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SF6-sensitized dissociation of UF6 with a pulsed CO2 laser
VolumelOl_ number 1
SF6SENSITZED
CHEMLCALPHYSICSLETTERS
7 October 1983
DISSOCIATION OF UF6 WITH A PULSED CO2 LASER
CHIN Chi-tsung, HOU Hui-qi, BAU Yi-han and LI Ting-hua of Nuclear Science. Fudan Univedy, Smnghai. People’s Repubifc of Cfiina
Deprrment
Received 15 March 1983; in foal form 26 May 1983
The dissociation of UFe sensitized by SFe excited with a pulsed CO* laser in the presence of Iis and Co as scavc~ge~s has been investigated. in the SF~--UF~-HZ system the dissociation yieIds have been determined as a function of the laser frezyncy, the fluence. and Hs partial pressure. A masimum dissociation yield has been found at a laser frequency of 935 cm _ No obvious dissociation of UFs \\asobserved in the UF6-SF6 system without F-atom scavengers.
I. Introduction The application of IR sensitizers to pulsed CO2 laser induced reactions has been studied for several
years Many of them appear to be typical high-temperature pyrolyses [l] _Hill et al. [2] used SF6 as an IR sensitizer to initiate the CH, I- 0, explosion reaction with a single CO2 laser pulse, and indicated that there was some non-thermal component in the combustion initiation. During the course of this work, two papers [3,4] demonstrated infrared photosensitized multiphoton dissociation of UFg by SF6 _ It is interesting that isotopic selectivity has been observed in the IR-sensitized dissociation of SF,Cl and CF, I by SF6 excited with a TEA CO, laser [S]_ We report here the SFg-sensitized dissociation of UFg with a pulsed CO, laser in the presence of Hz and CO as scavengers_ A detailed study of the dissociation of UFg as a function of laser frequency, fluence and H2 partial pressure has been performed.
2. Experimental A multimode TEA CO, laser was indigenously built. The pulse width was measured with a Rofm model 7415 photon drag detector, The main pulse had a 100 ns duration fwhm, followed by a long tail (1.5 ps duration) containing 50% of the total energy. The laser was tunable between 9 and 11 .um using a 0 009-2614/83/0000-0000/S
03.00 0 1983 North-Holland
150 lines/mm grating, and its frequency was calibrated with a CO, spectrum analyzer_ Tlte laser beam was directed through the sample cell, and focused 7 cm away from the cell by a gold-coated concave mirror cf= 35 cm)_ The beam spot was =1.6 cm2 at the entrance window_ The pulse energy was measured with a precalibrated calorimeter_ The average fluence used in our experiments was less than OS J/cmz, which was not high enough to cause dielectric breakdown of the gas samples_ The reaction cell (3-4 cm in diameter and IO cm long) was made of stainless steel and equipped with a pair of KC1 windows fued with viton “0” rings. An all-metal vacuum manifold and the cell were passivated by F2 for 4 days. After passivation the cell was evacuated to JO-3 Torr and f&ed with the g,as mixtures_ The pressure was measured with a membrane manometer_ The partial pressure of UF, in the cell was monitored with a Brucker IFS-l 13V FTIR. We have found that the g4 band of SF6 and the 9 band of UFs overlapped seriously, but at 1157 cm-l, the p2 + p3 combination band of UFg , the absorbance of SF5 was negligible. Our experiments showed that the absorbance of UF, at 1157 cm-l obeyed the Beer-Lambert law in the O-30 Torr pressure range, and was suitable for monitoring the UFs dissociation in the presence of SFs_ UF6 and SF6 were purified using a number of freeze-pump-thaw cycles at 77 and 193 K, respec69
CHEMlCAL
\‘ofun~ lOI_ number 1
tively. After purification no observable were observed in the 1R spectra_
PHYSICS
7 October
LETTERS
1983
impurities
3. Results In a preliminary t’?rpcriment 3 mkture of IS Torr SF, and 15 Torr UF6 without scavenger was irradiated dt different laser frequencies. No dissociation of UFs R as observed after 500 pulses with a fl uence of 0.3 Jjrm~. The yields observed in ref_ [3] might result from rile higher fluence they used (I J/cm2). Then l-I, (30 Torr) wx added to the previous gas mhturr as -an F-atom scavenger. The sample was irradiated b> the pulsed CO, laser at 935 cm-l _It was found that the dissociation of UFs occurred and increased with increasing fiuence. as shown in fig. 1 _ The dissociation yield.f= @“,,,~~~=~~~~~~~~_was determined by mwsuringp~, 6 and pufz, . the partial pressure of UF, in the cell before and after irradiation. In rhr presence of 1t , _ UF6 dissociated obviously even at a Iluence of 0.3 J.k~. while no measurable SF,, dissoci.k~n was observed. However. as the fluence increased to 0.5 J/cm~. the SF6 also dissociated, This result was similar to that obtained in ref. [4] with a higher iluence, The laser frequency dependence of the dissociation of Ul:, was detcrmmed with fluences of O.-E4 and 0.18 J/cm for rhe SFh--UF,-I-i, system. It can be seen
00
in fig. 2. that there is no measurable dissociation when the frequencies are higher than 946 cm-l, the dissociation yields increase with decreasing laser frequency <946 cm-l, and reach a rnasinnm~ at 935 cm-l _ The dependence of the dissociation of UF6 upon the partial pressure of H, in the gas mixture is shown in fig 3. The dissociation yield increases rapidly with pIIz for pH2 < 10 Torr, and then remains constant in the IO-30 Torr region. The dissociation of UF, was also strongly depen-
(J/Cm’)
i‘y. 1. L.wx !lucnrc dependence of rhe SFh-sensitized decompositian of UF, .tt 935 cm - ’ _ 50 laser pulses for each d&la point. G&s mixrure: IS Torr SF, + 15 Torr UF, + 30 Torr Hz.
70
(cm-‘)
0 50
0.2 5
Fluence
Frequency
Fig. 2. Laser frequency dependence of rhe SF&-sensitized decomposition of UF6. Gas mixture: 18 Torr SF6 + 13 Torr UF, + 30 Torr Hz. (*) 0-11 J/cm’ 1% ith 100 pulses for each daxs point: (0) 0.1s J/cm’ Qith 100 pulses for each data poinz. The CO? Iaser Imcs in the (OO” !-lODO) transitions are J&O indicxred.
El. 3. UFs dissoctation yield versus Hz pressure for SF, and UF, fi\ed at 18 and 15 Torr respective&_ Laser fluenee is 0.18 J/cm’ at 935 cm-’ . 100 pulses for each data point.
CHl3bfICAL
Volume 101, nllmbeI 1
PHYSICS
LETTERS
1983
7 October
s 0
950
940
CO, laser
Frequency km-‘) Fig_ 4. The infrared absorption spectrum of gas mixture before (solid) and after (dotted) laser irradiation with 200 pulses. Gas mixture: 18 Torr SF6 + 15 Torr UF, -I- 30 Torr CO. Laser fluence: 0.44 J/cm*. Laser frequency: 935 cm-‘_ The appearance of new peaks (l), (2) and (3) after irradiation indicates the formation of COF2.
dent on the laser frequency with CO instead of H2 as a scavenger_ It was found that the dissociation yield of SF6 was O-2 at 935 cm-l for an 18 Torr SFe, 15 Torr UF6 and 30 Torr CO gas mixture, but no measurable dissociation was observed at 944 cm--l with fluences of 0.44 J/cm’ and 200 pulses. The IR spectrum of an SF6-UF, -CO gas mixture before and after irradiation is shown in fig. 4_ It can be seen that UFe decomposition was observed after 200 pulses at 935 cm-l by monitoring the absorbance of UF, at 1157 cm-l, and that the appearance of new bands at 774, 1250, and 1928 cm-1 indicated the formation of COF, [6].
4. Discussion The presence of an F-atom scavenger is essential the laser IR photosensitization reaction for the SF&JFg system. Either H2 or CO can be used as a scavenger, but CO is less efficient than Ha_ Most infrared-photosensitized reactions with a pulsed CO, laser were explained as a result of laser heating effects, but in our experiments there are sevin
940
930
a -5 0
frequencxtcm‘o
Fig. 5. The absorbance of SF6 (20 Torr) as a function of CO2 laser frequency (solid line) and the UF6 dissociation yield curve (dotted line), the same curve as shown in fig. 2.
eral arguments in favour of a non-thermal nature of the process. (1) We did measure incident and transmitted energy at the experimental pressure of SF6 (20 Torr), and obtained the SF6 absorbance dependence on laser frequency as shown in fig_ 5. It can be seen that the absorbance curve does not match the dissociation curve of UF6. The absorbance of SF6 at the laser frequencies of 1OP(30) and P( 18) are nearly the same, but the UF, dissociation yields are quite different. These results indicate that purely thermal pyrolysis might not be the most probable mechanism_ (2) If in our experiments a full V-T,R transfer takes place rapidly during the laser pulse, the temperature rise, AT, in the system can be calculated using the following equation [7],
AT= Qo%F,psFJc CpPt
,
where Q. is the average laser fluence, usF, is the average multiphoton excitation cross section of SF6 [S], cP is the average heat capacity of gas mixture and the tOkd pressure pt =pSFB +pm, +p& _ For the system with 18 Torr SF6,15 Torr UF, and 30 Torr
H,,
AT calculated
was 212”
with a laser
fluence of 0.44 J/cm? and the average temperature of the gas mixture was ~500 K, which was lower than the dissociation temptrature, 1200 K for UF6 and 1500 K for SF6 _ The behavior of energy absorption of SF6 under intense IR laser radiation has been studied with the 71
optoacoustic effect [9] _ The laser fsequency and fluence dependence of the optoacoustic signals showed that in addition to the single-quantum transition resonance of ntultiquantunt transitions to the overtone levels of SF6 tttight play an iutportattt role. SF6 molecules could be escited to ~3, 2v3 and 3~3 states as well. There are many possible paths for vibrationally excited SF6 to transfer energy to UF6 molecules_ The probability of V-V energ)l transfer ltas been discussed by Knudtson and Flymt [lo] _Suppose tlte SF64JF6 V-V ener9 transfer is similar to the SF,-SF6 V-V transfer. the general process can be given by SF6 tyi) + UF6 (~~1 3
SF6(v6) i- UF6&)
- 24
Clll-l, (1)
SF6(2vj)
+ UF6
Aq=5
I. SF6 + UF6(3v7)
-
CO, laser
SF; _
10.6 Lcm
SF; + UP6 + SF6 + UF; ,
UF; &UF5+F.
10.6 pm
F-t-H,dHF+H 222UFj --f (UF&
or
ZF+CO*COF,,
.
Acknowledgement We wish to thank Professor C.B. Moore for helpful discussions. Many thanks are due to Dr. M. Yu and Mr. Y.Y. Yang for assistance in the laser facilities and 1R spectra measurements.
11 cm-‘.
(3 SF6 (3~3) + UF6 -------F
sF6 .
However. the mechanism described above is preliminary. a more detailed study is in progress_
SF6(vI)+UF6(vn2)+4E.
where 4q is the number of quanta exchanged. The proba~ity of energy transfer decreases as LIE and & increase. According to this criterion the nearly resonant V-V ener,gy transfer paths between SF6 and UF6 might be SF6 (v3 I+ UF6 z
7 October 1983
CHEhiICAL PHYSiCS LE’JTERS
Volume $01, number i
SFjVg)+UF6(-fVj)-66cm-i.
When UF6 molecules are excited to the 3~3 Ievel. the
density of states of UF6(3~3) is 723 states/cm-’ and UF6 mofecules. which have been excited into its quasi-continuum, could absorb non-resonantly by CO, photons and dissociate_ Thus the main steps for the UF6-IR-photosensitized dissociation in the presence of H, or CO as scavenger cat be written as follows:
References 111
J.J. Steinfefd. Laser induced chemicaf.processes (Plenum Press, Xew York, 1981) pp_86,252. I;. Hill and G.A. Laguns, Opt. Commun_32 (1980) 435 R.S. Karve. S.K. Sarkar. K_V_S_Ranta Rae and J-P. Mittal, Chem. Phys. Letters 78 (1951) 273. C. Angelic, hf. Cauchetier and J_ Parris, Chem. Phys. 66 (1982) 129. hf. Cauchetier. M. Lute and C. _&r~eiie.Chem. Phys. Lerters 88 (1982) 146. W.A. Yeranos and J.D. Graham, Spectrochim. Acta 23A (1967) 73,.
N.G. Basov et al_, in: Chemical and biochemical applications of lasers, Vol. 1, ed. C-B. hloore (Academic Press, New York, 1974) p_ 216. J.L. Lyman, J. Chem.Phys. 67 (1977) 1868. T. Fuhwmi, Chem. Letters (1961) 1431.
ISI PI [lOI J-T. Rnudtson and G.W. Flynn, J. Chem. Phys. 56 (1973) 1467.
72
SF6SENSITZED
CHEMLCALPHYSICSLETTERS
7 October 1983
DISSOCIATION OF UF6 WITH A PULSED CO2 LASER
CHIN Chi-tsung, HOU Hui-qi, BAU Yi-han and LI Ting-hua of Nuclear Science. Fudan Univedy, Smnghai. People’s Repubifc of Cfiina
Deprrment
Received 15 March 1983; in foal form 26 May 1983
The dissociation of UFe sensitized by SFe excited with a pulsed CO* laser in the presence of Iis and Co as scavc~ge~s has been investigated. in the SF~--UF~-HZ system the dissociation yieIds have been determined as a function of the laser frezyncy, the fluence. and Hs partial pressure. A masimum dissociation yield has been found at a laser frequency of 935 cm _ No obvious dissociation of UFs \\asobserved in the UF6-SF6 system without F-atom scavengers.
I. Introduction The application of IR sensitizers to pulsed CO2 laser induced reactions has been studied for several
years Many of them appear to be typical high-temperature pyrolyses [l] _Hill et al. [2] used SF6 as an IR sensitizer to initiate the CH, I- 0, explosion reaction with a single CO2 laser pulse, and indicated that there was some non-thermal component in the combustion initiation. During the course of this work, two papers [3,4] demonstrated infrared photosensitized multiphoton dissociation of UFg by SF6 _ It is interesting that isotopic selectivity has been observed in the IR-sensitized dissociation of SF,Cl and CF, I by SF6 excited with a TEA CO, laser [S]_ We report here the SFg-sensitized dissociation of UFg with a pulsed CO, laser in the presence of Hz and CO as scavengers_ A detailed study of the dissociation of UFg as a function of laser frequency, fluence and H2 partial pressure has been performed.
2. Experimental A multimode TEA CO, laser was indigenously built. The pulse width was measured with a Rofm model 7415 photon drag detector, The main pulse had a 100 ns duration fwhm, followed by a long tail (1.5 ps duration) containing 50% of the total energy. The laser was tunable between 9 and 11 .um using a 0 009-2614/83/0000-0000/S
03.00 0 1983 North-Holland
150 lines/mm grating, and its frequency was calibrated with a CO, spectrum analyzer_ Tlte laser beam was directed through the sample cell, and focused 7 cm away from the cell by a gold-coated concave mirror cf= 35 cm)_ The beam spot was =1.6 cm2 at the entrance window_ The pulse energy was measured with a precalibrated calorimeter_ The average fluence used in our experiments was less than OS J/cmz, which was not high enough to cause dielectric breakdown of the gas samples_ The reaction cell (3-4 cm in diameter and IO cm long) was made of stainless steel and equipped with a pair of KC1 windows fued with viton “0” rings. An all-metal vacuum manifold and the cell were passivated by F2 for 4 days. After passivation the cell was evacuated to JO-3 Torr and f&ed with the g,as mixtures_ The pressure was measured with a membrane manometer_ The partial pressure of UF, in the cell was monitored with a Brucker IFS-l 13V FTIR. We have found that the g4 band of SF6 and the 9 band of UFs overlapped seriously, but at 1157 cm-l, the p2 + p3 combination band of UFg , the absorbance of SF5 was negligible. Our experiments showed that the absorbance of UF, at 1157 cm-l obeyed the Beer-Lambert law in the O-30 Torr pressure range, and was suitable for monitoring the UFs dissociation in the presence of SFs_ UF6 and SF6 were purified using a number of freeze-pump-thaw cycles at 77 and 193 K, respec69
CHEMlCAL
\‘ofun~ lOI_ number 1
tively. After purification no observable were observed in the 1R spectra_
PHYSICS
7 October
LETTERS
1983
impurities
3. Results In a preliminary t’?rpcriment 3 mkture of IS Torr SF, and 15 Torr UF6 without scavenger was irradiated dt different laser frequencies. No dissociation of UFs R as observed after 500 pulses with a fl uence of 0.3 Jjrm~. The yields observed in ref_ [3] might result from rile higher fluence they used (I J/cm2). Then l-I, (30 Torr) wx added to the previous gas mhturr as -an F-atom scavenger. The sample was irradiated b> the pulsed CO, laser at 935 cm-l _It was found that the dissociation of UFs occurred and increased with increasing fiuence. as shown in fig. 1 _ The dissociation yield.f= @“,,,~~~=~~~~~~~~_was determined by mwsuringp~, 6 and pufz, . the partial pressure of UF, in the cell before and after irradiation. In rhr presence of 1t , _ UF6 dissociated obviously even at a Iluence of 0.3 J.k~. while no measurable SF,, dissoci.k~n was observed. However. as the fluence increased to 0.5 J/cm~. the SF6 also dissociated, This result was similar to that obtained in ref. [4] with a higher iluence, The laser frequency dependence of the dissociation of Ul:, was detcrmmed with fluences of O.-E4 and 0.18 J/cm for rhe SFh--UF,-I-i, system. It can be seen
00
in fig. 2. that there is no measurable dissociation when the frequencies are higher than 946 cm-l, the dissociation yields increase with decreasing laser frequency <946 cm-l, and reach a rnasinnm~ at 935 cm-l _ The dependence of the dissociation of UF6 upon the partial pressure of H, in the gas mixture is shown in fig 3. The dissociation yield increases rapidly with pIIz for pH2 < 10 Torr, and then remains constant in the IO-30 Torr region. The dissociation of UF, was also strongly depen-
(J/Cm’)
i‘y. 1. L.wx !lucnrc dependence of rhe SFh-sensitized decompositian of UF, .tt 935 cm - ’ _ 50 laser pulses for each d&la point. G&s mixrure: IS Torr SF, + 15 Torr UF, + 30 Torr Hz.
70
(cm-‘)
0 50
0.2 5
Fluence
Frequency
Fig. 2. Laser frequency dependence of rhe SF&-sensitized decomposition of UF6. Gas mixture: 18 Torr SF6 + 13 Torr UF, + 30 Torr Hz. (*) 0-11 J/cm’ 1% ith 100 pulses for each daxs point: (0) 0.1s J/cm’ Qith 100 pulses for each data poinz. The CO? Iaser Imcs in the (OO” !-lODO) transitions are J&O indicxred.
El. 3. UFs dissoctation yield versus Hz pressure for SF, and UF, fi\ed at 18 and 15 Torr respective&_ Laser fluenee is 0.18 J/cm’ at 935 cm-’ . 100 pulses for each data point.
CHl3bfICAL
Volume 101, nllmbeI 1
PHYSICS
LETTERS
1983
7 October
s 0
950
940
CO, laser
Frequency km-‘) Fig_ 4. The infrared absorption spectrum of gas mixture before (solid) and after (dotted) laser irradiation with 200 pulses. Gas mixture: 18 Torr SF6 + 15 Torr UF, -I- 30 Torr CO. Laser fluence: 0.44 J/cm*. Laser frequency: 935 cm-‘_ The appearance of new peaks (l), (2) and (3) after irradiation indicates the formation of COF2.
dent on the laser frequency with CO instead of H2 as a scavenger_ It was found that the dissociation yield of SF6 was O-2 at 935 cm-l for an 18 Torr SFe, 15 Torr UF6 and 30 Torr CO gas mixture, but no measurable dissociation was observed at 944 cm--l with fluences of 0.44 J/cm’ and 200 pulses. The IR spectrum of an SF6-UF, -CO gas mixture before and after irradiation is shown in fig. 4_ It can be seen that UFe decomposition was observed after 200 pulses at 935 cm-l by monitoring the absorbance of UF, at 1157 cm-l, and that the appearance of new bands at 774, 1250, and 1928 cm-1 indicated the formation of COF, [6].
4. Discussion The presence of an F-atom scavenger is essential the laser IR photosensitization reaction for the SF&JFg system. Either H2 or CO can be used as a scavenger, but CO is less efficient than Ha_ Most infrared-photosensitized reactions with a pulsed CO, laser were explained as a result of laser heating effects, but in our experiments there are sevin
940
930
a -5 0
frequencxtcm‘o
Fig. 5. The absorbance of SF6 (20 Torr) as a function of CO2 laser frequency (solid line) and the UF6 dissociation yield curve (dotted line), the same curve as shown in fig. 2.
eral arguments in favour of a non-thermal nature of the process. (1) We did measure incident and transmitted energy at the experimental pressure of SF6 (20 Torr), and obtained the SF6 absorbance dependence on laser frequency as shown in fig_ 5. It can be seen that the absorbance curve does not match the dissociation curve of UF6. The absorbance of SF6 at the laser frequencies of 1OP(30) and P( 18) are nearly the same, but the UF, dissociation yields are quite different. These results indicate that purely thermal pyrolysis might not be the most probable mechanism_ (2) If in our experiments a full V-T,R transfer takes place rapidly during the laser pulse, the temperature rise, AT, in the system can be calculated using the following equation [7],
AT= Qo%F,psFJc CpPt
,
where Q. is the average laser fluence, usF, is the average multiphoton excitation cross section of SF6 [S], cP is the average heat capacity of gas mixture and the tOkd pressure pt =pSFB +pm, +p& _ For the system with 18 Torr SF6,15 Torr UF, and 30 Torr
H,,
AT calculated
was 212”
with a laser
fluence of 0.44 J/cm? and the average temperature of the gas mixture was ~500 K, which was lower than the dissociation temptrature, 1200 K for UF6 and 1500 K for SF6 _ The behavior of energy absorption of SF6 under intense IR laser radiation has been studied with the 71
optoacoustic effect [9] _ The laser fsequency and fluence dependence of the optoacoustic signals showed that in addition to the single-quantum transition resonance of ntultiquantunt transitions to the overtone levels of SF6 tttight play an iutportattt role. SF6 molecules could be escited to ~3, 2v3 and 3~3 states as well. There are many possible paths for vibrationally excited SF6 to transfer energy to UF6 molecules_ The probability of V-V energ)l transfer ltas been discussed by Knudtson and Flymt [lo] _Suppose tlte SF64JF6 V-V ener9 transfer is similar to the SF,-SF6 V-V transfer. the general process can be given by SF6 tyi) + UF6 (~~1 3
SF6(v6) i- UF6&)
- 24
Clll-l, (1)
SF6(2vj)
+ UF6
Aq=5
I. SF6 + UF6(3v7)
-
CO, laser
SF; _
10.6 Lcm
SF; + UP6 + SF6 + UF; ,
UF; &UF5+F.
10.6 pm
F-t-H,dHF+H 222UFj --f (UF&
or
ZF+CO*COF,,
.
Acknowledgement We wish to thank Professor C.B. Moore for helpful discussions. Many thanks are due to Dr. M. Yu and Mr. Y.Y. Yang for assistance in the laser facilities and 1R spectra measurements.
11 cm-‘.
(3 SF6 (3~3) + UF6 -------F
sF6 .
However. the mechanism described above is preliminary. a more detailed study is in progress_
SF6(vI)+UF6(vn2)+4E.
where 4q is the number of quanta exchanged. The proba~ity of energy transfer decreases as LIE and & increase. According to this criterion the nearly resonant V-V ener,gy transfer paths between SF6 and UF6 might be SF6 (v3 I+ UF6 z
7 October 1983
CHEhiICAL PHYSiCS LE’JTERS
Volume $01, number i
SFjVg)+UF6(-fVj)-66cm-i.
When UF6 molecules are excited to the 3~3 Ievel. the
density of states of UF6(3~3) is 723 states/cm-’ and UF6 mofecules. which have been excited into its quasi-continuum, could absorb non-resonantly by CO, photons and dissociate_ Thus the main steps for the UF6-IR-photosensitized dissociation in the presence of H, or CO as scavenger cat be written as follows:
References 111
J.J. Steinfefd. Laser induced chemicaf.processes (Plenum Press, Xew York, 1981) pp_86,252. I;. Hill and G.A. Laguns, Opt. Commun_32 (1980) 435 R.S. Karve. S.K. Sarkar. K_V_S_Ranta Rae and J-P. Mittal, Chem. Phys. Letters 78 (1951) 273. C. Angelic, hf. Cauchetier and J_ Parris, Chem. Phys. 66 (1982) 129. hf. Cauchetier. M. Lute and C. _&r~eiie.Chem. Phys. Lerters 88 (1982) 146. W.A. Yeranos and J.D. Graham, Spectrochim. Acta 23A (1967) 73,.
N.G. Basov et al_, in: Chemical and biochemical applications of lasers, Vol. 1, ed. C-B. hloore (Academic Press, New York, 1974) p_ 216. J.L. Lyman, J. Chem.Phys. 67 (1977) 1868. T. Fuhwmi, Chem. Letters (1961) 1431.
ISI PI [lOI J-T. Rnudtson and G.W. Flynn, J. Chem. Phys. 56 (1973) 1467.
72