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Slow intermolecular redistribution of vibrational energy in SF6 gas excited by a tea CO2 laser
February 1980
OPTICS COMMUNICATIONS
Volume 32, number 2
SLOW INTERMOLECULAR
EEDISTRIBUTION
OF VIBRATIONAL
ENERGY
IN SF6 GAS EXCITED BY ti TEA CO, LASER G. KOREN, I. LEVIN and U.P. OPPENHEIM Physics Department,
Technion Israel Institute of Technology,
Haifa# Israel
Received 30 October 1979
Infra-red fluorescence (IRF) spectra of SF, excited by the 944.2 cm -t line of a pulsed CO2 laser were observed at various times after the time of the laser excitation. Each spectrum showed a strong IRF peak of the q mode which was red shifted relative to the room temperature fundamental (948 cm-i) by an amount which depended, apart from the level of excitation, on the different times employed. For a strong excitation with (n> IJ 11 photons absorbed per molecule, a significant decrease of red shift versus time was observed, indicating mainly excitation losses by IRF emission. For weak excitation with tn) a 1.4, almost an constant red shift versus time is observed. This result, and the previous finding that at weak excitation a nonthermal energy distribution in the ensemble of molecules exists, leads to the conclusion that intermolecular redistribution of vibrational energy in SF6 is slow, and does not exceed the observed fluorescence duration (-1 ms).
Previous
measurements
of the infra-red
fluorescence
(IRF) spectra of multiphoton excited SF, [l] have led to the conclusion that under weak excitation conditions ((n) < 3, where (n) is the average number of photons absorbed per molecule) a nonthermal distribution exists in the molecular ensemble.
energy The
experimental results in ref. [I] show that for (n) < 3 the laser induced red shift of the IRF peak of u3 is significantly larger than the red shift of SF, gas heated thermally to the same temperature [2]. This phenomenon was interpreted as resulting from the fact that during the excitation process only a part of the molecules is excited, while the other part remains unexcited due to a bottlenecking effect in the low energy levels at low values of fluence. The excited fraction of the molecules, which emits the IRF, therefore possesses a higher vibrational energy (nftw, where Aw is the energy of the laser photons) than the average vibrational energy per molecule ((n )Aw), and the resulting red shift is therefore higher than the thermal red shift. In the present study measurements complementary to ref. [l] with 1 < (n )< 2 are given. In addition, the state of nonthermal equilibrium of the molecular ensemble which exists for these (n) values, is used to investigate the intermolecular
vibrational energy transfer process by measuring the IRF spectra at different times from the time of the laser excitation and observing the decrease rate of the red shift with time. The experimental system used for the present measurements of the IRF spectra was described in detail in ref. [ 11. The pulsed CO, laser was operated on the P(20) line (944.2 cm-‘), and could be made to emit long pulses (“1 ps) with a nitrogen rich gas mixture and short pulses (-100 ns) using a mixture of CO, and He only. It was used to excite the SF6 gas in a parallel beam geometry which enabled the exact determination of the average number of photons absorbed per molecule (n ), using calorimeters as monitors. The IRF was measured in a direction perpendicular to that of the exciting beam. It was dispersed in a grating monochromator whose bandpass function was a triangle with 6 cm-’ width at half maximum. After dispersion the IRF was focused onto a HgCdTe detector, whose output was amplified and transferred to an oscilloscope. For each setting of the monochromator the IRF signal was observed in the oscilloscope. A typical IRF signal for 1.6 Torr of SF, is shown in fig. 1. Its risetime of a few microseconds is limited by our amplifier, and the ripple on the ex257
Volume 32, number 2
OPTICS COMMUNICATIONS
February 1980 <“>
20
I I
y
3 I
2 I
I6
torr
4 I
5 I
6
SF,
t tmsec) Fig. 1. Typical IRF oscilloscope trace of 1.6 Torr SF, and or) = 1.4.
Oo ponential decay is due to thermal shock waves created in the absorption cell. At times longer than about 0.5 ms the IRF signal is smaller than expected due to the decrease in the detectivity of the detector at low frequencies. The measurement was carried out by recording the height of the IRF signal as a function of time (t) from the time of the laser excitation. The fmite response time of the detection system, caused a delay in the appearance of the peak of the pulse, so the height of the signal at times approaching t = 0 was found by extrapolation back to the time of the laser excitation. Using the short (100 ns) laser pulses for excitation, a set of IRF spectra with I = 0 was obtained for 1 < (n) < 2. From these spectra the red shift of the v3 IRF peak was obtained and plotted versus (n) and versus the average vibrational energy per molecule (Ev) in fig. 2. This figure provides additional points on the curve given in fig. 4 of ref. [I], and shows that the measured red shifts in the region 1 < (n) < 2 fall close to the straight line that passes through the data point of Steinfeld et al. [3] and our previous data points. Therefore, our conclusion in ref. [l] that a nonthermal energy distribution exists in the gas under weak excitation conditions, is now based on a direct experimental result rather than on extrapolation of our experimental results, used previously. The state of excitation of the molecules is reflected by the IRF spectrum. The red shift of the ~3 IRF 258
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4000 < EV >
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6000
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Fig. 2. Red shifts of the IRF vs peaks versus (n) and (E,). Results shown are: present IRF results (o), IRF results from ref. [l] (=), double resonance absorption measurement of Steinfeld et al. [3] (a), and the shock tube absorption results of Nowak and Lyman [2] (0).
peak, which depends on the vibrational energy distribution in the gas, is the simplest measure of this excitation. A decrease in the red shift indicates either that the molecules are losing vibrational energy while maintaining a thermal distribution, or that a nonthermal distribution has become thermal by transferring energy from high energy levels to lower levels. It is also possible that a combination of these two processes takes place. Another possibility is that a decrease in the red shift is caused by heat diffusion out of the irradiated zone. Under the present experimental conditions (P = 1.6 Torr and 300 < T < 1500K) the thermal diffusivity coefficient D ranges between 20 and 50 cm2 s-l. This leads to a diffusion time of 10 5 Tdiff = a2/(1 .6 D) 2 30 ms, where a = 1 cm is the radius of the pump beam. Thus on the time scale of 1 ms observed in fig. 1 the heat diffusion effects are negligible. An excited molecule can lose its vibrational energy in a number of ways, the important ones being the V-V and V-T processes, and the emission of infrared radiation. Since the time scale of the present measure-
February 1980
OPTICS COMMUNICATIONS
Volume 32, number 2
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Fig. 3. IRF spectra of 1.6 Torr SF6 and (n) - 11 in the region of the vs peak, as measured at various times from the time of the laser excitation. All curves are on the same relative scale. Long laser pulses of 1 ~~1s duration and a fluence of 0.45 J/cm* were used for the irradiation. The arrows indicate the maximum IRF in each spectrum and the curves are a visual fit to the data points.
ments is relatively long (a few /.fs to 1 ms), it can safely be assumed that the fast intramolecular relaxations, taking less than a few microseconds [4,5], have reached equilibrium even before oure measurements starts. V-T relaxation at high laser intensities is also fast, and has a value of r < 20 ps Torr-’ at an intensity of -0.2 MW/cm’ [6] which is the lowest intensity used here. Thus one is left with the intermolecular vibrational relaxations and the IRF emission as the possible channels for losses of vibrational excitation. Figs. 3 and 4 show IRF spectra as measured at different times from the moment of laser excitation. The results in fig. 3 were measured under high excitation conditions (
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Fig. 4. IRF spectra of 1.6 Torr SF6 and tn) = 1.4 in the region of the vs peak as measured at different times from the time of the laser excitation. All curves are on the same relative scale. Short laser pulses of 100 ns duration and of 0.02 J/cm* were used for the irradiation. The arrows indicate the maximum IRF in each spectrum and the curves are visual tits.
distribution of the ensemble of molecules. The only channel left for losses of vibrational energy which will explain the observed decrease in red shift as a function of time in fig. 3 is thus the IRF radiative decay. The spontaneous emission lifetime for a transition between the harmonic oscillator levels n = 11 and n = = 0.321 X 102*/ 10 is equal to rspon = l/(lOAl+o) (SmTYuzpre v is in cm-’ units and Sl,o = 4800 [7]. The spontaneous lifetime may now be calculated from the v3 integrated band intensity s 1_,o = 4800 atm-’ cmP2 [7] and gives Tag,, = 0.8 ms. This time is relatively short and clearly allows for the IRF decay to cause the red shift decrease during the 1 ms measurement time. The results of fig. 4 were measured under low excitation conditions with (n) = 1.4. They show that the laser induced red shift is almost constant (-9 cm -‘) for all the times measured, and that this shift is much higher than the equivalent thermal red shift (-5 cm -’ at (m) = 1.4, as can be seen from fig. 2). As was explained above, this indicates that during 259
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OPTICS COMMUNICATIONS
the IRF decay the molecular ensemble remains in a state of nonthermal equilibrium [l] in which only a fraction of the molecules is excited. It therefore follows that no thermalizing relaxation processes are active during the observed period of the fluorescence. The fact that the radiation losses by fluorescence, which were found to affect the red shift of the u3 IRF peak under high excitation conditions (fig. 3) are of negligible influence on the red shift at low excitation levels (fig. 4) may be explained as follows. The lifetime of spontaneous emission for the transition IZ= 1 + n = 0 (rspon M 8 ms) is longer by a factor of 10 than the lifetime at n = 11 which was calculated before. Also, the cascade series of IRF transitions from n = 11 to n = 10, from n = 10 to n = 9, etc., is clearly much longer than that of a molecule with IZ= 2 or n = 1. Thus during the 1 ms measurement time, there exist many more optical transitions with varying red shifts in a molecule excited to n = 11 than for a molecule close to the ground state. The remaining relaxations that could lead to thermalization in fig. 4 are the intermolecular vibrational energy transfer processes. An intermolecular V-V process in which a total exchange of vibrational energy occurs, i.e. SF6(vj) + SF6(vi) + SFS(ui) t SFg(vi), cannot change the vibrational energy distribution of the gas and therefore does not affect the almost constant red shift as a function of time observed in fig. 4. One would expect that other intermolecular V-V energy transfer processes ,would affect the nonthermal energy distribution, by processes such as SF,(2v3) t SF6 + 2SFg(v3). The fact that this does not happen means that this relaxation is slower than the 1 ms duration of the fluorescence measurement, which constitutes an upper limit to this relaxation rate. The latter result is somewhat unexpected, since many reports have been published in which a V-V relaxation time of about 1 gs Torr-’ or less was found. In these reports double resonance [3-5,8 ] and IRF techniques [9] were employed, and the measurements were performed by excitation at one wavelength
260
February 1980
in the v3 mode, and probing at another wavelength. The risetime of the signal at the second wavelength, whether it was induced absorption or transparency in the double resonance experiments or IRF emission at a different vibrational mode (~4) in the IRF experiments, was used to determine the V-V relaxation time. The results of these experiments, however, are insensitive to the nature of the V-V relaxation process, which can be either an intramolecular or intermolecular relaxation process. Apparently intramolecular V-V relaxations were observed in these measurements, both induced by collisions and without them. Clearly, results of measurements in a collisionless regime at low pressures and with short laser pulse durations reflect intramolecular vibrational relaxations [4,5]. Thus the present finding of a slow intermolecular V-V relaxation process does not contradict the previous results. Note, however, that this process is of a very special nature and that it does not include the case of total exchange intermolecular V-V processes, which may be as fast as the gas kinetic collision rate.
References 111 G. Koren, I. Levin and U.P. Oppenheim, Optics Comm. 3 1 (1979) 321.
PI A.V. Nowak and J.L. Lyman, J. Quant. Spectrosc. Radiat. Transfer 15 (1975) 945. [31 J.I. Steinfeld, I. Burak, D.G. Sutton and A.V. Nowak, J. Chem. Phys. 42 (1970) 5421. [41 T.F. Deutsch and S.R.J. Brueck, J. Chem. Phys. 70 (1979) 2063. [51 D.S. Frankel, Jr., J. Chem. Phys. 65 (1976) 1696. WI S.A. Akhmanov, V.M. Gordienko, A.V. Mikheenko and V.Ya. Panchenko, JETP Lett. 26 (1977) 453. 171 S.C. Penner, Quantitative molecular spectroscopy and gas emissivities,(Addison-Wesley, London, 1959) pp. 21 and 98. 181 R.S. Taylor, T.A. Znotins, E.A. Ballik and B.K. Garside, J. Appl. Phys. 48 (1977) 4435. PI R.D. Bates, Jr., J.T. Knudtson, G.W. Flynn and A.M. Ronn, Chem. Phys. Lett. 8 (1971) 103.
OPTICS COMMUNICATIONS
Volume 32, number 2
SLOW INTERMOLECULAR
EEDISTRIBUTION
OF VIBRATIONAL
ENERGY
IN SF6 GAS EXCITED BY ti TEA CO, LASER G. KOREN, I. LEVIN and U.P. OPPENHEIM Physics Department,
Technion Israel Institute of Technology,
Haifa# Israel
Received 30 October 1979
Infra-red fluorescence (IRF) spectra of SF, excited by the 944.2 cm -t line of a pulsed CO2 laser were observed at various times after the time of the laser excitation. Each spectrum showed a strong IRF peak of the q mode which was red shifted relative to the room temperature fundamental (948 cm-i) by an amount which depended, apart from the level of excitation, on the different times employed. For a strong excitation with (n> IJ 11 photons absorbed per molecule, a significant decrease of red shift versus time was observed, indicating mainly excitation losses by IRF emission. For weak excitation with tn) a 1.4, almost an constant red shift versus time is observed. This result, and the previous finding that at weak excitation a nonthermal energy distribution in the ensemble of molecules exists, leads to the conclusion that intermolecular redistribution of vibrational energy in SF6 is slow, and does not exceed the observed fluorescence duration (-1 ms).
Previous
measurements
of the infra-red
fluorescence
(IRF) spectra of multiphoton excited SF, [l] have led to the conclusion that under weak excitation conditions ((n) < 3, where (n) is the average number of photons absorbed per molecule) a nonthermal distribution exists in the molecular ensemble.
energy The
experimental results in ref. [I] show that for (n) < 3 the laser induced red shift of the IRF peak of u3 is significantly larger than the red shift of SF, gas heated thermally to the same temperature [2]. This phenomenon was interpreted as resulting from the fact that during the excitation process only a part of the molecules is excited, while the other part remains unexcited due to a bottlenecking effect in the low energy levels at low values of fluence. The excited fraction of the molecules, which emits the IRF, therefore possesses a higher vibrational energy (nftw, where Aw is the energy of the laser photons) than the average vibrational energy per molecule ((n )Aw), and the resulting red shift is therefore higher than the thermal red shift. In the present study measurements complementary to ref. [l] with 1 < (n )< 2 are given. In addition, the state of nonthermal equilibrium of the molecular ensemble which exists for these (n) values, is used to investigate the intermolecular
vibrational energy transfer process by measuring the IRF spectra at different times from the time of the laser excitation and observing the decrease rate of the red shift with time. The experimental system used for the present measurements of the IRF spectra was described in detail in ref. [ 11. The pulsed CO, laser was operated on the P(20) line (944.2 cm-‘), and could be made to emit long pulses (“1 ps) with a nitrogen rich gas mixture and short pulses (-100 ns) using a mixture of CO, and He only. It was used to excite the SF6 gas in a parallel beam geometry which enabled the exact determination of the average number of photons absorbed per molecule (n ), using calorimeters as monitors. The IRF was measured in a direction perpendicular to that of the exciting beam. It was dispersed in a grating monochromator whose bandpass function was a triangle with 6 cm-’ width at half maximum. After dispersion the IRF was focused onto a HgCdTe detector, whose output was amplified and transferred to an oscilloscope. For each setting of the monochromator the IRF signal was observed in the oscilloscope. A typical IRF signal for 1.6 Torr of SF, is shown in fig. 1. Its risetime of a few microseconds is limited by our amplifier, and the ripple on the ex257
Volume 32, number 2
OPTICS COMMUNICATIONS
February 1980 <“>
20
I I
y
3 I
2 I
I6
torr
4 I
5 I
6
SF,
t tmsec) Fig. 1. Typical IRF oscilloscope trace of 1.6 Torr SF, and or) = 1.4.
Oo ponential decay is due to thermal shock waves created in the absorption cell. At times longer than about 0.5 ms the IRF signal is smaller than expected due to the decrease in the detectivity of the detector at low frequencies. The measurement was carried out by recording the height of the IRF signal as a function of time (t) from the time of the laser excitation. The fmite response time of the detection system, caused a delay in the appearance of the peak of the pulse, so the height of the signal at times approaching t = 0 was found by extrapolation back to the time of the laser excitation. Using the short (100 ns) laser pulses for excitation, a set of IRF spectra with I = 0 was obtained for 1 < (n) < 2. From these spectra the red shift of the v3 IRF peak was obtained and plotted versus (n) and versus the average vibrational energy per molecule (Ev) in fig. 2. This figure provides additional points on the curve given in fig. 4 of ref. [I], and shows that the measured red shifts in the region 1 < (n) < 2 fall close to the straight line that passes through the data point of Steinfeld et al. [3] and our previous data points. Therefore, our conclusion in ref. [l] that a nonthermal energy distribution exists in the gas under weak excitation conditions, is now based on a direct experimental result rather than on extrapolation of our experimental results, used previously. The state of excitation of the molecules is reflected by the IRF spectrum. The red shift of the ~3 IRF 258
L
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/’
I
I 2000
4000 < EV >
I
6000
(cm-r)
Fig. 2. Red shifts of the IRF vs peaks versus (n) and (E,). Results shown are: present IRF results (o), IRF results from ref. [l] (=), double resonance absorption measurement of Steinfeld et al. [3] (a), and the shock tube absorption results of Nowak and Lyman [2] (0).
peak, which depends on the vibrational energy distribution in the gas, is the simplest measure of this excitation. A decrease in the red shift indicates either that the molecules are losing vibrational energy while maintaining a thermal distribution, or that a nonthermal distribution has become thermal by transferring energy from high energy levels to lower levels. It is also possible that a combination of these two processes takes place. Another possibility is that a decrease in the red shift is caused by heat diffusion out of the irradiated zone. Under the present experimental conditions (P = 1.6 Torr and 300 < T < 1500K) the thermal diffusivity coefficient D ranges between 20 and 50 cm2 s-l. This leads to a diffusion time of 10 5 Tdiff = a2/(1 .6 D) 2 30 ms, where a = 1 cm is the radius of the pump beam. Thus on the time scale of 1 ms observed in fig. 1 the heat diffusion effects are negligible. An excited molecule can lose its vibrational energy in a number of ways, the important ones being the V-V and V-T processes, and the emission of infrared radiation. Since the time scale of the present measure-
February 1980
OPTICS COMMUNICATIONS
Volume 32, number 2
I 0933
I 935
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v” lcm-’
Y
(cm-11
Fig. 3. IRF spectra of 1.6 Torr SF6 and (n) - 11 in the region of the vs peak, as measured at various times from the time of the laser excitation. All curves are on the same relative scale. Long laser pulses of 1 ~~1s duration and a fluence of 0.45 J/cm* were used for the irradiation. The arrows indicate the maximum IRF in each spectrum and the curves are a visual fit to the data points.
ments is relatively long (a few /.fs to 1 ms), it can safely be assumed that the fast intramolecular relaxations, taking less than a few microseconds [4,5], have reached equilibrium even before oure measurements starts. V-T relaxation at high laser intensities is also fast, and has a value of r < 20 ps Torr-’ at an intensity of -0.2 MW/cm’ [6] which is the lowest intensity used here. Thus one is left with the intermolecular vibrational relaxations and the IRF emission as the possible channels for losses of vibrational excitation. Figs. 3 and 4 show IRF spectra as measured at different times from the moment of laser excitation. The results in fig. 3 were measured under high excitation conditions (
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Fig. 4. IRF spectra of 1.6 Torr SF6 and tn) = 1.4 in the region of the vs peak as measured at different times from the time of the laser excitation. All curves are on the same relative scale. Short laser pulses of 100 ns duration and of 0.02 J/cm* were used for the irradiation. The arrows indicate the maximum IRF in each spectrum and the curves are visual tits.
distribution of the ensemble of molecules. The only channel left for losses of vibrational energy which will explain the observed decrease in red shift as a function of time in fig. 3 is thus the IRF radiative decay. The spontaneous emission lifetime for a transition between the harmonic oscillator levels n = 11 and n = = 0.321 X 102*/ 10 is equal to rspon = l/(lOAl+o) (SmTYuzpre v is in cm-’ units and Sl,o = 4800 [7]. The spontaneous lifetime may now be calculated from the v3 integrated band intensity s 1_,o = 4800 atm-’ cmP2 [7] and gives Tag,, = 0.8 ms. This time is relatively short and clearly allows for the IRF decay to cause the red shift decrease during the 1 ms measurement time. The results of fig. 4 were measured under low excitation conditions with (n) = 1.4. They show that the laser induced red shift is almost constant (-9 cm -‘) for all the times measured, and that this shift is much higher than the equivalent thermal red shift (-5 cm -’ at (m) = 1.4, as can be seen from fig. 2). As was explained above, this indicates that during 259
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OPTICS COMMUNICATIONS
the IRF decay the molecular ensemble remains in a state of nonthermal equilibrium [l] in which only a fraction of the molecules is excited. It therefore follows that no thermalizing relaxation processes are active during the observed period of the fluorescence. The fact that the radiation losses by fluorescence, which were found to affect the red shift of the u3 IRF peak under high excitation conditions (fig. 3) are of negligible influence on the red shift at low excitation levels (fig. 4) may be explained as follows. The lifetime of spontaneous emission for the transition IZ= 1 + n = 0 (rspon M 8 ms) is longer by a factor of 10 than the lifetime at n = 11 which was calculated before. Also, the cascade series of IRF transitions from n = 11 to n = 10, from n = 10 to n = 9, etc., is clearly much longer than that of a molecule with IZ= 2 or n = 1. Thus during the 1 ms measurement time, there exist many more optical transitions with varying red shifts in a molecule excited to n = 11 than for a molecule close to the ground state. The remaining relaxations that could lead to thermalization in fig. 4 are the intermolecular vibrational energy transfer processes. An intermolecular V-V process in which a total exchange of vibrational energy occurs, i.e. SF6(vj) + SF6(vi) + SFS(ui) t SFg(vi), cannot change the vibrational energy distribution of the gas and therefore does not affect the almost constant red shift as a function of time observed in fig. 4. One would expect that other intermolecular V-V energy transfer processes ,would affect the nonthermal energy distribution, by processes such as SF,(2v3) t SF6 + 2SFg(v3). The fact that this does not happen means that this relaxation is slower than the 1 ms duration of the fluorescence measurement, which constitutes an upper limit to this relaxation rate. The latter result is somewhat unexpected, since many reports have been published in which a V-V relaxation time of about 1 gs Torr-’ or less was found. In these reports double resonance [3-5,8 ] and IRF techniques [9] were employed, and the measurements were performed by excitation at one wavelength
260
February 1980
in the v3 mode, and probing at another wavelength. The risetime of the signal at the second wavelength, whether it was induced absorption or transparency in the double resonance experiments or IRF emission at a different vibrational mode (~4) in the IRF experiments, was used to determine the V-V relaxation time. The results of these experiments, however, are insensitive to the nature of the V-V relaxation process, which can be either an intramolecular or intermolecular relaxation process. Apparently intramolecular V-V relaxations were observed in these measurements, both induced by collisions and without them. Clearly, results of measurements in a collisionless regime at low pressures and with short laser pulse durations reflect intramolecular vibrational relaxations [4,5]. Thus the present finding of a slow intermolecular V-V relaxation process does not contradict the previous results. Note, however, that this process is of a very special nature and that it does not include the case of total exchange intermolecular V-V processes, which may be as fast as the gas kinetic collision rate.
References 111 G. Koren, I. Levin and U.P. Oppenheim, Optics Comm. 3 1 (1979) 321.
PI A.V. Nowak and J.L. Lyman, J. Quant. Spectrosc. Radiat. Transfer 15 (1975) 945. [31 J.I. Steinfeld, I. Burak, D.G. Sutton and A.V. Nowak, J. Chem. Phys. 42 (1970) 5421. [41 T.F. Deutsch and S.R.J. Brueck, J. Chem. Phys. 70 (1979) 2063. [51 D.S. Frankel, Jr., J. Chem. Phys. 65 (1976) 1696. WI S.A. Akhmanov, V.M. Gordienko, A.V. Mikheenko and V.Ya. Panchenko, JETP Lett. 26 (1977) 453. 171 S.C. Penner, Quantitative molecular spectroscopy and gas emissivities,(Addison-Wesley, London, 1959) pp. 21 and 98. 181 R.S. Taylor, T.A. Znotins, E.A. Ballik and B.K. Garside, J. Appl. Phys. 48 (1977) 4435. PI R.D. Bates, Jr., J.T. Knudtson, G.W. Flynn and A.M. Ronn, Chem. Phys. Lett. 8 (1971) 103.