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XeCl laser operation with liquid chlorine donors: An experimental investigation on the gas composition
Volume 39, number 1,2
OPTICS COMMUNICATIONS
15 September 1981
XeCI LASER OPERATION WITH LIQUID CHLORINE DONORS: AN EXPERIMENTAL INVESTIGATION ON THE GAS COMPOSITION ~ R. SALIMBENI 1, M. MATERA 2 , M. VANNINI 3, p. BURLAMACCHI 1 1 Istituto di Elettronica Quantistica del CNR, Firenze, Italy 2 CNEN, Divisione Nuovo Attivitd, Centro di FrascatL Italy 3 Istituto di Fisica, Universitd di Firenze, Italy
Received 4 June 1981 An experimental investigation on the performance of a spark preionized high energy XeCI laser operating with liquid chlorine donors is reported. Output energy of 70 mJ and 110 mJ has been achieved with CC14 and SiCI4 respectively. In both cases the effective chlorine donor appears to be HCI synthetized by discharge-induced reactions during a preliminary run of discharges.
Laser action from XeC1 excimer in TEA lasers has in recent years achieved output energy and efficiency quite comparable with the KrF excuner [ 1 - 4 ] . Nevertheless in the initial stages o f development several attempts were performed to find the most suitable chlorine donor [ 5 - 8 ] . Our attention was attracted at that time by the excellent results obtained using chlorinated hydrocarbons as CI donors in a fast Blumlein type TEA laser [9,10]. Despite the relatively low energy output o f the laser, the efficiency was in effect comparable with that obtained using HCI, which is the most commonly used donor. The use of chlorocarbons was considered very attractive in order to avoid inherent problems in handling corrosive and toxic gases. In fact this solution is certainly effective in terms of reduced hazard. However different results have been reported on XeC1 laser operation with chlorocarbon halogen donors. While fast devices appeared to operate quite well [ 7 - 9 ] , only weak emission was reported from high energy XeCI lasers with volumetric UV preiortization [1 ]. In effect, large discharge volumes, high gas mixture densities and long lasting current pulses provide restrictive conditions for the preionization level which is needed for a uniform and stable glow discharge. Unfortunately these conditions can hardly be met in Work performed at lstituto di Elettroniea Quantistica, Via Panciatichi, 56/30, 50127 Firenze, ltalia.
gas mixtures containing VUV absorbing species like chlorometanes [1 ]. With these premises we have investigated the performance of a high energy XeC1 laser, operating with liquid phase chlorine donors. In this work we describe the results obtained using CCI4 and SiCI4 in a UV preionized XeC1 laser. It turned out in any case that HCI was formed following hydrolysis of the chlorine carrier molecules during the electric discharge. After a few thousand shots, in which reaction took place, the output energy of the laser rose to a stationary level. For CC14 the maximum output was about 2/3 of the level obtained with pure HC1, while for SiC14 it was quite comparable. Maximum energy o f 70 mJ and 110 mJ has been obtained for the two donors. At 20 Hz the system gave about 1.2 W and 2 W average power. The experimental set-up and the cross section of the laser head are shown in fig. 1. The discharge chamber is built entirely o f aluminum and lucite with a total o f about 15 liters. The electrodes are smoothly contoured, 3 cm wide and 80 cm long. Electrode separation is 2 cm. To operate the laser up to 30 Hz repetition rate a transverse flow of the gas mixture is provided by means of a set of radial fans mounted on the same axis and powered by a vacuum-tight rotating feed-through. A spark gap starts both the charge-transfer LC generator and the preionizing sliding-sparks bar, which has been located downstream, as close as possible
0 0 3 0 - 4 0 1 8 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
75
Volume 39, number 1,2
OPTICS COMMUNICATIONS
15 Scptenlher 1981
VP \ , i
CI
H .
.
.
.
.
c2 c3il _
_
SG
Xe ccl
4.1sici4
0 HV He. At
I.~. 1. Schematicdiagram of the experimental apparatus. Legend: C l preionization capacitor, C2 primary storage capacitor. (-'3 peaking capacitor, F fan, PC premixing chamber, SG spark gap. to the electrodes. The main discharge is intrinsically delayed by the higher breakdown level of the electrodes gap as compared with that of the preionization bar. The gases used in the experiment were Matheson Technical grade HC1 (99%), C.P. grade Xe (99.95%), High Purity grade He and Ar (99.995%). The energy output was measured by a GEN-TEC ED 500 energy meter. With 30 J stored in the primary capacitor the laser emitted a maximum energy of 115 mJ with 3 Torr HCI, 40 Torr Xe, 1500 Torr He, and 135 mJ with 3 Torr HC1, 40 Torr Xe, 900 Torr At. In order to accurately control the vapor concentration inside the laser chamber the usual gas handling system has been modified slightly by adding a small premixing chamber. The ratio of the premixing chamber volume to the laser volume was such that a partial vapor pressure of 10-1 Torr could easily be determined with the use of a standard vacuum gauge. Consequently the filling procedure becomes: 1) evacuation of the entire system by a double stage rotative pump; 2) filling with 40 Torr Xe; 3) vapor transfer from the storage bottle inside the premixing chamber; 4) filling with He or Ar through the premixing chamber. With the above descried vaporization system connected to the laser, CCI4 and SiC14 were tested. The first was considered the most appropriate among chlorocarbons because in a previous investigation [8] it appeared that a lower concentration was needed. This fact should minimize the absorption of preionizing radiation and, on a long term run, the deposit of chlo76
rocarbon compounds inside the laser cell. In this respect the second, despite some toxicity, was supposed to be less detrimental on the long term operation. In fact discharge-induced reactions, proceeding along a chloro silicon polymer chain, should lead to the formation of solid particles instead of oily chlorocarbon polymers. Both compounds have been tested in the same operating conditions, while the discharge parameters and the noble gas pressures were previously optimized with HC1. While using HC1 the discharge appeared very uniform with slight streamers periodically distributed along the electrodes, glow discharges could be obtained only with low pressure of CCI4 vapor, but no lasing was achieved in this condition. When 0.25 Torr of CCI4 was introduced into the laser chamber, the discharge started, extremely striated by multiple arcs with a weak emission of about 10 mJ. By running the laser at 10 Hz r.r. the laser emission rose to a maximum lev~l of 70 mJ (fig. 2), while the streamers in the discharge gradually disappeared. Although at the beginning of the discharge run the absorption in the photoionization region can reasonably be considered responsible for the discharge instability, less could be said about the subsequent evolution of the laser emission. To investigate the corresponding gas composition evolution, IR absorption spectra technique has been employed, by collecting samples of the gas mixture every 5 minutes from the start of the discharges. IR spectra were ob-
Volume 39, number 1,2
OPTICS COMMUNICATIONS
band, spectrum (a) shows traces of HCI, probably released by the cell walls. In spectrum (b) a relatively strong vibrational spectrum of HC1 increased at 2900 cm -1, while CC14 absorption peak is reduced to about one third of the original value and a well resolved vibrational spectrum of CO comes out at 2100 cm -1. In both conditions water traces were detected by the characteristic vibro-rotational structures around 1600 cm -1 and 3750 cm -1 not reported in fig. 3. These results suggest a new understanding of the XeC1 laser operation with CC14 that can be summarized in the following points: l) CC14 is never substantially involved in the formation kinetics of the XeC1 excimer. 2) The discharge induces a set of chemical reactions, involving CC14 and H20 along a hydrolysis path. As an example the primary step could be:
- - 250-
w
200E
o 150-
=
-
I00-
50-
//
/
~/
.,
//3
S,C,,
0.25 mb CCI4 o 0.35 mb CCI4
i
I
05m
2
3
i
i
t
i
i
i
4
5
6
7
8
9
1()
CC14 + H20 +hv ~ 2HCI + Cl2 + CO.
10 Hz Operation Time (mini
Fig. 2. Output energy level as a function of the operation time for the chlorine donors under test. tained with a Perkin Elmer mod. 225 spectrometer. Fig. 3 shows the absorption spectrum of the gas mixture immediately after the introduction in the laser chamber (a) and after a 10 minute run at 10 p.p.s. (b). Besides a band at 795 On -1 characteristic of the C-CI Wavelength 3
F
4
I
.
.
{
5 .
.
.
I_
pm
I
10 _
15
_ i. . . . . . . .
li'
-
I -
7 - ' "
3000
I
2o'oo 1o'oo
Wavenumber
- - -
20
L
g
B~o
15 September 1981
do
(cm -I )
Fig. 3. IR transmittance spectra of two independent samples of a gas mixture containing 0.25 Torr CC14. Spectrum a undischarged, spectrum b after 6000 shots (shifted for clarity).
3) The increase of laser emission during the preliminary run of discharges is produced by the growth of the HCI concentration. 4) Residual components, like Cl2, CO and CC14 have a lower influence on the laser performance, but they can prevent the attainment of the power levels reached with pure HC1. A similar evolution in the discharge characteristics was observed when 0.5 Torr of SiC14 vapor was.introduced inside the laser chamber. As shown in fig.2, the laser starts with no emission at all and after a few thousand shots the output energy per pulse reached 110 mJ. IR spectra of samples taken before (a) and after (b) this evolution are shown in fig. 4. Besides the Si-Cl band at 621 cm -1, spectrum (a) shows that, even without any discharge, HC1 is present in a quantity large enough to give strong laser emission. In effect SiC14 is a very reactive molecule and, in the presence of trace quantity of water, rapidly hydrolyzes producing HC1 and other silicon compounds not resolved in the spectrum. On the other hand the absorption of preionizing photons by SiCI4 is quite strong and no glow discharge is observed in the first shots. Spectrum (b), obtained after a few thousands shots, does not show any observable variation in the HC1 concentration, while SiC14 has almost completely disappeared. According to experimental evidence the quick dissociation of chlorosilicon molecules turns into precipitation of solid phase heavy compounds, which in fact could be col77
Volume 39, number 1,2
OPTICS COMMUNICATIONS
Wavelenght 3
4
]
I
J
(pm) 10
5
1'5
J------ I
I
20
L
/ b A
m v
I
2 I
-'2
l
•
30100
l---I
2000 1000
Wavenumber
000
KO0
(crn -I)
Fig. 4.1R transmittance spectra of two independent samples of a gas mixture containing 0.5 Torr SiCI4. Spectrum a undischarged. Spectrum b after 6000 shots (shifted for clarity). lected at the b o t t o m o f the laser chamber, after several refills. These results confirm those previously obtained with CC14. Once more the effective CI donor for excimer formation is HC1 synthesized by hydrolysis of SiC14. Considering the relative tolerance o f the laser performance to trace level o f impurities like water or CO, we also tried to operate the laser using as liquid donor an aqueous solution o f HC1 at the standard concentration of 37%. As shown in table 1 the laser level was surprisingly unaffected b y the doubtless presence of H20 vaporized inside the laser chamber together with HC1. In conclusion, with the simple investigation presentTable 1 XeCI laser performance using different chlorine donors with 40 Tort Xe, 1500 Torr He and 30 J stored energy Chlorine donor (Torr)
Output energy (m J)
Pulse width (ns)
Refill lifet ime (shots)
1tC1 (3) CC14 (0.25) SiCI4 (0.5) 37% water solution of HCI (2)
110 70 110
25 20 25
6 x 104 5 x 104 6x 104
110
25
6 x 104
78
15 September 1981
ed here, the following points have been clarified. The foreseen difficulty in obtaining glow discharges with convenient concentrations of chlorocompounds as CC14 or SiC14 is confirmed. Nevertheless, with moderate concentrations of CCI 4 ( 0 . 2 - 0 . 3 Torr), dischargeinduced chemical reactions, while restoring a sufficient transmission for the preionizing radiation, synthesize HC1 which emerges as the effective chlorine donor for the intense lasing observed after this evolution. These reactions are likely to follow a hydrolysis path, producing HC1 and burning water, which at trace level persists in the laser cell after evacuation due to wall outgassing. The laser performance in these conditions is d e s c r i e d b y the results listed in table 1. These resuits demonstrate the possibility of operating a XeCI laser at high energy and high average power reducing to a minimum the otherwise unavoidable hazard of handling compressed gaseous HC1. Considering the widespread diffusion of the XeC1 excimer laser as an ideal dye laser pump, this opportunity could be very helpful in becoming familiar with these UV sources and to meet safety requirements in every type o f location. Furthermore the possibility of utilizing dischargeinduced reactions in other excimer systems like KrC1 or XeBr seems promising. Further work in this direction is in progress. The authors wish to thank Prof. S. Califano and Dr. P. Salvi for many useful exchanges and suggestions.
References [1 ) R. Burnham, OpticsComm. 24 (1978) 161. [21 R.C. Sze, J. Appl. Phys. 50 (1979) 4596. [3] A.J. Kearsley, A.J. Andrews and C.E. Webb, Optics Comm. 31 (1975) 181. [41 R.S. Taylor, S. Watanabe, A.J. Alcock, K.E. Leopold and P.B. Corkum, paper presented at CLEO 1981, Washingt o n. [5 ] J.J. Ewing and C.A. Brau, Appl. Phys. Lett. 27 (1975) 350. [6] J. Tellinghuisen, J.M. Hoffman, G.C. Tisone and A.K. Hays, J. Chem. Phys. 64 (1976) 2484. [7] Yu.A. Kudryavtsev and N.P. Kuz'mina, Sov. J. Quantum Electron. 7 (1977) 131. [81 V.N. Ishchenko, V.N. Lisitsyn and A.M. Razhev, Optics Comm. 21 (1977) 30. [91 P. Burlamacchi and R. Salimbeni, Optics Comm. 26 (1978) 233. [ 101 L. Burlamacchi, P. Burlamacchi and R. Salimbeni, Appl. Phys. Lett. 34 (1979) 33.
OPTICS COMMUNICATIONS
15 September 1981
XeCI LASER OPERATION WITH LIQUID CHLORINE DONORS: AN EXPERIMENTAL INVESTIGATION ON THE GAS COMPOSITION ~ R. SALIMBENI 1, M. MATERA 2 , M. VANNINI 3, p. BURLAMACCHI 1 1 Istituto di Elettronica Quantistica del CNR, Firenze, Italy 2 CNEN, Divisione Nuovo Attivitd, Centro di FrascatL Italy 3 Istituto di Fisica, Universitd di Firenze, Italy
Received 4 June 1981 An experimental investigation on the performance of a spark preionized high energy XeCI laser operating with liquid chlorine donors is reported. Output energy of 70 mJ and 110 mJ has been achieved with CC14 and SiCI4 respectively. In both cases the effective chlorine donor appears to be HCI synthetized by discharge-induced reactions during a preliminary run of discharges.
Laser action from XeC1 excimer in TEA lasers has in recent years achieved output energy and efficiency quite comparable with the KrF excuner [ 1 - 4 ] . Nevertheless in the initial stages o f development several attempts were performed to find the most suitable chlorine donor [ 5 - 8 ] . Our attention was attracted at that time by the excellent results obtained using chlorinated hydrocarbons as CI donors in a fast Blumlein type TEA laser [9,10]. Despite the relatively low energy output o f the laser, the efficiency was in effect comparable with that obtained using HCI, which is the most commonly used donor. The use of chlorocarbons was considered very attractive in order to avoid inherent problems in handling corrosive and toxic gases. In fact this solution is certainly effective in terms of reduced hazard. However different results have been reported on XeC1 laser operation with chlorocarbon halogen donors. While fast devices appeared to operate quite well [ 7 - 9 ] , only weak emission was reported from high energy XeCI lasers with volumetric UV preiortization [1 ]. In effect, large discharge volumes, high gas mixture densities and long lasting current pulses provide restrictive conditions for the preionization level which is needed for a uniform and stable glow discharge. Unfortunately these conditions can hardly be met in Work performed at lstituto di Elettroniea Quantistica, Via Panciatichi, 56/30, 50127 Firenze, ltalia.
gas mixtures containing VUV absorbing species like chlorometanes [1 ]. With these premises we have investigated the performance of a high energy XeC1 laser, operating with liquid phase chlorine donors. In this work we describe the results obtained using CCI4 and SiCI4 in a UV preionized XeC1 laser. It turned out in any case that HCI was formed following hydrolysis of the chlorine carrier molecules during the electric discharge. After a few thousand shots, in which reaction took place, the output energy of the laser rose to a stationary level. For CC14 the maximum output was about 2/3 of the level obtained with pure HC1, while for SiC14 it was quite comparable. Maximum energy o f 70 mJ and 110 mJ has been obtained for the two donors. At 20 Hz the system gave about 1.2 W and 2 W average power. The experimental set-up and the cross section of the laser head are shown in fig. 1. The discharge chamber is built entirely o f aluminum and lucite with a total o f about 15 liters. The electrodes are smoothly contoured, 3 cm wide and 80 cm long. Electrode separation is 2 cm. To operate the laser up to 30 Hz repetition rate a transverse flow of the gas mixture is provided by means of a set of radial fans mounted on the same axis and powered by a vacuum-tight rotating feed-through. A spark gap starts both the charge-transfer LC generator and the preionizing sliding-sparks bar, which has been located downstream, as close as possible
0 0 3 0 - 4 0 1 8 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
75
Volume 39, number 1,2
OPTICS COMMUNICATIONS
15 Scptenlher 1981
VP \ , i
CI
H .
.
.
.
.
c2 c3il _
_
SG
Xe ccl
4.1sici4
0 HV He. At
I.~. 1. Schematicdiagram of the experimental apparatus. Legend: C l preionization capacitor, C2 primary storage capacitor. (-'3 peaking capacitor, F fan, PC premixing chamber, SG spark gap. to the electrodes. The main discharge is intrinsically delayed by the higher breakdown level of the electrodes gap as compared with that of the preionization bar. The gases used in the experiment were Matheson Technical grade HC1 (99%), C.P. grade Xe (99.95%), High Purity grade He and Ar (99.995%). The energy output was measured by a GEN-TEC ED 500 energy meter. With 30 J stored in the primary capacitor the laser emitted a maximum energy of 115 mJ with 3 Torr HCI, 40 Torr Xe, 1500 Torr He, and 135 mJ with 3 Torr HC1, 40 Torr Xe, 900 Torr At. In order to accurately control the vapor concentration inside the laser chamber the usual gas handling system has been modified slightly by adding a small premixing chamber. The ratio of the premixing chamber volume to the laser volume was such that a partial vapor pressure of 10-1 Torr could easily be determined with the use of a standard vacuum gauge. Consequently the filling procedure becomes: 1) evacuation of the entire system by a double stage rotative pump; 2) filling with 40 Torr Xe; 3) vapor transfer from the storage bottle inside the premixing chamber; 4) filling with He or Ar through the premixing chamber. With the above descried vaporization system connected to the laser, CCI4 and SiC14 were tested. The first was considered the most appropriate among chlorocarbons because in a previous investigation [8] it appeared that a lower concentration was needed. This fact should minimize the absorption of preionizing radiation and, on a long term run, the deposit of chlo76
rocarbon compounds inside the laser cell. In this respect the second, despite some toxicity, was supposed to be less detrimental on the long term operation. In fact discharge-induced reactions, proceeding along a chloro silicon polymer chain, should lead to the formation of solid particles instead of oily chlorocarbon polymers. Both compounds have been tested in the same operating conditions, while the discharge parameters and the noble gas pressures were previously optimized with HC1. While using HC1 the discharge appeared very uniform with slight streamers periodically distributed along the electrodes, glow discharges could be obtained only with low pressure of CCI4 vapor, but no lasing was achieved in this condition. When 0.25 Torr of CCI4 was introduced into the laser chamber, the discharge started, extremely striated by multiple arcs with a weak emission of about 10 mJ. By running the laser at 10 Hz r.r. the laser emission rose to a maximum lev~l of 70 mJ (fig. 2), while the streamers in the discharge gradually disappeared. Although at the beginning of the discharge run the absorption in the photoionization region can reasonably be considered responsible for the discharge instability, less could be said about the subsequent evolution of the laser emission. To investigate the corresponding gas composition evolution, IR absorption spectra technique has been employed, by collecting samples of the gas mixture every 5 minutes from the start of the discharges. IR spectra were ob-
Volume 39, number 1,2
OPTICS COMMUNICATIONS
band, spectrum (a) shows traces of HCI, probably released by the cell walls. In spectrum (b) a relatively strong vibrational spectrum of HC1 increased at 2900 cm -1, while CC14 absorption peak is reduced to about one third of the original value and a well resolved vibrational spectrum of CO comes out at 2100 cm -1. In both conditions water traces were detected by the characteristic vibro-rotational structures around 1600 cm -1 and 3750 cm -1 not reported in fig. 3. These results suggest a new understanding of the XeC1 laser operation with CC14 that can be summarized in the following points: l) CC14 is never substantially involved in the formation kinetics of the XeC1 excimer. 2) The discharge induces a set of chemical reactions, involving CC14 and H20 along a hydrolysis path. As an example the primary step could be:
- - 250-
w
200E
o 150-
=
-
I00-
50-
//
/
~/
.,
//3
S,C,,
0.25 mb CCI4 o 0.35 mb CCI4
i
I
05m
2
3
i
i
t
i
i
i
4
5
6
7
8
9
1()
CC14 + H20 +hv ~ 2HCI + Cl2 + CO.
10 Hz Operation Time (mini
Fig. 2. Output energy level as a function of the operation time for the chlorine donors under test. tained with a Perkin Elmer mod. 225 spectrometer. Fig. 3 shows the absorption spectrum of the gas mixture immediately after the introduction in the laser chamber (a) and after a 10 minute run at 10 p.p.s. (b). Besides a band at 795 On -1 characteristic of the C-CI Wavelength 3
F
4
I
.
.
{
5 .
.
.
I_
pm
I
10 _
15
_ i. . . . . . . .
li'
-
I -
7 - ' "
3000
I
2o'oo 1o'oo
Wavenumber
- - -
20
L
g
B~o
15 September 1981
do
(cm -I )
Fig. 3. IR transmittance spectra of two independent samples of a gas mixture containing 0.25 Torr CC14. Spectrum a undischarged, spectrum b after 6000 shots (shifted for clarity).
3) The increase of laser emission during the preliminary run of discharges is produced by the growth of the HCI concentration. 4) Residual components, like Cl2, CO and CC14 have a lower influence on the laser performance, but they can prevent the attainment of the power levels reached with pure HC1. A similar evolution in the discharge characteristics was observed when 0.5 Torr of SiC14 vapor was.introduced inside the laser chamber. As shown in fig.2, the laser starts with no emission at all and after a few thousand shots the output energy per pulse reached 110 mJ. IR spectra of samples taken before (a) and after (b) this evolution are shown in fig. 4. Besides the Si-Cl band at 621 cm -1, spectrum (a) shows that, even without any discharge, HC1 is present in a quantity large enough to give strong laser emission. In effect SiC14 is a very reactive molecule and, in the presence of trace quantity of water, rapidly hydrolyzes producing HC1 and other silicon compounds not resolved in the spectrum. On the other hand the absorption of preionizing photons by SiCI4 is quite strong and no glow discharge is observed in the first shots. Spectrum (b), obtained after a few thousands shots, does not show any observable variation in the HC1 concentration, while SiC14 has almost completely disappeared. According to experimental evidence the quick dissociation of chlorosilicon molecules turns into precipitation of solid phase heavy compounds, which in fact could be col77
Volume 39, number 1,2
OPTICS COMMUNICATIONS
Wavelenght 3
4
]
I
J
(pm) 10
5
1'5
J------ I
I
20
L
/ b A
m v
I
2 I
-'2
l
•
30100
l---I
2000 1000
Wavenumber
000
KO0
(crn -I)
Fig. 4.1R transmittance spectra of two independent samples of a gas mixture containing 0.5 Torr SiCI4. Spectrum a undischarged. Spectrum b after 6000 shots (shifted for clarity). lected at the b o t t o m o f the laser chamber, after several refills. These results confirm those previously obtained with CC14. Once more the effective CI donor for excimer formation is HC1 synthesized by hydrolysis of SiC14. Considering the relative tolerance o f the laser performance to trace level o f impurities like water or CO, we also tried to operate the laser using as liquid donor an aqueous solution o f HC1 at the standard concentration of 37%. As shown in table 1 the laser level was surprisingly unaffected b y the doubtless presence of H20 vaporized inside the laser chamber together with HC1. In conclusion, with the simple investigation presentTable 1 XeCI laser performance using different chlorine donors with 40 Tort Xe, 1500 Torr He and 30 J stored energy Chlorine donor (Torr)
Output energy (m J)
Pulse width (ns)
Refill lifet ime (shots)
1tC1 (3) CC14 (0.25) SiCI4 (0.5) 37% water solution of HCI (2)
110 70 110
25 20 25
6 x 104 5 x 104 6x 104
110
25
6 x 104
78
15 September 1981
ed here, the following points have been clarified. The foreseen difficulty in obtaining glow discharges with convenient concentrations of chlorocompounds as CC14 or SiC14 is confirmed. Nevertheless, with moderate concentrations of CCI 4 ( 0 . 2 - 0 . 3 Torr), dischargeinduced chemical reactions, while restoring a sufficient transmission for the preionizing radiation, synthesize HC1 which emerges as the effective chlorine donor for the intense lasing observed after this evolution. These reactions are likely to follow a hydrolysis path, producing HC1 and burning water, which at trace level persists in the laser cell after evacuation due to wall outgassing. The laser performance in these conditions is d e s c r i e d b y the results listed in table 1. These resuits demonstrate the possibility of operating a XeCI laser at high energy and high average power reducing to a minimum the otherwise unavoidable hazard of handling compressed gaseous HC1. Considering the widespread diffusion of the XeC1 excimer laser as an ideal dye laser pump, this opportunity could be very helpful in becoming familiar with these UV sources and to meet safety requirements in every type o f location. Furthermore the possibility of utilizing dischargeinduced reactions in other excimer systems like KrC1 or XeBr seems promising. Further work in this direction is in progress. The authors wish to thank Prof. S. Califano and Dr. P. Salvi for many useful exchanges and suggestions.
References [1 ) R. Burnham, OpticsComm. 24 (1978) 161. [21 R.C. Sze, J. Appl. Phys. 50 (1979) 4596. [3] A.J. Kearsley, A.J. Andrews and C.E. Webb, Optics Comm. 31 (1975) 181. [41 R.S. Taylor, S. Watanabe, A.J. Alcock, K.E. Leopold and P.B. Corkum, paper presented at CLEO 1981, Washingt o n. [5 ] J.J. Ewing and C.A. Brau, Appl. Phys. Lett. 27 (1975) 350. [6] J. Tellinghuisen, J.M. Hoffman, G.C. Tisone and A.K. Hays, J. Chem. Phys. 64 (1976) 2484. [7] Yu.A. Kudryavtsev and N.P. Kuz'mina, Sov. J. Quantum Electron. 7 (1977) 131. [81 V.N. Ishchenko, V.N. Lisitsyn and A.M. Razhev, Optics Comm. 21 (1977) 30. [91 P. Burlamacchi and R. Salimbeni, Optics Comm. 26 (1978) 233. [ 101 L. Burlamacchi, P. Burlamacchi and R. Salimbeni, Appl. Phys. Lett. 34 (1979) 33.