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Vacuum ultraviolet spark emission spectra of fluorine and some polyatomic fluorides in noble gases
Volume 58, number 3
CHEWCAL
PHYSICS LEMERS
1 October 1978
VACUUM ULTRAVIOLET SPARK EMISSION SPECI’RA OF FLUORINE AND SOME POLYATOMIC FLUORIDES IN NOBLE GASES* R. SCHEPSZ and F. M. RYAN Wtxtingkouse R & D Center, Pittsburgh. Pennsylvania I523-5. USA
Received 2 June 1978 Spark emissionspectra in tie vacuum ultraviolet are reported for severalpoiyatomic fluorides in He + Ar mhtures, and for pure fluorine_ These results are discussed in terms of the potential of a given fluorine compound for ArF formation. The 158 run emission in fluorine is tentatively assised to the 3Ug + 3nu.
We have measured spark emission spectra in the vacuum UV from gas mixtures of He, Ar, and various fluorine compounds as part of a fluorine donor selection study for the rare gas fluoride self-sustained discharge laser. Many fluorine bearing molecules can form rare gas fluoride molecules through the ion-ion recombination and so-called “harpooning” channels so that mixtures for th3 test were selected on the basis of their applicability to the excimer formation process. We found that spark spectra provide a fast and reliable test of excimer formation in a given mixture: The appearance or absence of ArF fluorescence in the spark emission spectrum correlated quite well with excimer fluorescence resulting from a glow discharge_ Fluorine samples observed were F2. SF6, S02F2. NF3, CHsF, N2Fq and BF3 in 700 torr He and 2-20 torr Ar. In addition we observed emission at 158 MI from FdHe, F2/Ne and unmixed fluorine samples_ Emission intensity was observed as a function of fluorine and noble gas density to better understand the population mechanism for the excited F2 level. in this experiment, a sp&k source is mounted near the entrance slit of a 03 m vacuum monochromator and isolated from the vacuum by a MgF2 window. Research grade purity gases are premixed in the inlet * This work was funded in part by Advanced Research Projects Agency under Contract * DASG60-77-C-0015. r Present address: Maxwell Laboratories, Inc., San Diego, California 92193,
USA.
manifold and flow past the spark gap at rates of between 500 and 2500 scc/min. The spark pulse circuit is of the capacitive discharge type. It consists of a 0.0025 PF capacitor charged to 11 kV in series with a 20.5 Q resistor and switched by an SC22 ‘rhyratron in the common grounded carbode configuration_ This circuit provides a 200 A peak 150 ns pulse to the nickel spark electrodes, which were adjusted to a 1 mm gap. A sodium salicylate scintillator on a photodetector is mounted on the exit slit of the monochromator. The spark gap is pulsed at 20 Hz and the phototube output is time averaged before entering the electrometer. in most cases, absolute intensities are reported and were measured with an NBS calibrated vacuum photodiode. The noble gas/F2 data are reported in arbitrary intensity units since they were taken with an uncalibrated trialkali photomultiplier_ In mixtures containing only nob!e gases the spark emission spectrum between 120 and 200 nm is dominated by molecular hydrogen emission. The hydrogen was present in the spark chamber and could be observed throughout the course of their experiments. It is most likely a dissociation product of trace water molecules adsorbed on the walls, and this well known
H2 emission spectrum could easily be subtracted from the total emission spectrum_ The only noble gas emissions observed were the molecular continua of Xe at 150-180 nm and Kr at 125-170 run. 617
Volume 58, number 3
1 October 1978
CHEMICAL PHYSICAL LETTERS
omer IO&Wity
H. Lyman 7RJTorrHe ZQTorrAr
m)TorrHe ZoTorrAr
i
Other
5 Tom SE6
Other unl
Y Lyman
Ii
Jl.;
II
r” I
Sulfur EmAsfons 700 iorr He ZQ Tow AI 5 Tom F2
Fig_ l_ Spark emission spectra in various gas mixtures, as indicated. Emissionin He/Ar was taken at a resolution of 0.01 nm and shows the de*& of the molecular continuum of H,, present in the spark chamber. Other spectra are taken 2t 0.6 nm resolution and oxdy the intense emissions are shown. Where Endicated, other elementalemissio% are Izbelled2nd arise from dissociationof the parentmolecuie. Vertical scale gives absolute emission intetitier Emission from ArF was observed in many samples and is shown in figs. 1 and 2_ This emission was most intense in Fz, but was not detectable in any cf the freons or BF3. The upper limit of ArF emission in these latter cases would then be 10d2 of that in F2_
Previous studies in KrF and XeF [ 1,2] demonstrate a variation in gain and efficiency with fluorine donor molecule that is in qualitative agreement with the variation in ArF fluorescence intensities observed in this work_ We have folund further confnmation for the relevance cf spark induced excimer fluorescence to laser efficiencies in our own d--e sustained laser studies [3], indicating tke spark studies can provide a reliable screening procedure for fluorine donor selection. The application of this tectique to other excimer lasers that require discharge initiated
molecular formation prior to stimulated emission appears promising_ Spark emission spectra in F2 mixtures are shown in fig_ 3 and are similar to spectra produced in e-beam 418
excited noble gas/F;?mixtures [4] _ We were aIso able to obtain the 158 run emission in pure F2, but not in Ar/F2 mixtures. The emission was observed as a function of nob!e gas and fluorine density. Relative to pure fluorine at 40--100 torr, some small enhancement of emissicn was observed when several hundred torr of He or Ne was added to the spark chamber, presumably due to third body equiliiration in the upper bound level. In the case of unmixed F3, little variation in optical emission was observed-as a function of fluorine pressure. The fluorine gas used contained (LMathesonGas Company, reported typical impurity analysis) about 1% N2 as an impurity, and emission from atomic nitrogen at 149.5 run, corresponding to the ‘P + 2D transition, could be used as a rough normalization factor for constant spark gap energy, relative to this nitrogen emission, the F2 emission at 158 nm was constant to within 5% at fluorine pressures between 20 and 1.50torr. Taken together
VoIume 58, number 3
CHEMICAL PHYSICS LETTERS
,
I
I
1
I
L
1
1 October 1978 I
I
t
1
t
700 mrr He 40 rorr Ar 210rr 3Ft
,I
t Source A
k-1
I
t
I
I
000
700 Torr Ct$F. Source A
DDO
700 Torr CH3F. Source B
’
1
I
1
I
/
/’
0
in
Wavelength.A
Fig. 2. Spark emission
spectra
taken at 0.6 nm resolution.See caption fig. 1 for details.
6
with the previously mentioned minor intensity
s
variation with noble gas density, the data argues against the formation of *he upper level occurring by molecular collisions, and supports a one step electron impact exci;ation mechanism for F%. The 158 nm band has not been previously observed in absorption_ It is not likely that the lower
Q
coLvrF2
=
f3 = d I2
2
5 I
16s
A
1s
* 155
lso
us
Alma,
6
Fig. 3. Sparkemissionspectrataken at 1.5 nm resolution. Verticalscab~ vbitrasyunits,unaxrectedforchaugein ek&_rondensitywithgacmixture.
transition Ievel is the X ‘Eg’ state since the required analogous absorption is not observed [5] at photon energies as high as 13 eV. Furthermore, the recently reported [4] stimulated emission at 158 run would require a more deeply bound upper level than the ground state. Not only are no such valence or ionic states predided [6,71, but the observed emission is uncharacteristic of bound-bound radiation_ Representative fluorine levels are given in fig_ 4. The lowest lying ionic state is the 311g, calculated to he at 10.2 eV above the ground state. The 31iu was measured [7] at a vertical energy of 3.2 eV at 1.4 A= re for X ‘IZg. The potential minimum in the ionic level is surely at larger internuclear separation, so that the vertical energy difference between the ‘lTg and 31T,, couid very well correspond to the 419
CHEMICAL
Voiume 5S, number 3
PHYSICS LEl-fERS
1 October 1978
band in 12, whic& has been tentatively assigned [8] to the 3II2u + 3112g transition.
Refenaces
Fig_ 4. Schematic representation of some enew molecular fluorine, from refs. [6,7 ] _
levels in
would make the observed emission analogous to the 301 S nm observed
420
7.9 eV
transition
energy_ This
IS1
161 171
D-E. Roth and R-A. G~bsen, Opt. commun 22 (1977) 265. V. Hasson, CM. Lee, R. Exberger, K-W. BBlman and P.D_ Rowley. Apple Phyr Letters 31<1977) 167. L.J. Denes, L.E. Kline, R.J. Spreadbury. N.T. Melamed, F.Y. Ryan, D.R Suhre, R.R. Mitchell, J.L. Pack,C.L. Chen, P-J. Chantry, R. Scheps and H.S. Feld, Research Program on W Initiated Rare Gas Halide Excimer Lasers, Final TechnicaJ Report, Contract No. DASG60-77-C-0015, Report 77-9C3-UVLASRI, Dec. 1977, Westinghouse R&D Center, Pittshur& PA 15235. J.K. Rice, A.K. Hays and J.R. Woodworth, AppL Phyr Letters 31 (1977) 31. J-L._Cole and J.L. Margrave, J. Mol. Spectry. 43 (1972) 6.5. P.J. Hay and DC_ Cartwright, Chem. Phys Letters 41 (1976) 80. K_ Hijikata. J_ Chem. Phys- 34 (1961) 221; 34 (1961) 231_
181 RS. MuBiken, J. Chem. Phyr 55 (1971) 288.
CHEWCAL
PHYSICS LEMERS
1 October 1978
VACUUM ULTRAVIOLET SPARK EMISSION SPECI’RA OF FLUORINE AND SOME POLYATOMIC FLUORIDES IN NOBLE GASES* R. SCHEPSZ and F. M. RYAN Wtxtingkouse R & D Center, Pittsburgh. Pennsylvania I523-5. USA
Received 2 June 1978 Spark emissionspectra in tie vacuum ultraviolet are reported for severalpoiyatomic fluorides in He + Ar mhtures, and for pure fluorine_ These results are discussed in terms of the potential of a given fluorine compound for ArF formation. The 158 run emission in fluorine is tentatively assised to the 3Ug + 3nu.
We have measured spark emission spectra in the vacuum UV from gas mixtures of He, Ar, and various fluorine compounds as part of a fluorine donor selection study for the rare gas fluoride self-sustained discharge laser. Many fluorine bearing molecules can form rare gas fluoride molecules through the ion-ion recombination and so-called “harpooning” channels so that mixtures for th3 test were selected on the basis of their applicability to the excimer formation process. We found that spark spectra provide a fast and reliable test of excimer formation in a given mixture: The appearance or absence of ArF fluorescence in the spark emission spectrum correlated quite well with excimer fluorescence resulting from a glow discharge_ Fluorine samples observed were F2. SF6, S02F2. NF3, CHsF, N2Fq and BF3 in 700 torr He and 2-20 torr Ar. In addition we observed emission at 158 MI from FdHe, F2/Ne and unmixed fluorine samples_ Emission intensity was observed as a function of fluorine and noble gas density to better understand the population mechanism for the excited F2 level. in this experiment, a sp&k source is mounted near the entrance slit of a 03 m vacuum monochromator and isolated from the vacuum by a MgF2 window. Research grade purity gases are premixed in the inlet * This work was funded in part by Advanced Research Projects Agency under Contract * DASG60-77-C-0015. r Present address: Maxwell Laboratories, Inc., San Diego, California 92193,
USA.
manifold and flow past the spark gap at rates of between 500 and 2500 scc/min. The spark pulse circuit is of the capacitive discharge type. It consists of a 0.0025 PF capacitor charged to 11 kV in series with a 20.5 Q resistor and switched by an SC22 ‘rhyratron in the common grounded carbode configuration_ This circuit provides a 200 A peak 150 ns pulse to the nickel spark electrodes, which were adjusted to a 1 mm gap. A sodium salicylate scintillator on a photodetector is mounted on the exit slit of the monochromator. The spark gap is pulsed at 20 Hz and the phototube output is time averaged before entering the electrometer. in most cases, absolute intensities are reported and were measured with an NBS calibrated vacuum photodiode. The noble gas/F2 data are reported in arbitrary intensity units since they were taken with an uncalibrated trialkali photomultiplier_ In mixtures containing only nob!e gases the spark emission spectrum between 120 and 200 nm is dominated by molecular hydrogen emission. The hydrogen was present in the spark chamber and could be observed throughout the course of their experiments. It is most likely a dissociation product of trace water molecules adsorbed on the walls, and this well known
H2 emission spectrum could easily be subtracted from the total emission spectrum_ The only noble gas emissions observed were the molecular continua of Xe at 150-180 nm and Kr at 125-170 run. 617
Volume 58, number 3
1 October 1978
CHEMICAL PHYSICAL LETTERS
omer IO&Wity
H. Lyman 7RJTorrHe ZQTorrAr
m)TorrHe ZoTorrAr
i
Other
5 Tom SE6
Other unl
Y Lyman
Ii
Jl.;
II
r” I
Sulfur EmAsfons 700 iorr He ZQ Tow AI 5 Tom F2
Fig_ l_ Spark emission spectra in various gas mixtures, as indicated. Emissionin He/Ar was taken at a resolution of 0.01 nm and shows the de*& of the molecular continuum of H,, present in the spark chamber. Other spectra are taken 2t 0.6 nm resolution and oxdy the intense emissions are shown. Where Endicated, other elementalemissio% are Izbelled2nd arise from dissociationof the parentmolecuie. Vertical scale gives absolute emission intetitier Emission from ArF was observed in many samples and is shown in figs. 1 and 2_ This emission was most intense in Fz, but was not detectable in any cf the freons or BF3. The upper limit of ArF emission in these latter cases would then be 10d2 of that in F2_
Previous studies in KrF and XeF [ 1,2] demonstrate a variation in gain and efficiency with fluorine donor molecule that is in qualitative agreement with the variation in ArF fluorescence intensities observed in this work_ We have folund further confnmation for the relevance cf spark induced excimer fluorescence to laser efficiencies in our own d--e sustained laser studies [3], indicating tke spark studies can provide a reliable screening procedure for fluorine donor selection. The application of this tectique to other excimer lasers that require discharge initiated
molecular formation prior to stimulated emission appears promising_ Spark emission spectra in F2 mixtures are shown in fig_ 3 and are similar to spectra produced in e-beam 418
excited noble gas/F;?mixtures [4] _ We were aIso able to obtain the 158 run emission in pure F2, but not in Ar/F2 mixtures. The emission was observed as a function of nob!e gas and fluorine density. Relative to pure fluorine at 40--100 torr, some small enhancement of emissicn was observed when several hundred torr of He or Ne was added to the spark chamber, presumably due to third body equiliiration in the upper bound level. In the case of unmixed F3, little variation in optical emission was observed-as a function of fluorine pressure. The fluorine gas used contained (LMathesonGas Company, reported typical impurity analysis) about 1% N2 as an impurity, and emission from atomic nitrogen at 149.5 run, corresponding to the ‘P + 2D transition, could be used as a rough normalization factor for constant spark gap energy, relative to this nitrogen emission, the F2 emission at 158 nm was constant to within 5% at fluorine pressures between 20 and 1.50torr. Taken together
VoIume 58, number 3
CHEMICAL PHYSICS LETTERS
,
I
I
1
I
L
1
1 October 1978 I
I
t
1
t
700 mrr He 40 rorr Ar 210rr 3Ft
,I
t Source A
k-1
I
t
I
I
000
700 Torr Ct$F. Source A
DDO
700 Torr CH3F. Source B
’
1
I
1
I
/
/’
0
in
Wavelength.A
Fig. 2. Spark emission
spectra
taken at 0.6 nm resolution.See caption fig. 1 for details.
6
with the previously mentioned minor intensity
s
variation with noble gas density, the data argues against the formation of *he upper level occurring by molecular collisions, and supports a one step electron impact exci;ation mechanism for F%. The 158 nm band has not been previously observed in absorption_ It is not likely that the lower
Q
coLvrF2
=
f3 = d I2
2
5 I
16s
A
1s
* 155
lso
us
Alma,
6
Fig. 3. Sparkemissionspectrataken at 1.5 nm resolution. Verticalscab~ vbitrasyunits,unaxrectedforchaugein ek&_rondensitywithgacmixture.
transition Ievel is the X ‘Eg’ state since the required analogous absorption is not observed [5] at photon energies as high as 13 eV. Furthermore, the recently reported [4] stimulated emission at 158 run would require a more deeply bound upper level than the ground state. Not only are no such valence or ionic states predided [6,71, but the observed emission is uncharacteristic of bound-bound radiation_ Representative fluorine levels are given in fig_ 4. The lowest lying ionic state is the 311g, calculated to he at 10.2 eV above the ground state. The 31iu was measured [7] at a vertical energy of 3.2 eV at 1.4 A= re for X ‘IZg. The potential minimum in the ionic level is surely at larger internuclear separation, so that the vertical energy difference between the ‘lTg and 31T,, couid very well correspond to the 419
CHEMICAL
Voiume 5S, number 3
PHYSICS LEl-fERS
1 October 1978
band in 12, whic& has been tentatively assigned [8] to the 3II2u + 3112g transition.
Refenaces
Fig_ 4. Schematic representation of some enew molecular fluorine, from refs. [6,7 ] _
levels in
would make the observed emission analogous to the 301 S nm observed
420
7.9 eV
transition
energy_ This
IS1
161 171
D-E. Roth and R-A. G~bsen, Opt. commun 22 (1977) 265. V. Hasson, CM. Lee, R. Exberger, K-W. BBlman and P.D_ Rowley. Apple Phyr Letters 31<1977) 167. L.J. Denes, L.E. Kline, R.J. Spreadbury. N.T. Melamed, F.Y. Ryan, D.R Suhre, R.R. Mitchell, J.L. Pack,C.L. Chen, P-J. Chantry, R. Scheps and H.S. Feld, Research Program on W Initiated Rare Gas Halide Excimer Lasers, Final TechnicaJ Report, Contract No. DASG60-77-C-0015, Report 77-9C3-UVLASRI, Dec. 1977, Westinghouse R&D Center, Pittshur& PA 15235. J.K. Rice, A.K. Hays and J.R. Woodworth, AppL Phyr Letters 31 (1977) 31. J-L._Cole and J.L. Margrave, J. Mol. Spectry. 43 (1972) 6.5. P.J. Hay and DC_ Cartwright, Chem. Phys Letters 41 (1976) 80. K_ Hijikata. J_ Chem. Phys- 34 (1961) 221; 34 (1961) 231_
181 RS. MuBiken, J. Chem. Phyr 55 (1971) 288.