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Infrared optogalvanic effects in xenon
Volume 64, number 1
OPTICS COMMUNICATIONS
1 October 1987
INFRARED OPTOGALVANIC EFFECTS IN X E N O N Randy D. MAY Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
Received 27 April 1987
A color-centerlaser operating near 2.7 ~tm has been used to study opto-galvaniceffects in a xenon dc glow discharge. Exciting eleven transitions between the higher lying Rydberglevels, with one exception, resulted in an increase in the discharge ionization rate. Absorption on the 5d[3/2 ] ~-6p[1/2 ]0 transition in xenon caused the ionization rate to decrease. The mechanism for this effect is related to the extremelyshort radiative lifetime of the 5d [ 3/2 ] i level.
1. Introduction
The optogalvanic ( O G ) effect, a change in the impedance of an electrical discharge in response to absorption of radiation by a species in the discharge, has been extensively investigated during the last ten years using visible and infrared lasers as discharge probes. Atomic [ 1 ], molecular [2 ], and ionic [ 3 ] species have all been studied using OG techniques. In addition to providing spectroscopic information, such measurements give insight into the various, often complex, physical processes occurring in the discharge. The rare gases, in particular neon, have served as prototype systems for detailed studies of mechanisms of the O G effect in dc glow discharges [4-6]. Although the results of many mechanistic studies utilizing laser sources in the visible region have been reported, very few studies of infrared O G effects in the rare gases have appeared. In earlier infrared work by Kaplafka, Merkelo, and Goldstein [ 7 ] the glow discharge was used simply as a detector of incident 10.6 ~tm CO2 laser radiation, following similar studies in the microwave region [8,9]. Illumination of the discharge caused an increase in the dc discharge current, and the mechanism responsible was believed to be direct photoionization of highly excited atoms. Begemann and Saykally [ 10 ], using a color-center laser operating in the 2.5-2.8 ~tm wavelength region, observed and assigned many transitions between higher lying Rydberg levels of neon and argon excited in commercial 36
hollow cathode lamps. Absorption of all of the assigned transitions resulted in an increase in the dc discharge current. The mechanism postulated in this case was a higher probability of ionization for the upper level relative to the lower level involved in the transition, similar to mechanisms proposed previously for illumination of rare gas discharge with visible lasers [ 4 ]. Jackson et al. [ 11 ] investigated infrared O G effects in helium and neon discharges, also using a colorcenter laser operating near 2.6 ~tm. The emphasis in these studies was on the determination of fine structure intervals and line broadening mechanisms, and detailed studies of mechanisms of the O G effect itself were not pursued. A1-Chalabi et al. [ 12] made a quantitative study of the infrared O G effect in helium at 2.64 ~tm, again using a tunable color-center laser. Results were obtained on the ionization efficiency of the process, and on the effects of quenching by hydrogen gas added to the discharge. The spatial dependence of OG effects in a helium positive column discharge were investigated by Tam [ 13 ] using high powered helium lamps as sources of radiation at 2.058 ~tm. Absorption at this wavelength originates from the 2 1S metastable level of helium. Except for irradiation of the negative glow region of the discharge, the OG signal polarity corresponded to a decrease in the dc discharge current. Tam presented quantitative measurements and discussed possible mechanisms. In this communication we report observations in 0 030-4018/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 64, number l
OPTICS COMMUNICATIONS
a xenon hollow cathode discharge of a strong O G effect at 3771 cm -~ (2.652 ~tm) which consistently exhibited a polarity corresponding to a decrease in the discharge current over the range of discharge conditions investigated. O G effects associated with a decrease in the discharge current are generally attributed to laser induced changes in the metastable atom density. However, the xenon transition studied in this experiment involves upper and lower levels which are both free to radiate. A mechanism for the observed O G effect is proposed.
1 October 1987
X E N O N OG
p = 3 Torr = 6.2 m A
(+)
I
r
I
3763
3777
3791
(-)
i
I
3805
~(cm - 1)
5d[3/211 - 6p[1/2]0
2. Experimental O G spectra were recorded using a commercial infrared color-center laser (Burleigh FCL-20) as the discharge probe. A mechanical chopper amplitude modulated the laser beam at a frequency of 400 Hz and provided the reference signal for a lock-in amplifier. The laser operated predominantly on two longitudinal modes, a fundamental mode and a spatial hole-burning mode separated from the fundamental by 10 G H z (0.3 c m - ~). The laser output was tuned over the 3650-4100 cm-~ region by rotation of an intracavity grating. The grating served as both end mirror and output coupler, and was controlled by a stepping motor and reduction gear assembly. The grating sine bar drive was calibrated using the atmospheric H20 spectrum, with line positions taken from the AFGL linelist [ 14]. The maximum output power was about 20 mW near 2.64 ~tm using a 3W Ar ÷ laser operating on all lines as the pump source. The beam was directed along the axis of a pyrex discharge cell fitted with CaF2 Brewster windows. The cathode was a 4 cm long stainless steel cylinder having an inner diameter of 0.4 cm. Pyrex disks placed around both ends of the cathode, and extending outward to the cell walls, restricted the discharge to the interior of the cathode and helped to eliminate noisy operation at higher discharge currents [ 15]. Two stainless steel rings, 1.2 cm inner diameter × 0.2 cm width were spaced 1 cm from either end of the cathode in a double anode configuration. Care was taken during measurements to ensure that the laser beam did not strike the surfaces of the electrodes. The discharge cell was sealed during measurements and was operated at xenon pressures of 2-4
Fig. 1. Optogalvanic spectrum of xenon near 3800 cm ~. ( + ) and ( - ) refer to the direction 0fchange in the discharge current.
torr and dc currents of 3-30 mA. No positive column was developed under these conditions due to the small anode-cathode spacing. The xenon gas was used as supplied by Matheson and had a stated purity of 99.995%. The cell was placed in series with a l0 K ballast resistor, and O G signals extracted from the high voltage anodes via a 0.05 ~tF capacitor. O G spectra were processed by a lock-in amplifier and displayed on a strip chart recorder. The strongest OG signals could be observed directly on an oscilloscope, and these signals were used to determine the relationship between the polarity of the lock-in amplifier output, and the polarity of the voltage change across the discharge.
3. Results Fig. 1 shows a typical O G spectrum obtained for discharge conditions of 3 tort xenon, and 6.2 mA dc current. The relative intensities of the observed transitions would vary somewhat with operating conditions and with exact positioning of the laser beam, as expected from the structured nature of the hollow cathode discharge [ 16 ]. However, the individual signal polarities were constant over the operating range. Most of the observed lines appeared as doublets due to the dual mode operation of the laser. The xenon transitions can be readily assigned using the 37
Volume 64, number 1
OPTICS COMMUNICATIONS
list o f transitional frequencies given by Outred [ 17 ] in conjunction with Moore's [18] tabulation of energy levels. Observed transitions, together with assignments and approximate intensities, are listed in table 1. Except for the strong signal at 3771 cm l, all observed O G signals corresponded to a laserinduced increase in the discharge current, similar to the observations of Begemann and Saykally [ 10 ] for transitions among higher Rydberg levels of neon and argon. The strong, negative O G signal at 3771 cm -~, corresponds to the 5 d [ 3 / 2 ] l - 6 p [ 1 / 2 ] o transition in xenon. The polarity of this signal did not change with discharge current over the range 3-30 mA, nor was its intensity a strong function of the current. The voltage change associated with the 3771 cm-~ O G signal was typically 2 5 - 5 0 mV rms, corresponding to rms changes in the discharge current o f a few microamperes. Qualitatively similar spectra were obtained using a commercial (Jarrell-Ash) hollow cathode lamp having an iron cathode, and modified to allow refilling o f the buffer gas. A weak, positive polarity continuum O G signal, proportional to the laser power, was observed when using the commercial lamp. This continuum was assumed to be due to effects resulting from direct irradiation of the cathode surface, as no wavelength independent O G signal was observed using the open cathode cell o f fig. 1.
1 October 1987
4. Discussion The present study was concerned primarily with the mechanism responsible for the polarity o f the 5d[ 3/2 ] l - 6 p [ 1/2 ]o O G signal in xenon. In studies reported to date, a strong O G signal corresponding to a decrease in the ionization rate in a pure rare gas discharge has been associated with absorption from one of the long-lived metastable levels o f the lowest excited np5 (n + 1 ) s configuration. The basic mechanism in this case can be briefly described as follows. Irradiation of the discharge at a suitable wavelength raises the ns[ 1/2 ]o or ns[3/2 ]2 metastable atom to a higher lying excited level o f n p S ( n + 1 )p configuration. F r o m the upper p-level the atom may be ionized, or deactivated via collisional processes, or it may decay radiatively to one o f the short-lived resonance levels (ns [ 1/2 ] ~ or ns [ 3/2 ] t ) of the original ns manifold. The resonance levels rapidly decay to the t So ground state through emission o f a photon in the v a c u u m ultraviolet region. Fig. 2 shows this process schematically for a single transition. If the laser pumping rate combined with the two step radiative decay rate exceeds the normal (in the absence of external illumination) metastable production rate, the result is a net loss of metastable atoms from the irradiated region. Because the longlived metastable atoms play a vital role in secondary ionization processes in the discharge at low current densities [19], a decrease in the metastable population will cause the steady-state ionization rate, or current, to decrease. This is observed experimentally
Table 1 Observed optogalvanic signals in xenon; VS=very strong ( > 5 mV), S=strong ( ~ 1 mV), M=medium ( ~ 500 ~tV), W=weak ( <200 ~tV). The polarity changes denote the direction of change in the discharge current.
38
v (cm- t )
Assignment
Intensity
Polarity
3770.7 3776.1 3805.9 3839.1 3843.4 3866.6 3871.4 3872.8 3885.6 3933.2 3975.7
5d[3/2 ] r6p[ 1/2 ]o 7d[ 1/2 ] ~-7p[5/2 ]2 5d[ 5/2 ]z-6p[ 5/2 ]2 7s[ 3/2 ] ~-6p'[ 1/2 ]~ 5f[9/2 ]4-6d[ 5/2 ]3 5f[ 5/2 ]3-6d[5/2 ]3 7d[3/2 ] j-Tp[7/2 ]o 13p[5/2 ] 3-7d[ 5/2 ]3 5f[7/2 ] 3.,-6d[ 5/2 ]3 7d[ 5/2 ] 2-7p[ 3/2 ] ~ 7d[ 7/2 ]4-7p[ 5/2 ]3
VS W S S M M S W M M W
+ + + + + + + + + +
Volume 64, number 1
OPTICS COMMUNICATIONS
1 October 1987
13.433 IONIZATIONLIMITS
i ! (2Pl/2)
13
2p312
l
LIMITS
t
12
12.127
ei
e-
np5(n+ 1)p I
VlS y/LASER [112]1~ [1/2]0 ~ [312]1~ [31212~
PUMP
11
e-
e-
I
>0 ILl z u.I 10
5a 13/211 6s'
r m np5(n+ 1)s r m
[11211"----- v
[11210
?
k.~ 1192,,~ 6s_~ [3/211 ~ ( F A S T , [3•2]2 ~
of
vuv 1% RARE GAS
Fig. 2. Simplifiedenergylevel diagram for the rare gases (except helium) to illustrate the mechanism whereby laser absorption from a metastable level causes a decrease in discharge current, m and r refer to the metastable and resonance levels, respectively, of the lowest excited np5(n + 1) s configuration. Wavylines indicate radiative transitions. (e-) signifies electron collisional ionization.
[4,5]. At low pressures the process may also result in a global loss of energy from the discharge system due to escape of the resonance level emission. The net loss from radiative decay is determined by the degree of radiative trapping [20]. Other collisional and radiative processes, whose rates are, in general, different functions of the operating environment [ 19 ], may contribute to the O G effect as well. The net result of these competing processes determines the polarity of the (time integrated) O G signal. A similar mechanism can account for the observed polarity of the 5 d [ 3 / 2 ] ~ - 6 p [ l / 2 ] o transition in xenon. Fig. 3 shows a partial energy level diagram for xenon. The upper 5d[3/2]~ level has a direct radiative path to ground through emission of a pho-
T
iso
T
XENON Fig. 3. Simplifiedenergyleveldiagramfor xenon. Symbolsare definedas in fig. 2. Absorptionat 3771 cm-t is followedby fast radiativedecayto groundfromthe upper 5d[3/2 ] ~level,resulting in a decreaseddensityof excitedatomsin the discharge. ton at 1192 A, as well as less probable decay routes to levels of the 6p manifold. The lower 6p[ 1/2 ]o level is of the same parity as the xenon ground state (both are even parity) and thus has allowed transitions only to the J= 1 resonance levels of the 6s manifold. In the discharge, excited s and d levels are populated efficiently by electron impact excitation, while the p levels are populated primarily by cascading from higher lying s and d levels [21,22]. Irradiation of the xenon discharge at 3771 c m transfers population from ithe 6p [ 1/2 ] o level (z = 40 ns) [23] to the much sl~orter-lived 5d[3/2 ]l level (z= 1.4 ns) [24]. The upper 5d[3/2 ]~ level decays radiatively to the ground state in a time comparable to electron collisional rates [ 19], and short compared with the rate of (tWo step) decay to ground from the lower 6p [ 1/2 ] o level. Thus, the transfer of population results in a net loss of excited atoms from 39
Volume 64, number 1
OPTICS COMMUNICATIONS
the discharge, and an a c c o m p a n y i n g decrease in the ionization rate, as the 5d[3/2 ]1 a t o m s convert to ground state atoms. For the xenon 5 d [ 3 / 2 ] l - 6 p [ 1 / 2 ] o transition the process is efficient because radiative decay to the a t o m i c ground state is a single step process. Lawless [25] has modelled this situation using an equivalent circuit description o f the discharge system. The lack o f accurate collision cross sections for the xenon Rydberg levels however, precludes application o f the Lawless model here. Because o f radiative trapping [20] the effective lifetime o f the 5d[ 3/2 ]1 level m a y greatly exceed the natural radiative lifetime. At 3 Torr pressure radiative trapping will decrease the excited a t o m loss rate from escape o f the 5 d [ 3 / 2 ]~ emission. However, as shown in the neon system [5,6,19], escape o f V U V radiation can represent a significant energy loss under the experimental conditions (pressure-radius product ~ 0.6 cm Torr) e m p l o y e d here. A second 5 d - 6 p transition observed at 3805.9 cm-~ (fig. 1 ) exhibited an O G signal polarity corresponding to an increase in the ionization rate. This observation indicates that the 5 d - 6 p transitions do not cause a n o m a l o u s O G effects as a group. F o r example, p o p u l a t i o n inversions, which enable cw lasing on several o f the 5 d - 6 p xenon transitions [26], can d r a m a t i c a l l y alter the characteristics o f the O G effect [6 ]. Other possible m e c h a n i s m s responsible for the observed decrease in discharge current for the 5d[3/2 ]~-6p[ 1/2 ]o transition, such as i n v o l v e m e n t in d i m e r f o r m a t i o n or destruction processes [ 27,28 ] cannot be ruled out. Such effects, however, seem unlikely at the low pressures used to obtain these data. The work described in this p a p e r was p e r f o r m e d at the University o f N o r t h Carolina at Chapel Hill, sponsored by the National Science F o u n d a t i o n under grant No. CHES111131. The writing a n d publication o f this p a p e r was s u p p o r t e d by the Jet Propulsion Laboratory, California Institute o f Technology, under a contract with the N a t i o n a l Aeronautics and Space A d m i n i s t r a t i o n .
40
1 October 1987
References [1] J.E.M. Goldsmith and J.E. Lawler, Contemp. Phys. 22 (1981) 235. [2] C.R. Webster and C.T. Rettner, Laser Focus (February, 1983) pp. 41-52. [ 3 ] J. de Physique, Intern. Colloquium on Optogalvanic spectroscopy and its applications, Colloque No. 7, C7 (1983). [4] K.C. Smyth and P.K. Schenck, Chem. Phys. Lett. 55 (1978) 466. [5] D.K. Doughty and J.E. Lawler, Phys. Rev. A 28 (1983) 773. [6] A. Ben-Amar, G. Erez and R. Shuker, J. Appl. Phys. 54 (1983) 3688. [7] J.P. Kaplafka, H. Merkelo and L. Goldstein, Appl. Phys. Lett. 15 (1969) 113; 19 (1971) 197. [8] N.H. Farhat, Proc. IEEE 52 (1964) 1053; 62 (1974) 279. [9] N.S. Kopeika, Proc. IEEE 63 (1975) 981. [ 10 ] M.H. Begemann and R.J. Saykally,Optics Comm. 40 (1982) 277. [ 11 ] D.J. Jackson, H. Gerhardt and T.W. Hansch, Optics Comm. 37 (1981) 23. [12] S.A.M. Al-Chalabi, R.S. Steard, R. Illingworth and I.S. Ruddock, J. Phys. D 16 (1983) 115. [ 13] A.C. Tam, IEEE Trans. Plasma Science PS-10 (1982) 252. [ 14] AFGL Spectral Data Tapes, see L.S. Rothman et al., Appl. Optics 20 (1981) 791. [ 15 ] This idea was taken from the design of commercial (Jarrell Ash) hollow cathode lamps in which a stack of mica disks is used for this purpose. [16] B. Chapman, Glow discharge processes (Wiley and Sons, New York, 1980). [ 17 ] M. Outred, J. Phys. Chem. Ref. Data 7 ( 1978 ) pp. 1-262. [18] C.E. Moore, Atomic energy levels, National Bureau of Standards, Circular No. 467 (1949). [ 19 ] B.E. Cherrington, Gaseous electronics and gas lasers (Pergamon Press, Oxford, 1979). [20] F.E. Irons, J. Quant. Spectrosc. Rad. Trans. 22 (1979) 1; 22 (1979) 21. [21] W.R. Bennett, Jr., Appl. Optics, Supplement on Optical Masers ( 1962 ) pp. 24-61. [ 22 ] W.R. Bennett, Jr., P.J. Kindlemann and G.W. Mercer, Appl. Optics, Supplement 2 of Chemical Lasers ( 1965) pp. 35-58. [23] L. Allen, D.G.C. Jones and D.G. Schofield,J. Opt. Soc. Am. 59 (1969) 842. [24] E. Matthias, R.A. Rosenberg, E.D. Poliakoff, M.G. White, S.T. Lee and D.A. Shirley, Chem. Phys. Lett. 52 (1977) 239. [25] J.L. Lawless, J. Appl. Physics 55 (1984) 3226. [26] C.C. Davis and T.A. King, J. Quant. Spectrosc. Rad. Transfer 13 (1973) 825. [27] R.S. Mulliken, J. Chem. Phys. 52 (1970) 5170. [28 ] W. Gornik, S. Kindt, E. Matthias and D. Schmidt, J. Chem. Phys. 75 (1981) 68.
OPTICS COMMUNICATIONS
1 October 1987
INFRARED OPTOGALVANIC EFFECTS IN X E N O N Randy D. MAY Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
Received 27 April 1987
A color-centerlaser operating near 2.7 ~tm has been used to study opto-galvaniceffects in a xenon dc glow discharge. Exciting eleven transitions between the higher lying Rydberglevels, with one exception, resulted in an increase in the discharge ionization rate. Absorption on the 5d[3/2 ] ~-6p[1/2 ]0 transition in xenon caused the ionization rate to decrease. The mechanism for this effect is related to the extremelyshort radiative lifetime of the 5d [ 3/2 ] i level.
1. Introduction
The optogalvanic ( O G ) effect, a change in the impedance of an electrical discharge in response to absorption of radiation by a species in the discharge, has been extensively investigated during the last ten years using visible and infrared lasers as discharge probes. Atomic [ 1 ], molecular [2 ], and ionic [ 3 ] species have all been studied using OG techniques. In addition to providing spectroscopic information, such measurements give insight into the various, often complex, physical processes occurring in the discharge. The rare gases, in particular neon, have served as prototype systems for detailed studies of mechanisms of the O G effect in dc glow discharges [4-6]. Although the results of many mechanistic studies utilizing laser sources in the visible region have been reported, very few studies of infrared O G effects in the rare gases have appeared. In earlier infrared work by Kaplafka, Merkelo, and Goldstein [ 7 ] the glow discharge was used simply as a detector of incident 10.6 ~tm CO2 laser radiation, following similar studies in the microwave region [8,9]. Illumination of the discharge caused an increase in the dc discharge current, and the mechanism responsible was believed to be direct photoionization of highly excited atoms. Begemann and Saykally [ 10 ], using a color-center laser operating in the 2.5-2.8 ~tm wavelength region, observed and assigned many transitions between higher lying Rydberg levels of neon and argon excited in commercial 36
hollow cathode lamps. Absorption of all of the assigned transitions resulted in an increase in the dc discharge current. The mechanism postulated in this case was a higher probability of ionization for the upper level relative to the lower level involved in the transition, similar to mechanisms proposed previously for illumination of rare gas discharge with visible lasers [ 4 ]. Jackson et al. [ 11 ] investigated infrared O G effects in helium and neon discharges, also using a colorcenter laser operating near 2.6 ~tm. The emphasis in these studies was on the determination of fine structure intervals and line broadening mechanisms, and detailed studies of mechanisms of the O G effect itself were not pursued. A1-Chalabi et al. [ 12] made a quantitative study of the infrared O G effect in helium at 2.64 ~tm, again using a tunable color-center laser. Results were obtained on the ionization efficiency of the process, and on the effects of quenching by hydrogen gas added to the discharge. The spatial dependence of OG effects in a helium positive column discharge were investigated by Tam [ 13 ] using high powered helium lamps as sources of radiation at 2.058 ~tm. Absorption at this wavelength originates from the 2 1S metastable level of helium. Except for irradiation of the negative glow region of the discharge, the OG signal polarity corresponded to a decrease in the dc discharge current. Tam presented quantitative measurements and discussed possible mechanisms. In this communication we report observations in 0 030-4018/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 64, number l
OPTICS COMMUNICATIONS
a xenon hollow cathode discharge of a strong O G effect at 3771 cm -~ (2.652 ~tm) which consistently exhibited a polarity corresponding to a decrease in the discharge current over the range of discharge conditions investigated. O G effects associated with a decrease in the discharge current are generally attributed to laser induced changes in the metastable atom density. However, the xenon transition studied in this experiment involves upper and lower levels which are both free to radiate. A mechanism for the observed O G effect is proposed.
1 October 1987
X E N O N OG
p = 3 Torr = 6.2 m A
(+)
I
r
I
3763
3777
3791
(-)
i
I
3805
~(cm - 1)
5d[3/211 - 6p[1/2]0
2. Experimental O G spectra were recorded using a commercial infrared color-center laser (Burleigh FCL-20) as the discharge probe. A mechanical chopper amplitude modulated the laser beam at a frequency of 400 Hz and provided the reference signal for a lock-in amplifier. The laser operated predominantly on two longitudinal modes, a fundamental mode and a spatial hole-burning mode separated from the fundamental by 10 G H z (0.3 c m - ~). The laser output was tuned over the 3650-4100 cm-~ region by rotation of an intracavity grating. The grating served as both end mirror and output coupler, and was controlled by a stepping motor and reduction gear assembly. The grating sine bar drive was calibrated using the atmospheric H20 spectrum, with line positions taken from the AFGL linelist [ 14]. The maximum output power was about 20 mW near 2.64 ~tm using a 3W Ar ÷ laser operating on all lines as the pump source. The beam was directed along the axis of a pyrex discharge cell fitted with CaF2 Brewster windows. The cathode was a 4 cm long stainless steel cylinder having an inner diameter of 0.4 cm. Pyrex disks placed around both ends of the cathode, and extending outward to the cell walls, restricted the discharge to the interior of the cathode and helped to eliminate noisy operation at higher discharge currents [ 15]. Two stainless steel rings, 1.2 cm inner diameter × 0.2 cm width were spaced 1 cm from either end of the cathode in a double anode configuration. Care was taken during measurements to ensure that the laser beam did not strike the surfaces of the electrodes. The discharge cell was sealed during measurements and was operated at xenon pressures of 2-4
Fig. 1. Optogalvanic spectrum of xenon near 3800 cm ~. ( + ) and ( - ) refer to the direction 0fchange in the discharge current.
torr and dc currents of 3-30 mA. No positive column was developed under these conditions due to the small anode-cathode spacing. The xenon gas was used as supplied by Matheson and had a stated purity of 99.995%. The cell was placed in series with a l0 K ballast resistor, and O G signals extracted from the high voltage anodes via a 0.05 ~tF capacitor. O G spectra were processed by a lock-in amplifier and displayed on a strip chart recorder. The strongest OG signals could be observed directly on an oscilloscope, and these signals were used to determine the relationship between the polarity of the lock-in amplifier output, and the polarity of the voltage change across the discharge.
3. Results Fig. 1 shows a typical O G spectrum obtained for discharge conditions of 3 tort xenon, and 6.2 mA dc current. The relative intensities of the observed transitions would vary somewhat with operating conditions and with exact positioning of the laser beam, as expected from the structured nature of the hollow cathode discharge [ 16 ]. However, the individual signal polarities were constant over the operating range. Most of the observed lines appeared as doublets due to the dual mode operation of the laser. The xenon transitions can be readily assigned using the 37
Volume 64, number 1
OPTICS COMMUNICATIONS
list o f transitional frequencies given by Outred [ 17 ] in conjunction with Moore's [18] tabulation of energy levels. Observed transitions, together with assignments and approximate intensities, are listed in table 1. Except for the strong signal at 3771 cm l, all observed O G signals corresponded to a laserinduced increase in the discharge current, similar to the observations of Begemann and Saykally [ 10 ] for transitions among higher Rydberg levels of neon and argon. The strong, negative O G signal at 3771 cm -~, corresponds to the 5 d [ 3 / 2 ] l - 6 p [ 1 / 2 ] o transition in xenon. The polarity of this signal did not change with discharge current over the range 3-30 mA, nor was its intensity a strong function of the current. The voltage change associated with the 3771 cm-~ O G signal was typically 2 5 - 5 0 mV rms, corresponding to rms changes in the discharge current o f a few microamperes. Qualitatively similar spectra were obtained using a commercial (Jarrell-Ash) hollow cathode lamp having an iron cathode, and modified to allow refilling o f the buffer gas. A weak, positive polarity continuum O G signal, proportional to the laser power, was observed when using the commercial lamp. This continuum was assumed to be due to effects resulting from direct irradiation of the cathode surface, as no wavelength independent O G signal was observed using the open cathode cell o f fig. 1.
1 October 1987
4. Discussion The present study was concerned primarily with the mechanism responsible for the polarity o f the 5d[ 3/2 ] l - 6 p [ 1/2 ]o O G signal in xenon. In studies reported to date, a strong O G signal corresponding to a decrease in the ionization rate in a pure rare gas discharge has been associated with absorption from one of the long-lived metastable levels o f the lowest excited np5 (n + 1 ) s configuration. The basic mechanism in this case can be briefly described as follows. Irradiation of the discharge at a suitable wavelength raises the ns[ 1/2 ]o or ns[3/2 ]2 metastable atom to a higher lying excited level o f n p S ( n + 1 )p configuration. F r o m the upper p-level the atom may be ionized, or deactivated via collisional processes, or it may decay radiatively to one o f the short-lived resonance levels (ns [ 1/2 ] ~ or ns [ 3/2 ] t ) of the original ns manifold. The resonance levels rapidly decay to the t So ground state through emission o f a photon in the v a c u u m ultraviolet region. Fig. 2 shows this process schematically for a single transition. If the laser pumping rate combined with the two step radiative decay rate exceeds the normal (in the absence of external illumination) metastable production rate, the result is a net loss of metastable atoms from the irradiated region. Because the longlived metastable atoms play a vital role in secondary ionization processes in the discharge at low current densities [19], a decrease in the metastable population will cause the steady-state ionization rate, or current, to decrease. This is observed experimentally
Table 1 Observed optogalvanic signals in xenon; VS=very strong ( > 5 mV), S=strong ( ~ 1 mV), M=medium ( ~ 500 ~tV), W=weak ( <200 ~tV). The polarity changes denote the direction of change in the discharge current.
38
v (cm- t )
Assignment
Intensity
Polarity
3770.7 3776.1 3805.9 3839.1 3843.4 3866.6 3871.4 3872.8 3885.6 3933.2 3975.7
5d[3/2 ] r6p[ 1/2 ]o 7d[ 1/2 ] ~-7p[5/2 ]2 5d[ 5/2 ]z-6p[ 5/2 ]2 7s[ 3/2 ] ~-6p'[ 1/2 ]~ 5f[9/2 ]4-6d[ 5/2 ]3 5f[ 5/2 ]3-6d[5/2 ]3 7d[3/2 ] j-Tp[7/2 ]o 13p[5/2 ] 3-7d[ 5/2 ]3 5f[7/2 ] 3.,-6d[ 5/2 ]3 7d[ 5/2 ] 2-7p[ 3/2 ] ~ 7d[ 7/2 ]4-7p[ 5/2 ]3
VS W S S M M S W M M W
+ + + + + + + + + +
Volume 64, number 1
OPTICS COMMUNICATIONS
1 October 1987
13.433 IONIZATIONLIMITS
i ! (2Pl/2)
13
2p312
l
LIMITS
t
12
12.127
ei
e-
np5(n+ 1)p I
VlS y/LASER [112]1~ [1/2]0 ~ [312]1~ [31212~
PUMP
11
e-
e-
I
>0 ILl z u.I 10
5a 13/211 6s'
r m np5(n+ 1)s r m
[11211"----- v
[11210
?
k.~ 1192,,~ 6s_~ [3/211 ~ ( F A S T , [3•2]2 ~
of
vuv 1% RARE GAS
Fig. 2. Simplifiedenergylevel diagram for the rare gases (except helium) to illustrate the mechanism whereby laser absorption from a metastable level causes a decrease in discharge current, m and r refer to the metastable and resonance levels, respectively, of the lowest excited np5(n + 1) s configuration. Wavylines indicate radiative transitions. (e-) signifies electron collisional ionization.
[4,5]. At low pressures the process may also result in a global loss of energy from the discharge system due to escape of the resonance level emission. The net loss from radiative decay is determined by the degree of radiative trapping [20]. Other collisional and radiative processes, whose rates are, in general, different functions of the operating environment [ 19 ], may contribute to the O G effect as well. The net result of these competing processes determines the polarity of the (time integrated) O G signal. A similar mechanism can account for the observed polarity of the 5 d [ 3 / 2 ] ~ - 6 p [ l / 2 ] o transition in xenon. Fig. 3 shows a partial energy level diagram for xenon. The upper 5d[3/2]~ level has a direct radiative path to ground through emission of a pho-
T
iso
T
XENON Fig. 3. Simplifiedenergyleveldiagramfor xenon. Symbolsare definedas in fig. 2. Absorptionat 3771 cm-t is followedby fast radiativedecayto groundfromthe upper 5d[3/2 ] ~level,resulting in a decreaseddensityof excitedatomsin the discharge. ton at 1192 A, as well as less probable decay routes to levels of the 6p manifold. The lower 6p[ 1/2 ]o level is of the same parity as the xenon ground state (both are even parity) and thus has allowed transitions only to the J= 1 resonance levels of the 6s manifold. In the discharge, excited s and d levels are populated efficiently by electron impact excitation, while the p levels are populated primarily by cascading from higher lying s and d levels [21,22]. Irradiation of the xenon discharge at 3771 c m transfers population from ithe 6p [ 1/2 ] o level (z = 40 ns) [23] to the much sl~orter-lived 5d[3/2 ]l level (z= 1.4 ns) [24]. The upper 5d[3/2 ]~ level decays radiatively to the ground state in a time comparable to electron collisional rates [ 19], and short compared with the rate of (tWo step) decay to ground from the lower 6p [ 1/2 ] o level. Thus, the transfer of population results in a net loss of excited atoms from 39
Volume 64, number 1
OPTICS COMMUNICATIONS
the discharge, and an a c c o m p a n y i n g decrease in the ionization rate, as the 5d[3/2 ]1 a t o m s convert to ground state atoms. For the xenon 5 d [ 3 / 2 ] l - 6 p [ 1 / 2 ] o transition the process is efficient because radiative decay to the a t o m i c ground state is a single step process. Lawless [25] has modelled this situation using an equivalent circuit description o f the discharge system. The lack o f accurate collision cross sections for the xenon Rydberg levels however, precludes application o f the Lawless model here. Because o f radiative trapping [20] the effective lifetime o f the 5d[ 3/2 ]1 level m a y greatly exceed the natural radiative lifetime. At 3 Torr pressure radiative trapping will decrease the excited a t o m loss rate from escape o f the 5 d [ 3 / 2 ]~ emission. However, as shown in the neon system [5,6,19], escape o f V U V radiation can represent a significant energy loss under the experimental conditions (pressure-radius product ~ 0.6 cm Torr) e m p l o y e d here. A second 5 d - 6 p transition observed at 3805.9 cm-~ (fig. 1 ) exhibited an O G signal polarity corresponding to an increase in the ionization rate. This observation indicates that the 5 d - 6 p transitions do not cause a n o m a l o u s O G effects as a group. F o r example, p o p u l a t i o n inversions, which enable cw lasing on several o f the 5 d - 6 p xenon transitions [26], can d r a m a t i c a l l y alter the characteristics o f the O G effect [6 ]. Other possible m e c h a n i s m s responsible for the observed decrease in discharge current for the 5d[3/2 ]~-6p[ 1/2 ]o transition, such as i n v o l v e m e n t in d i m e r f o r m a t i o n or destruction processes [ 27,28 ] cannot be ruled out. Such effects, however, seem unlikely at the low pressures used to obtain these data. The work described in this p a p e r was p e r f o r m e d at the University o f N o r t h Carolina at Chapel Hill, sponsored by the National Science F o u n d a t i o n under grant No. CHES111131. The writing a n d publication o f this p a p e r was s u p p o r t e d by the Jet Propulsion Laboratory, California Institute o f Technology, under a contract with the N a t i o n a l Aeronautics and Space A d m i n i s t r a t i o n .
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1 October 1987
References [1] J.E.M. Goldsmith and J.E. Lawler, Contemp. Phys. 22 (1981) 235. [2] C.R. Webster and C.T. Rettner, Laser Focus (February, 1983) pp. 41-52. [ 3 ] J. de Physique, Intern. Colloquium on Optogalvanic spectroscopy and its applications, Colloque No. 7, C7 (1983). [4] K.C. Smyth and P.K. Schenck, Chem. Phys. Lett. 55 (1978) 466. [5] D.K. Doughty and J.E. Lawler, Phys. Rev. A 28 (1983) 773. [6] A. Ben-Amar, G. Erez and R. Shuker, J. Appl. Phys. 54 (1983) 3688. [7] J.P. Kaplafka, H. Merkelo and L. Goldstein, Appl. Phys. Lett. 15 (1969) 113; 19 (1971) 197. [8] N.H. Farhat, Proc. IEEE 52 (1964) 1053; 62 (1974) 279. [9] N.S. Kopeika, Proc. IEEE 63 (1975) 981. [ 10 ] M.H. Begemann and R.J. Saykally,Optics Comm. 40 (1982) 277. [ 11 ] D.J. Jackson, H. Gerhardt and T.W. Hansch, Optics Comm. 37 (1981) 23. [12] S.A.M. Al-Chalabi, R.S. Steard, R. Illingworth and I.S. Ruddock, J. Phys. D 16 (1983) 115. [ 13] A.C. Tam, IEEE Trans. Plasma Science PS-10 (1982) 252. [ 14] AFGL Spectral Data Tapes, see L.S. Rothman et al., Appl. Optics 20 (1981) 791. [ 15 ] This idea was taken from the design of commercial (Jarrell Ash) hollow cathode lamps in which a stack of mica disks is used for this purpose. [16] B. Chapman, Glow discharge processes (Wiley and Sons, New York, 1980). [ 17 ] M. Outred, J. Phys. Chem. Ref. Data 7 ( 1978 ) pp. 1-262. [18] C.E. Moore, Atomic energy levels, National Bureau of Standards, Circular No. 467 (1949). [ 19 ] B.E. Cherrington, Gaseous electronics and gas lasers (Pergamon Press, Oxford, 1979). [20] F.E. Irons, J. Quant. Spectrosc. Rad. Trans. 22 (1979) 1; 22 (1979) 21. [21] W.R. Bennett, Jr., Appl. Optics, Supplement on Optical Masers ( 1962 ) pp. 24-61. [ 22 ] W.R. Bennett, Jr., P.J. Kindlemann and G.W. Mercer, Appl. Optics, Supplement 2 of Chemical Lasers ( 1965) pp. 35-58. [23] L. Allen, D.G.C. Jones and D.G. Schofield,J. Opt. Soc. Am. 59 (1969) 842. [24] E. Matthias, R.A. Rosenberg, E.D. Poliakoff, M.G. White, S.T. Lee and D.A. Shirley, Chem. Phys. Lett. 52 (1977) 239. [25] J.L. Lawless, J. Appl. Physics 55 (1984) 3226. [26] C.C. Davis and T.A. King, J. Quant. Spectrosc. Rad. Transfer 13 (1973) 825. [27] R.S. Mulliken, J. Chem. Phys. 52 (1970) 5170. [28 ] W. Gornik, S. Kindt, E. Matthias and D. Schmidt, J. Chem. Phys. 75 (1981) 68.