2 molecd~

scribed by a FC transition from the undisturbed target molecular ground state_ With decreasing cross sections a perturbation of the molecule prior to charge transfer becomes increasingly important_ This perturbation cannot be described by polarization forces only, instead Short range forces have to be taken into account. The size of the. cross sections for C+, p, and O* with Nz is found to depend on the combined effects of energy resonance, Franck-Condon overlap, and spin conservation.

Acknowledgement , Fiiancial support of this work by the Deutsche Forschungsgemeinschaft is gratefully acknowiedged. We also thank Professor D.W. Setser for many valuable comments. Numerical calculations were performed on the computer of the Gesellschaft fiir wissenschaftIi&e Datenverarbeitung GGttingen.

A&&ix

A: Correction for ion beam attenuation

torr), I,, is obtained asle,=Io/21fl. At thiS.attenuation the approtiation made iri tie derivation of this simpIe correction involves less than 1% error.

Appendix 3: Analysis of O* ion be& compo&on From the Of data in fig- 4, for ion source conditions (2), the fractions& = 1 -f12) = 0.86 and fl f f2 =f12)= 0.14 are known, and we read w = 0.0332 mtorr-l from the high-pressure slopes of the attenuation curves. Let cq and 1x2< txl be the attenuation coefficients ofthe strongly and the less strongly attenuated metastable beam components, respectively. Then ozI must be greater than 6.8a0 and ct2 smaller than S-5%, where 6.81~~0 and 5.5aro are the measured coefficients of curves (2a) and (3a) in fig. 4, to which both aI and 4 contribute. Wecontinue the analysisby relating the attenuation across the mass selector chamber, fig. 5, to the equivalent attenuation across the collision cell, fig. 4. Defming a collision cell pressure p which would give the same attenuation as is measured across the mass selector chamber, we write

Considering the exponential decrease of the ion beam intensity within the cell, @I = 10 exp(-yl) ,

(A.11

Zcff was determined as a spatial average aiang the beam direction:

Converting the three component ion intensities lo light intensities by multiplying with tie corresponding relative cross sections Oil we obtain for the light intensity 2:

2 zlrg =ig fioi eXPt_tuip) * Tfre integral is extended over rhat portion l2 - II of the ion path which Is seen by the optical spectrometer. In our apparatus this detection region is 12 mm long

and centered between the entrance and the exit slit of the collisionchamber. It can be shown that feff is appruximatefy equal to the ion beam intensityI((C~f lz)/2), Le. the intensity at the midpoint of the derection region: Jeff =‘lu exp(4/2) =I, [~@Mcp I

(A.31

CA.51

From (A.4) and (AS):

which is the quantity plotted in fig. 5. At Ij’IO = 1, correspondingtop = 0 in 9.6), Z/I as-

sumesthe value Eizo &a, (becauseof I&o 4 = 1Xin the units of fig_5,

where L is the total length of the collision cell. i(L)

and I, are directly measurableas the ion currenr1, defmed above, with and without collision gas in the

cell, respectively.Thus at 50%attenuation (20 X IO-3

(A-7) which &es the normalization of the ai as used in the following.

Ch. Ottinger, J. Simonis/Charge transfer of C+, N’and 0’ ions with Nz molecules

In the limit of high pressure (I/IO + 0), both metastabIe components are removed from the ion beam and only component 0 is left in eq. (A.6). In this limit 211 tends towards the constant value u,,, which is read frbm fig. 5 as o. = 0.06. At 1110
:0.22:O.i5.

For ion source conditions (1) no detailed analysis was made, because here the absoIute uncertainties in the beam attenuation measurements are as great as the total metastable fraction (#I) = 3%) itself. The sets of parameters were checked by backcalcuMing some of the data displayed in figs. 2-5, using eqs. (A-4) - (A.6). The most stringent test is fig. 5. Here all three beam components make major contributions, and all,nine parameters are involved. As the cal-

111

culated points in fig. 5 show, the light yield curve can ‘be reproduced quite accurately. The good agreement over the whole range, not only at the fit points, lends strong support to the reliability of the parameter set. In fig. 4, the backcalculafed curves illustrate the tit with the measured points that is achieved using the “i and figiven above. Besides, even the data of fig. 2, which were not used in the derivation ol the parameters, can be reproduced satisfactorily. The calculated points shown in fig. 2 were obtained on the assumption that the light yield for luminescent charge transfer depends linearly on the N2 pressure for each of the three O+ species involved.

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112 -_

-

CR. Ottingw, % SimonisjGarge tti&feiof

C’: N’and O?ioniwith ~~~rnol&&

.-

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[27] E. &stafssq~-;d

_219.

_: . _.,

-

--

E.-Lindbobn; -&r&v Fysik1;

. I

:.;

.., (1960)

~. -- I __~._ -_I_

; _--

[ZS] J& Ruthtifqrd-%nd D-6 Vc?om,,J: [Ch~m;Phys. 55, (1971)5622..!, [iSI Q Of&+&id j. S&&s, to be pub&i&. .. 1301 R.N. Z+re, University of California Radi&on Lab&tdry Report, UCRL- 10925.(1963)._ ; -~. _-- -~ -[31] J.J.Leventh&J.D.EailandH.H.Hauis,Phys.Re~. Letters35 (1975>719.. [32] L.G. Pip&, L. Gundel, LE. Velazco and D.dr. setser, J. -. < Chem.Ptiys.62 (1975)3883.-. [33] E. Wigner and E.E. Witmer, Z. Phys. 51(1928) 859: :