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New hydrogen fluoride chemical lasers
Volume
3, number
CREMJCAL
4
NEW
HYDROGEN
PHYSICS
FLUORIDE
LETTERS
April
CHEMICAL
1969
LASERS
-Karl L. KOMPA, Peter GENSEL and Jochen WANNER Institut fir Anorganisc.‘l-* Chemie der UniversitiG 1Miinchen. Germany Received
13 March
1969
Hydrogen fluoride vibration-rotation laser emission has been observed in the flash photolysis of xenon tetrafluoride and antimony pentafluoride mixed with hydrogen or methane. The observed emission lines, gain estimates, and information on the chemical excitation scheme are reported.
Vibration-rotation laser emission of hydrogen and deuterium fluoride was first observed in the flash photolysis of uranium hexafluoride/hydrogen (deuterium) mixtures by Kompa and Pimentel [l] and of uranium hexafluoride/hydrocarbon mixtures by Parker and Pimentel[2]. In the course of this work it was noted that antimony pentafluoride can be used as a photolytic fluorine atom source, too [I]. We have found that xenon tetrafluoride photolysis can initiate the formation of another strong HF chemical laser, and we wish to report here some new results for the systems xenon tetrafluoride with hydrogen or methane and antimony pentafluoride with hydrog=The experimental set-up was different from the -ones used resolution
in previous
work.
and the possibility
For
better
of vacuum
time
chemical
210
laser
chemicallaseremissionfrom
SbF5/JJ
emission
(arbitrary
intensity scale).
UV
photolysis the flash photolysis design of Welge, Wanner, Stuhl and Heindrichs [3] was adapted utilizing b nitrogen spark gaps (each one connected to a 1 pFd/2C) kV r:~pX discharge capacitor) instead of a xenon flash tube. The spark .gaps were arranged in two rows along two sides of the Ieser tube in a vacuum-tight cylindrical Iacite discharge vessel which was coated with Eastman white reflectance standard. At a nitrogen pressure of 300 - 400 mm and with a total discharge energy of up to 1600 Joules the unit produced flashes of 1.2 psec half-peak duration (fig. 1) *. The Suprasil laser tube had an illuminated length of 30 cm, an inner diameter of 12 mm, and was equipped with Brewster’s zmgle sapphire windows. The laser cavity was formed by two 10 m radius gold-coated mirrors spaced 80 cm. About 3% of the energy (per traversal)
* Details of the apparatus
Fig. 1. HF
photolysis, Upper trace fI ash signal, lower trace tser
and performance data for studies will be published elsewhere.
was coupled out of the cavity by a sodium chloride disk. Detector and monochromator arrangement were as described before [1,2]. The monochromator was a O.Sm-grating instrument (McPherson model 218). We used a PEM InSb-detector for the reference signal and a Ge:Au photoconductive detector to record the single-line emission which passed the monochromator. A foto field effect transistor recorded the flash profile. Xenon tetraJ7uori&/hydyogen (methane). When mixtures of 2.5 mm XeF4 and 3-7 mm H2 or CH4 were exposed to the light of a 1000 Joules flash the HF emission lines listed in table 1 are observed. The emission occurs in the P branch. No emission was detected on corresponding R-branch transitions. Operation of the laser was only possible in a carefully cleaned (rinsing with lfi hydrofluoric acid for 5 min) vacuum line and laser tube and after “conditioning”
Table 1 Stimulated emission lines of hydrogen fluoride Line Obsefped SbF5/HS XeF4/HS XeF4/CH4 (cm ) 3504.2 3709.4
P3+3(3)* P2-+1(2)
-
Literature (cm-l)
-3 .+(3)
(3)
(3)
3666.4 [Xl
3623.2
(4)
(4)
(4)
3622.6 [S]
3578.1
(5)
(5)
(5)
3577.5 [8]
(6)
(6)
3531.2 [S]
3484.2
(7)
(7)
3483.7 [S]
3435.6
(8)
(8)
Pa-l@)
3531.7
q-+0(3)
3788.8
(4)
3435.1 [S] 3834.0 [9] 3788.5 [S]
* very weak. by repeated exposure to XeF4. With these experimental conditions the laser emission appears 0.2 psec after the beginning of the flash having a pulse duration of about 1 wsec. Even when one of the cavity mirrors was taken off strong laser signals could still be detected which points to a very high gain of this laser. From the same XeFd/CHd-filling the HF laser could be operated up to 8 times while only one emission pulse was observable with a XeFq/H2 mixture showing complete decomposition of XeF4 in this case. A discussion of the results has to be based on the following reaction scheme [1,2] in which the fluorine atoms come from XeF4 photolysisr F -t GH4
-HFtCR3 AH = -31.6
F +Hg
AH=
H +XeFq-
kcal ;
+HF+H -31.8 kcal ;
ing by (2) would bear a resemblance to the HCL chemical laser from H2/C12 photolysis where only the second reaction step H t Cl2 -HCL c CL is effective in pumping the laser [5].
3503.7 [8] 3708.9 [8]
3666 .s
3834.4
ApriI 1969
CHEMICAL PHYSICS LETTERS
Volume 3, num5er 4
(lb)
iI HF + XeF2 (and subsequent steps).
The spectral similarities of the 2 - 1 transitions for H2 and CH4 (table 1) show reaction (1) to be the dominant laser excitation step. This confirms earlier results for the UFg/H2(CH,$ chemical laser systems [1,2] where almost the same emission Iincs as in this work have been founTI. However, the appearance - although very weak - of one 3 - 2 line suggests a small contribution of another pumping reaction. This might be reaction (2) since HF (v= 3) can hardly be formed in process (1) with only 32 kcal exothermicity. On the other hand, ample reaction heat is provided by (2) for this excitation in view of less than 40 kcal Xe-F bond energy 141. Pump-
When mixtures of SbFg and H2 (3 mm each) were photolysed by flashes of 1000 Joules energy laser emission was recorded consisting of four 2- 1 lines and two 1 - 0 lines (table I). Thus antimony pentafluoride/hydrogen forms the first HF laser showing 1 - 0 transitions. Unless higher ratios of H2 to SbF5 are used the emission appears only after the flash has nearly terminated. A typical record is shown in fig. I. This suggests that the rate of HF formation is now determined by the F + H2 rate constant rather than by the photolytic F production rate as was probably the case before. The laser could stiI.l be observed when one cavity ndrror was replaced by a germanium flat tith only about 50% reflectivity. From this the optical gain can be estimated to be more thar. 10 dB/m. For reproducibility of results the lazer tube had to be cleaned after about 30 experiments. The laser could be operated more than 10 times from the same SbF5 /Hz mixture without much decrease in intensity indicating very little decomposition of SbFS in one experiment. This is consistent with the smali absorption coefficient of SbFg in the spectral region accessible Ear photolysis; absorption starts only at 1900 A rising to E = 2000 l/mole* cm at 1650 6. An HF laser couLd also be observed when methane was substituted for hydrogen. This shows again that reaction (1) is suZric:entto produce laser action_ The lasing action of lines with very Low J VAues like P(2) in these spectra suggests that the population inversion between the vibrationalrotational states invob~ed is total rather than only partial [6]. Detailed gain determinations which are in progress should he@ to evaluate the exact population ratios. We feel that mare information is needed on other photoLytic fhtorine sources [7] in order to identify the various exciFation and de-excitation processes in HF Laser systems. Such investigations are currently being done end will be reported at a Later time. Support of this research by the Stiftung Volkswagenwerk and Deutsche Forschungsgemeinschaft is gratefully acknowledged. We thank Herrn H. Kaiser for providing us with the vacuum W spectrum of antimony pentafluoride. ,:I7 211
Volume 3, number 4
CHEMICAL PHYSICS LETTERS
REFERENCES [1] K.L.Kompa and G.C. Pimentel, J. Chem. Phys. 47 (1967) 857; K.L.Kompa. J.H.ParkerandG.C.Pimentel. J. Chem. Phys. (in press). [2] J. H. Parker and G. C. Pimentel, J. Chem. Phys. 48 (1963) 5273. 131 K. H. Welge. J. Wanner, F. StuN and A. Heindrichs, Rev. Sci. Instr. 38 (1967) 1728. r<; J. G. Malm. H. Selig, J. Jortner and S. A-Rice, Chem. Rev. 65 (1965) 199.
212
April 1969
[5] J. V. V. Kasper and G. C. Pimentel. Phys. Rev. Letters 14 (1965) 352; P. H.Corneil and G.C. Pimentel, J. Chem. Phys. (in press). [g J. C. Polanyi, Appl. Opt. Suppl. 2 (1965): Chemical Lasers. p. 109. [?‘I Compare also: R.W.F.Gross, N.Cohen and T.A. Jacobs, J. Chem. Phys. 48 (1968) 3821. 181 D.E.IUann. B.A.Thrush. D.R.Lide Jr., J.J.Ball and N.Aquista. J. Chem. Phys. 34 (1961) 420. [S] G.A.Kui&s..J. Molec. Spectr. 2 (i958j 75.
3, number
CREMJCAL
4
NEW
HYDROGEN
PHYSICS
FLUORIDE
LETTERS
April
CHEMICAL
1969
LASERS
-Karl L. KOMPA, Peter GENSEL and Jochen WANNER Institut fir Anorganisc.‘l-* Chemie der UniversitiG 1Miinchen. Germany Received
13 March
1969
Hydrogen fluoride vibration-rotation laser emission has been observed in the flash photolysis of xenon tetrafluoride and antimony pentafluoride mixed with hydrogen or methane. The observed emission lines, gain estimates, and information on the chemical excitation scheme are reported.
Vibration-rotation laser emission of hydrogen and deuterium fluoride was first observed in the flash photolysis of uranium hexafluoride/hydrogen (deuterium) mixtures by Kompa and Pimentel [l] and of uranium hexafluoride/hydrocarbon mixtures by Parker and Pimentel[2]. In the course of this work it was noted that antimony pentafluoride can be used as a photolytic fluorine atom source, too [I]. We have found that xenon tetrafluoride photolysis can initiate the formation of another strong HF chemical laser, and we wish to report here some new results for the systems xenon tetrafluoride with hydrogen or methane and antimony pentafluoride with hydrog=The experimental set-up was different from the -ones used resolution
in previous
work.
and the possibility
For
better
of vacuum
time
chemical
210
laser
chemicallaseremissionfrom
SbF5/JJ
emission
(arbitrary
intensity scale).
UV
photolysis the flash photolysis design of Welge, Wanner, Stuhl and Heindrichs [3] was adapted utilizing b nitrogen spark gaps (each one connected to a 1 pFd/2C) kV r:~pX discharge capacitor) instead of a xenon flash tube. The spark .gaps were arranged in two rows along two sides of the Ieser tube in a vacuum-tight cylindrical Iacite discharge vessel which was coated with Eastman white reflectance standard. At a nitrogen pressure of 300 - 400 mm and with a total discharge energy of up to 1600 Joules the unit produced flashes of 1.2 psec half-peak duration (fig. 1) *. The Suprasil laser tube had an illuminated length of 30 cm, an inner diameter of 12 mm, and was equipped with Brewster’s zmgle sapphire windows. The laser cavity was formed by two 10 m radius gold-coated mirrors spaced 80 cm. About 3% of the energy (per traversal)
* Details of the apparatus
Fig. 1. HF
photolysis, Upper trace fI ash signal, lower trace tser
and performance data for studies will be published elsewhere.
was coupled out of the cavity by a sodium chloride disk. Detector and monochromator arrangement were as described before [1,2]. The monochromator was a O.Sm-grating instrument (McPherson model 218). We used a PEM InSb-detector for the reference signal and a Ge:Au photoconductive detector to record the single-line emission which passed the monochromator. A foto field effect transistor recorded the flash profile. Xenon tetraJ7uori&/hydyogen (methane). When mixtures of 2.5 mm XeF4 and 3-7 mm H2 or CH4 were exposed to the light of a 1000 Joules flash the HF emission lines listed in table 1 are observed. The emission occurs in the P branch. No emission was detected on corresponding R-branch transitions. Operation of the laser was only possible in a carefully cleaned (rinsing with lfi hydrofluoric acid for 5 min) vacuum line and laser tube and after “conditioning”
Table 1 Stimulated emission lines of hydrogen fluoride Line Obsefped SbF5/HS XeF4/HS XeF4/CH4 (cm ) 3504.2 3709.4
P3+3(3)* P2-+1(2)
-
Literature (cm-l)
-3 .+(3)
(3)
(3)
3666.4 [Xl
3623.2
(4)
(4)
(4)
3622.6 [S]
3578.1
(5)
(5)
(5)
3577.5 [8]
(6)
(6)
3531.2 [S]
3484.2
(7)
(7)
3483.7 [S]
3435.6
(8)
(8)
Pa-l@)
3531.7
q-+0(3)
3788.8
(4)
3435.1 [S] 3834.0 [9] 3788.5 [S]
* very weak. by repeated exposure to XeF4. With these experimental conditions the laser emission appears 0.2 psec after the beginning of the flash having a pulse duration of about 1 wsec. Even when one of the cavity mirrors was taken off strong laser signals could still be detected which points to a very high gain of this laser. From the same XeFd/CHd-filling the HF laser could be operated up to 8 times while only one emission pulse was observable with a XeFq/H2 mixture showing complete decomposition of XeF4 in this case. A discussion of the results has to be based on the following reaction scheme [1,2] in which the fluorine atoms come from XeF4 photolysisr F -t GH4
-HFtCR3 AH = -31.6
F +Hg
AH=
H +XeFq-
kcal ;
+HF+H -31.8 kcal ;
ing by (2) would bear a resemblance to the HCL chemical laser from H2/C12 photolysis where only the second reaction step H t Cl2 -HCL c CL is effective in pumping the laser [5].
3503.7 [8] 3708.9 [8]
3666 .s
3834.4
ApriI 1969
CHEMICAL PHYSICS LETTERS
Volume 3, num5er 4
(lb)
iI HF + XeF2 (and subsequent steps).
The spectral similarities of the 2 - 1 transitions for H2 and CH4 (table 1) show reaction (1) to be the dominant laser excitation step. This confirms earlier results for the UFg/H2(CH,$ chemical laser systems [1,2] where almost the same emission Iincs as in this work have been founTI. However, the appearance - although very weak - of one 3 - 2 line suggests a small contribution of another pumping reaction. This might be reaction (2) since HF (v= 3) can hardly be formed in process (1) with only 32 kcal exothermicity. On the other hand, ample reaction heat is provided by (2) for this excitation in view of less than 40 kcal Xe-F bond energy 141. Pump-
When mixtures of SbFg and H2 (3 mm each) were photolysed by flashes of 1000 Joules energy laser emission was recorded consisting of four 2- 1 lines and two 1 - 0 lines (table I). Thus antimony pentafluoride/hydrogen forms the first HF laser showing 1 - 0 transitions. Unless higher ratios of H2 to SbF5 are used the emission appears only after the flash has nearly terminated. A typical record is shown in fig. I. This suggests that the rate of HF formation is now determined by the F + H2 rate constant rather than by the photolytic F production rate as was probably the case before. The laser could stiI.l be observed when one cavity ndrror was replaced by a germanium flat tith only about 50% reflectivity. From this the optical gain can be estimated to be more thar. 10 dB/m. For reproducibility of results the lazer tube had to be cleaned after about 30 experiments. The laser could be operated more than 10 times from the same SbF5 /Hz mixture without much decrease in intensity indicating very little decomposition of SbFS in one experiment. This is consistent with the smali absorption coefficient of SbFg in the spectral region accessible Ear photolysis; absorption starts only at 1900 A rising to E = 2000 l/mole* cm at 1650 6. An HF laser couLd also be observed when methane was substituted for hydrogen. This shows again that reaction (1) is suZric:entto produce laser action_ The lasing action of lines with very Low J VAues like P(2) in these spectra suggests that the population inversion between the vibrationalrotational states invob~ed is total rather than only partial [6]. Detailed gain determinations which are in progress should he@ to evaluate the exact population ratios. We feel that mare information is needed on other photoLytic fhtorine sources [7] in order to identify the various exciFation and de-excitation processes in HF Laser systems. Such investigations are currently being done end will be reported at a Later time. Support of this research by the Stiftung Volkswagenwerk and Deutsche Forschungsgemeinschaft is gratefully acknowledged. We thank Herrn H. Kaiser for providing us with the vacuum W spectrum of antimony pentafluoride. ,:I7 211
Volume 3, number 4
CHEMICAL PHYSICS LETTERS
REFERENCES [1] K.L.Kompa and G.C. Pimentel, J. Chem. Phys. 47 (1967) 857; K.L.Kompa. J.H.ParkerandG.C.Pimentel. J. Chem. Phys. (in press). [2] J. H. Parker and G. C. Pimentel, J. Chem. Phys. 48 (1963) 5273. 131 K. H. Welge. J. Wanner, F. StuN and A. Heindrichs, Rev. Sci. Instr. 38 (1967) 1728. r<; J. G. Malm. H. Selig, J. Jortner and S. A-Rice, Chem. Rev. 65 (1965) 199.
212
April 1969
[5] J. V. V. Kasper and G. C. Pimentel. Phys. Rev. Letters 14 (1965) 352; P. H.Corneil and G.C. Pimentel, J. Chem. Phys. (in press). [g J. C. Polanyi, Appl. Opt. Suppl. 2 (1965): Chemical Lasers. p. 109. [?‘I Compare also: R.W.F.Gross, N.Cohen and T.A. Jacobs, J. Chem. Phys. 48 (1968) 3821. 181 D.E.IUann. B.A.Thrush. D.R.Lide Jr., J.J.Ball and N.Aquista. J. Chem. Phys. 34 (1961) 420. [S] G.A.Kui&s..J. Molec. Spectr. 2 (i958j 75.