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Chemical efficiency in a pulsed HF laser
Volume
16, number
15 September 1972
CHEMICAL PHYSICS LETTERS
1
CHEMICAL
EFFICIENCY
IN A PULSED HF LASER+
W.H. BEATTIE, G.P. ARNOLD and R.G. WENZEL Los dlanzos Sciattific Laborarory, Los .4lamos, New Afexico 8 7544. USA Received 9 May 19 72
Quantitative analysis for HF production in the product gases of an ekctrically initiated H?:SF6 laser has been by aqueous titration. hfeasuremcnts were taken at an HF laser output of approximately one joule per pulse. Gas chromatographic analysis has revealed the presence of SFJ. A likely mechanism to describe the decomposition of SF6 in an electric discharge, and mechanisms to account for Hf: production in the lase: and titrimeter ze given. Based upon totai f3F production in the laser, 7.2% of the chcmicof energy avaiilable from the reaction F + ii2 + HF* + H appears as laser right autput. As the atomic ratio of F to H in the SFe + Hz mix is increased from 3 to 65, the chemical efficiency is increased and the HF production is decreased, suggesting that col!isionnl deactivation of HF* by Hz limits the laser efficiency. pxformcd
interest in the study of the chemical kinetics involved. Also of interest is the possibility of designing HF lasers with efficiencies appreciabbly greater than are presentiy available. As multijoule pulsed HF lasers are
1. Introduction
A rapid elektrical discharge in a mixture of SF6 and I-I, has frequently been used to provide free fluorine for the reaction,
developed, higher efficiency becomes more important. The HF production and laser pulse energy have
This is the usud technique to obtain the Iasing medium for pulsed HF chemical lasers. Reaction (1) is exothermic by 3 I.9 kcal/moIe. The reaction, SF,+SF,
(2)
+F,
is endothermic by 78.3 kcai/moIe, and the reaction, SF6 + SF, + 2F is endothermic by 142.9 k&/mole [If. Further decompcsition of SF, requires a greater energy input per F oblained. The chemical efficiency (the efficiency of conversion of chemical energy to Iaser Light) derived from reaction (1) will be shown to be < 48%. Thus, the maximurn possible electrical efficiency for a !aser operated with this technique is 0.48 X 63.8i142.9, or 2i%. Experimentally, eLectrical efficiencies in excess of 2% are obtained with some difficulty. The difference between the theoretical and practical efficiency is of 7 Work pcrforked~under ‘Energy.Commission. ,.’
.;I64_:. .__ ..;. ;:,y ,’ -. .. .‘. ,,, .:
,, ‘.“_ . . .; ,I.’
the auspices of the W?J. Atomic .’ ., ... ..,
.’
,, .,
,.. : ‘.
been measured for an L-IFlaser in order to determine the fraction of HF contributing to the lasing process. Measurements
have been performed
at several ratios
of atomic F to H to determine the effect of the mixture on the chemical efficiency. Determination of small quantities of HF in gases is hampered by its high adsorbability and reactivity. Representative sampling was obtained by flowing the effluent stream from a regufarly pulsed laser into a titration vessel.
2. Experimental The double discharge laser used in these measurements is similar to that previously described 121, with several modifications to improve efficiency, the most important being replacement of the original KRS-5 output coupler with an uncoated T&Cl flat and use of a capacitor bank with lower inductance. The laser tube, made of acrylic plastic, is about
Volume 16, number 1
CHEMICALPHYSICS LETTERS
15 Septcmbcr
1972
with inlet and outlet connections near the calcium fluoride end windows at the Brewster angle. The active volume between the brass electrodes is about 250 cm3. The optical cavity consists of a 30 m gold-coated concave reflector one meter from the NaCl output coupler. A two-stage Marx bank is used for electrical initiation of the flowing gas mixture, The bank is made up of eight 0.01 PF capacitors which can supply up to 100 J pulses of 400 nsec duration at a discharge voltage of 100 kV. In the present work, at 90 kV with an SF6:H1 mixture, output pulses 2s high as 1.3 J with a total duration of 300 nsec have been obtained. The quantity of HF produced by the laser was determined titrimetrically by bubbling the effluent gas through 2 column containing standardized NaOH and phenolphthalein indicator. The titrimeter consists of a borosilicate glass column coated on the inside with polyethylene as shown in fig. 1. In order to avoid contact of the gas with metal or glass, Teflon inlet
5 cm depth was used to break up the gas bubbles which produced rapid miving of the solution. Preliminary titrations in a fully packed column showed that I-IF is absorbed rapidly at the bottom of the co]umn. The laser was fired with 2 tot21 flow rate of ==3 l/min and total pressure of 120 torr. Decreasing the flow rate by a factor of two did not affect results. The ratio of SF6 to H2 was varied over a range of F/H from 3 to 65. In a titration the laser ~2s fired at one minute intervals. Possible errors due to adsorption of HF on rhe walls of the laser were avoided by allowing the gas to bypass the titrimeter during three preliminary shots. (it w2s determined that one to two shots were sufficient to sttain ar!sorption equilibrium. Additional preliminary shots did not affect the end point.) The titrimeter was filled with 3 to 10 ml of 0.1 hl NaOH, phenolphthalein indicator, and 150 to 250 ml of distilled water. Immediately following the preliminary shots, the gas was allowed to flow through the titrimeter for an integral number of minutes, stopping after a color change was observed. The number of shots required varied from 5 to 13. The excess HF ~2s titrated with additional 0.1 M NaOH to the end point. Interference by the acids of sulphur was considered. A slight odor of H,S was occasionally detected, but H,S is too weak an acid to interfere at the phenolphthalein end point. A small amount of !-IzSO3 was determined by boiling an acidified sample to remove any H,S, osidation to H2S04 with Br2, and determination by routine sulphate analysis. A powdery deposit appearing on the exhaust-end window after several laser shots is assumed to have been S as it ~2s soluble in CSz. Cleaning the window did not significantly change the laser output. A sample was obtained for gas chromatographic analysis by filling the laser tube with a static charge of the SF6:H2 mix at an F/H of 15. The !aser was fired 10 times. Lasing action ~2s observed for all shots. The energy of the first pulse was normal. It decreased an average of 6% per pulse thereafter due to the buildup of HF in the tube. The contents of the laser tube were then forced into a 300 ml stainless steel cylinder with He at a pressure of 600 torr. A 2.4 m column packed with Poropak Q and operated at 25°C with a
tubes and valves are used. Teflon chip packing
He flow
Fig, 1. Titration vessel 3.7 cm diameter by 35 cm long showing (1) bypass valve, (2) pressure equalization valve, (3) wter, (4) Teflon chips 70 cm long
to B
rate of47
ml/min
was used. Quantitative
de-
Volume 16, number 1
CHEMICAL PHYSICS LETTERS
15 September 1972
Table !
Summary of runs Run no. 1
2 3 4 5 6 7 8 9 IO
Gas
Input
composition P/H
energy U)
HF production (mmoles/J input)
Output(J)/ mmoles HF
Electrical efficiency (%)
81 81 81 81 81 81 81 49 40 a) 80 a)
2.6 2.1 1.7 1.7 1.6 1.5 1.3 1.8 0.65 0.49
2.7 5.8 7.8 7.6 7.8 8.4 9.6 6.3 8.4 5.2
0.70 1.2 1.3 1.3 1.3 1.3 1.1 1.1 0.55 0.26
8
8 1.5 15 18 34 65 15 35 35 -~-~-
a) In runs 9 and 10 a capacitor bank with higher inductance was used.
.010 -2.5
-1.0 2
Fig. 2. Variation of electrical efficiency wit!! ratio of F to H. terminations of Hz, SF6, and SF4 were made. No other major components were found, and H$ was not detected.
3. ResuIts The resuhs of a series of titrations are summarized in table 1. The F,‘H ratio was obtained from the pressure ratio of SF6 to H,. The variation of electrical efficiency with gas composition, shown in fig. 2, indicates that maximum output is 0btaine.l when F/H is 15. 166
,002 34
600
20
3040
6060
F/H
Fig. 3. Variation of HF production (left) and laser output per unit HF (right) with ratio of F to H.
Fig. 3 shows that the H F production is increased with increasing fraction of H,. On the other hand, the laser output per unit HF produced is decreased with increasing fraction of Hz. The latter trend suggests that collisional deactivation of HF* by H, may occur. These combined effects cause the e!ectrical efficiency to decrease at either high or low values of F/H. Comparison of run 6 with 9 and 10 in table 1 indicates that the use of a capacitor bank with higher inductance than the one described here decreased the
Volume 16, number 1
Parameters
CXEMICAL Table 2 of an nvcrage shot
lnput power Output power HF in titrimeter H2S03 SF4
Electrical efficiency
Chemical efficiency
81 J 1.07 J 0.126 mmole 0.012 mmole 0.0062 mmole 1.3%
7.2%
HF production. This is probably due to arc formation in the discharge. As mentioned in ref. [3], the double discharge frequently forms arcs in SF,:H, mixtures. Greater inductance results in longer discharge times which in turn result in arcs of greater ener,y. High energy arcs are inefficient in the decomposition of SF,. They may also influence the output per unit HF. The characteristics of an average shot with F/H at 15 are given in table 2. The HF in the titrimeter was obtained by subtracting the number of moles of H,SOs from the total acid determined titrimetrically (since only the first ionization of HaSO, occurs at the phenolphthalein
end point).
action (1) with the release of 14.8 J. The resulting chemical efficiency is 7.3%. Of the HF produced in the laser, 0.0124 mmoles must result from decomposition of 0.0062 mmoIes of SF, intc SF,. The remaining 0.102 mmoles of HF apparently result from total decomposition of l/6 as many moles of SF6 . Thus 0.023 mmoles of SF6 are decomposed per shot, both by forming SF, and by total decomposition. This compares well with the 0.0 i8 mmoles of S which appear as SF4 and H2S03. The production of elemental S presumably accounts for the difference between these numbers. Only F atoms produced during the electrical discharge appear early enough to t&e part in the lasing resulting from reaction (!). Decomposition of intermediates subsequent to the electrical discharge will produce HF molecules too slowly to maintain the
population Inversion. The mechanism of decomposition of SF, is not known, but mass spectrometric analysis has shown that an electric discharge breaks it into predominantly ions and radicals
of SF5 and F [4,5]. Edelson [6] detected S2 F2 and SF2 after electrical discharges through pure SF, _He proposed that SF, and other large frag-
Since
The chemical efficiency is proportional to the laser output per mmole HF in table 1. Comparison of runs 3 and 4 with 8 indicates that decreasing the input energy (by decreasing the capacitor bank voltage) caused the chemical efficiency to decrease, but had no effect upon the HF production. A possible explanation for the dependence of chemical efficiency upon the discharge energy is electrical pumping of the HF formed. Alternatively, reduced discharge voltage may result
in the production of F-, with i-L,. Investigations
1972
ments may combine to form excited S2F10, which in turn decomposes into S2F2 and F, and finally SF,.
4. Discussion
rapidly
15 September
PHYSICS LETTERS
which does not react
of these and other
possible mechanisms are needed. The SF, produced in reaction (2) may decompose into SF,, lower fluorides of S, and S. The HF determined titrietricahy (table 2) is the sum of the HF produced in the laser and the HF produced in the titr-imeter by the reaction [3], SF, + H20 + SOF, f 2HF. Thus, 0.0062 mmoles of SF, releases 0.0124 mmoles of HF in the titrirneter. The remaining 0.114 mmoles of HF are produced in the laser by re-
these
species
were
not
found
in this: work
with
H2 present, it appears that they react rapidly with Hz. If it is assumed that the initial step in the decomposition of SF6 produces one atom of F per SF,, the chemical position
energy released of 0.023 mmole
in reaction (1) after decomof SF6 is 3.07 J. If it is
further assumed that the electrical pumping of HF is negligible, the chemical efficierrcy, based on the HF production capable of lasing, is found to be 1.07/3.07, or 3570. The 3 1.9 kcal/mole produced by reaction (1) is sufficiently exothermic to excite HF to predominantly vibrational level 3. The population ceases to be inverted when there are approximately equal populations in levels 0, 1, 2, and 3, at an average energy of 16.5 kcal/mole. Therefore, the theoretical maximum chemical efficiency is (3 1.9 - 16.5)/3 1.9 or 48%. The difference between the theoretical maximum efficiency and the experimental value based on the HF producticn capable of lasing is probably due to collisional deactivation of HF* and light losses in the laser optical cavity.
167
VoIurne 16, number 1
CHEMICAL PHYSICS LETTERS
References [l] D.R. Stull and H. Prophet, JANAF Thermochemical Tables, NSRDS-NBS 37, 2nd Ed. (U.S. Dept. Commerce, 1971); R.L. Wilkins,
Aerospace
Carp.
Rept. No. TR-015s
i3240-20)-19 (1968). [2] R.G. Wenzel and G.P. Arnold, J. Quantum QE-8 (1972) 27.
168
Electron.
1.S September
1972
131 G.H. Cady, Advan. Inorg. Chem. Radiochem. 2 (1960) 107. [41 V.H. Dibeler and F.L. Mohler, J. Res. Natl. Bur. Std. 40 (1948) 2.5. [51 R.E. FOX and RX. Cuaran, J. Chem. Phys. 34 (1961) 1595. [61 D. Ed&on, CA. Bieling and G.T. Kohnan, Ind. Eng. Chem. 4.5 (1953) 2094.
16, number
15 September 1972
CHEMICAL PHYSICS LETTERS
1
CHEMICAL
EFFICIENCY
IN A PULSED HF LASER+
W.H. BEATTIE, G.P. ARNOLD and R.G. WENZEL Los dlanzos Sciattific Laborarory, Los .4lamos, New Afexico 8 7544. USA Received 9 May 19 72
Quantitative analysis for HF production in the product gases of an ekctrically initiated H?:SF6 laser has been by aqueous titration. hfeasuremcnts were taken at an HF laser output of approximately one joule per pulse. Gas chromatographic analysis has revealed the presence of SFJ. A likely mechanism to describe the decomposition of SF6 in an electric discharge, and mechanisms to account for Hf: production in the lase: and titrimeter ze given. Based upon totai f3F production in the laser, 7.2% of the chcmicof energy avaiilable from the reaction F + ii2 + HF* + H appears as laser right autput. As the atomic ratio of F to H in the SFe + Hz mix is increased from 3 to 65, the chemical efficiency is increased and the HF production is decreased, suggesting that col!isionnl deactivation of HF* by Hz limits the laser efficiency. pxformcd
interest in the study of the chemical kinetics involved. Also of interest is the possibility of designing HF lasers with efficiencies appreciabbly greater than are presentiy available. As multijoule pulsed HF lasers are
1. Introduction
A rapid elektrical discharge in a mixture of SF6 and I-I, has frequently been used to provide free fluorine for the reaction,
developed, higher efficiency becomes more important. The HF production and laser pulse energy have
This is the usud technique to obtain the Iasing medium for pulsed HF chemical lasers. Reaction (1) is exothermic by 3 I.9 kcal/moIe. The reaction, SF,+SF,
(2)
+F,
is endothermic by 78.3 kcai/moIe, and the reaction, SF6 + SF, + 2F is endothermic by 142.9 k&/mole [If. Further decompcsition of SF, requires a greater energy input per F oblained. The chemical efficiency (the efficiency of conversion of chemical energy to Iaser Light) derived from reaction (1) will be shown to be < 48%. Thus, the maximurn possible electrical efficiency for a !aser operated with this technique is 0.48 X 63.8i142.9, or 2i%. Experimentally, eLectrical efficiencies in excess of 2% are obtained with some difficulty. The difference between the theoretical and practical efficiency is of 7 Work pcrforked~under ‘Energy.Commission. ,.’
.;I64_:. .__ ..;. ;:,y ,’ -. .. .‘. ,,, .:
,, ‘.“_ . . .; ,I.’
the auspices of the W?J. Atomic .’ ., ... ..,
.’
,, .,
,.. : ‘.
been measured for an L-IFlaser in order to determine the fraction of HF contributing to the lasing process. Measurements
have been performed
at several ratios
of atomic F to H to determine the effect of the mixture on the chemical efficiency. Determination of small quantities of HF in gases is hampered by its high adsorbability and reactivity. Representative sampling was obtained by flowing the effluent stream from a regufarly pulsed laser into a titration vessel.
2. Experimental The double discharge laser used in these measurements is similar to that previously described 121, with several modifications to improve efficiency, the most important being replacement of the original KRS-5 output coupler with an uncoated T&Cl flat and use of a capacitor bank with lower inductance. The laser tube, made of acrylic plastic, is about
Volume 16, number 1
CHEMICALPHYSICS LETTERS
15 Septcmbcr
1972
with inlet and outlet connections near the calcium fluoride end windows at the Brewster angle. The active volume between the brass electrodes is about 250 cm3. The optical cavity consists of a 30 m gold-coated concave reflector one meter from the NaCl output coupler. A two-stage Marx bank is used for electrical initiation of the flowing gas mixture, The bank is made up of eight 0.01 PF capacitors which can supply up to 100 J pulses of 400 nsec duration at a discharge voltage of 100 kV. In the present work, at 90 kV with an SF6:H1 mixture, output pulses 2s high as 1.3 J with a total duration of 300 nsec have been obtained. The quantity of HF produced by the laser was determined titrimetrically by bubbling the effluent gas through 2 column containing standardized NaOH and phenolphthalein indicator. The titrimeter consists of a borosilicate glass column coated on the inside with polyethylene as shown in fig. 1. In order to avoid contact of the gas with metal or glass, Teflon inlet
5 cm depth was used to break up the gas bubbles which produced rapid miving of the solution. Preliminary titrations in a fully packed column showed that I-IF is absorbed rapidly at the bottom of the co]umn. The laser was fired with 2 tot21 flow rate of ==3 l/min and total pressure of 120 torr. Decreasing the flow rate by a factor of two did not affect results. The ratio of SF6 to H2 was varied over a range of F/H from 3 to 65. In a titration the laser ~2s fired at one minute intervals. Possible errors due to adsorption of HF on rhe walls of the laser were avoided by allowing the gas to bypass the titrimeter during three preliminary shots. (it w2s determined that one to two shots were sufficient to sttain ar!sorption equilibrium. Additional preliminary shots did not affect the end point.) The titrimeter was filled with 3 to 10 ml of 0.1 hl NaOH, phenolphthalein indicator, and 150 to 250 ml of distilled water. Immediately following the preliminary shots, the gas was allowed to flow through the titrimeter for an integral number of minutes, stopping after a color change was observed. The number of shots required varied from 5 to 13. The excess HF ~2s titrated with additional 0.1 M NaOH to the end point. Interference by the acids of sulphur was considered. A slight odor of H,S was occasionally detected, but H,S is too weak an acid to interfere at the phenolphthalein end point. A small amount of !-IzSO3 was determined by boiling an acidified sample to remove any H,S, osidation to H2S04 with Br2, and determination by routine sulphate analysis. A powdery deposit appearing on the exhaust-end window after several laser shots is assumed to have been S as it ~2s soluble in CSz. Cleaning the window did not significantly change the laser output. A sample was obtained for gas chromatographic analysis by filling the laser tube with a static charge of the SF6:H2 mix at an F/H of 15. The !aser was fired 10 times. Lasing action ~2s observed for all shots. The energy of the first pulse was normal. It decreased an average of 6% per pulse thereafter due to the buildup of HF in the tube. The contents of the laser tube were then forced into a 300 ml stainless steel cylinder with He at a pressure of 600 torr. A 2.4 m column packed with Poropak Q and operated at 25°C with a
tubes and valves are used. Teflon chip packing
He flow
Fig, 1. Titration vessel 3.7 cm diameter by 35 cm long showing (1) bypass valve, (2) pressure equalization valve, (3) wter, (4) Teflon chips 70 cm long
to B
rate of47
ml/min
was used. Quantitative
de-
Volume 16, number 1
CHEMICAL PHYSICS LETTERS
15 September 1972
Table !
Summary of runs Run no. 1
2 3 4 5 6 7 8 9 IO
Gas
Input
composition P/H
energy U)
HF production (mmoles/J input)
Output(J)/ mmoles HF
Electrical efficiency (%)
81 81 81 81 81 81 81 49 40 a) 80 a)
2.6 2.1 1.7 1.7 1.6 1.5 1.3 1.8 0.65 0.49
2.7 5.8 7.8 7.6 7.8 8.4 9.6 6.3 8.4 5.2
0.70 1.2 1.3 1.3 1.3 1.3 1.1 1.1 0.55 0.26
8
8 1.5 15 18 34 65 15 35 35 -~-~-
a) In runs 9 and 10 a capacitor bank with higher inductance was used.
.010 -2.5
-1.0 2
Fig. 2. Variation of electrical efficiency wit!! ratio of F to H. terminations of Hz, SF6, and SF4 were made. No other major components were found, and H$ was not detected.
3. ResuIts The resuhs of a series of titrations are summarized in table 1. The F,‘H ratio was obtained from the pressure ratio of SF6 to H,. The variation of electrical efficiency with gas composition, shown in fig. 2, indicates that maximum output is 0btaine.l when F/H is 15. 166
,002 34
600
20
3040
6060
F/H
Fig. 3. Variation of HF production (left) and laser output per unit HF (right) with ratio of F to H.
Fig. 3 shows that the H F production is increased with increasing fraction of H,. On the other hand, the laser output per unit HF produced is decreased with increasing fraction of Hz. The latter trend suggests that collisional deactivation of HF* by H, may occur. These combined effects cause the e!ectrical efficiency to decrease at either high or low values of F/H. Comparison of run 6 with 9 and 10 in table 1 indicates that the use of a capacitor bank with higher inductance than the one described here decreased the
Volume 16, number 1
Parameters
CXEMICAL Table 2 of an nvcrage shot
lnput power Output power HF in titrimeter H2S03 SF4
Electrical efficiency
Chemical efficiency
81 J 1.07 J 0.126 mmole 0.012 mmole 0.0062 mmole 1.3%
7.2%
HF production. This is probably due to arc formation in the discharge. As mentioned in ref. [3], the double discharge frequently forms arcs in SF,:H, mixtures. Greater inductance results in longer discharge times which in turn result in arcs of greater ener,y. High energy arcs are inefficient in the decomposition of SF,. They may also influence the output per unit HF. The characteristics of an average shot with F/H at 15 are given in table 2. The HF in the titrimeter was obtained by subtracting the number of moles of H,SOs from the total acid determined titrimetrically (since only the first ionization of HaSO, occurs at the phenolphthalein
end point).
action (1) with the release of 14.8 J. The resulting chemical efficiency is 7.3%. Of the HF produced in the laser, 0.0124 mmoles must result from decomposition of 0.0062 mmoIes of SF, intc SF,. The remaining 0.102 mmoles of HF apparently result from total decomposition of l/6 as many moles of SF6 . Thus 0.023 mmoles of SF6 are decomposed per shot, both by forming SF, and by total decomposition. This compares well with the 0.0 i8 mmoles of S which appear as SF4 and H2S03. The production of elemental S presumably accounts for the difference between these numbers. Only F atoms produced during the electrical discharge appear early enough to t&e part in the lasing resulting from reaction (!). Decomposition of intermediates subsequent to the electrical discharge will produce HF molecules too slowly to maintain the
population Inversion. The mechanism of decomposition of SF, is not known, but mass spectrometric analysis has shown that an electric discharge breaks it into predominantly ions and radicals
of SF5 and F [4,5]. Edelson [6] detected S2 F2 and SF2 after electrical discharges through pure SF, _He proposed that SF, and other large frag-
Since
The chemical efficiency is proportional to the laser output per mmole HF in table 1. Comparison of runs 3 and 4 with 8 indicates that decreasing the input energy (by decreasing the capacitor bank voltage) caused the chemical efficiency to decrease, but had no effect upon the HF production. A possible explanation for the dependence of chemical efficiency upon the discharge energy is electrical pumping of the HF formed. Alternatively, reduced discharge voltage may result
in the production of F-, with i-L,. Investigations
1972
ments may combine to form excited S2F10, which in turn decomposes into S2F2 and F, and finally SF,.
4. Discussion
rapidly
15 September
PHYSICS LETTERS
which does not react
of these and other
possible mechanisms are needed. The SF, produced in reaction (2) may decompose into SF,, lower fluorides of S, and S. The HF determined titrietricahy (table 2) is the sum of the HF produced in the laser and the HF produced in the titr-imeter by the reaction [3], SF, + H20 + SOF, f 2HF. Thus, 0.0062 mmoles of SF, releases 0.0124 mmoles of HF in the titrirneter. The remaining 0.114 mmoles of HF are produced in the laser by re-
these
species
were
not
found
in this: work
with
H2 present, it appears that they react rapidly with Hz. If it is assumed that the initial step in the decomposition of SF6 produces one atom of F per SF,, the chemical position
energy released of 0.023 mmole
in reaction (1) after decomof SF6 is 3.07 J. If it is
further assumed that the electrical pumping of HF is negligible, the chemical efficierrcy, based on the HF production capable of lasing, is found to be 1.07/3.07, or 3570. The 3 1.9 kcal/mole produced by reaction (1) is sufficiently exothermic to excite HF to predominantly vibrational level 3. The population ceases to be inverted when there are approximately equal populations in levels 0, 1, 2, and 3, at an average energy of 16.5 kcal/mole. Therefore, the theoretical maximum chemical efficiency is (3 1.9 - 16.5)/3 1.9 or 48%. The difference between the theoretical maximum efficiency and the experimental value based on the HF producticn capable of lasing is probably due to collisional deactivation of HF* and light losses in the laser optical cavity.
167
VoIurne 16, number 1
CHEMICAL PHYSICS LETTERS
References [l] D.R. Stull and H. Prophet, JANAF Thermochemical Tables, NSRDS-NBS 37, 2nd Ed. (U.S. Dept. Commerce, 1971); R.L. Wilkins,
Aerospace
Carp.
Rept. No. TR-015s
i3240-20)-19 (1968). [2] R.G. Wenzel and G.P. Arnold, J. Quantum QE-8 (1972) 27.
168
Electron.
1.S September
1972
131 G.H. Cady, Advan. Inorg. Chem. Radiochem. 2 (1960) 107. [41 V.H. Dibeler and F.L. Mohler, J. Res. Natl. Bur. Std. 40 (1948) 2.5. [51 R.E. FOX and RX. Cuaran, J. Chem. Phys. 34 (1961) 1595. [61 D. Ed&on, CA. Bieling and G.T. Kohnan, Ind. Eng. Chem. 4.5 (1953) 2094.