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The atomic fluorine laser: Spectral pressure dependence
Volume
2 1, number
OPTICS COMMUNICATIONS
2
May 1971
THEATOMICFLUORINELASER:SPECTRALPRESSUREDEPENDENCE* T.R. LOREE and R.C. SZE University of Gdifor??ia, Los Alamos Scientific Laboratory, P.O. Box 1663, Los Alamos, New Mexico 87545, USA
Received
22 February
1971
Atomic fluorine has been lased at pressures up to 30 psia in a double discharge laser with vigorous flashboard pre-ionization. The spectral output of the laser is a function of pressure, with low pressure favoring doublet transitions and high pressure inducing new laser transitions in the quartet manifold.
The lasing of atomic fluorine has been sporadically investigated [l-8] under a variety of lasing conditions, none of which resulted in the same spectral output. We have utilized yet another type of device to lase this species, but our results offer some clues that aid in understanding the spectral behavior of this laser. In the course of a series of experiments on the lasing of ArF in a modified Tachisto double discharge laser ]9] , we required the background fluorescent emission from a standard ArF gas mix with the Ar removed. This mix (0.2% F2/1 .O% Ne/98.8% He) lased superradiantly in the red, although the ArF is not superradiant. Spectral analysis of the lasing, and gas mix variations, convinced us that the lasing was from atomic fluorine, although the initial output (at 30 psia) contained only one one line (of four) in common with the reported fluorine laser wavelengths. The Tachisto device contains 60-cm discharge electrodes separated by 1.5 cm, with preionizing uv provided by a flashboard at one side on the mid-plane. It was designed as a TEA CO2 laser, but has rather fast (-50 ns),discharge risetime, making it useful for lasing some excimers [lo] . For this atomic fluorine experiment the flashboard was fired by the discharge of a 40-nF capacitor, resulting in an optical pulse risetime of about 0.5 ps. After some parametric studies we obtained maximum energy output from a mix of 2.0% F2 in He. * Work performed
under
the auspices
of the U.S. ERDA.
Under normal conditions the main discharge capacitors were charged to 27 kV and the discharge was tired 2 ps after the flashboard. At 30 psia the lasing output was rather insensitive to this delay, and became less sensitive as the pressure decreased until at 3 psia the flashboard was not required. The optimum energy into the flashboard obviously varied with pressure; at 30 psia maximum lasing energy was obtained when the flashboard supply was charged to about 18 kV. Under these conditions 0.60 mJ was obtained on all lasing lines in an 8 ns (FWHM) pulse, giving on the order of 75 kW. The same optical cavity was used at all pressures, consisting of a 5-m radius of curvature broadband high reflector (R > 99%, 635-780 nm) and a flat uncoated quartz output coupler. The remarkable results that emerged from the pressure variation studies are illustrated in fig. 1, in which are shown the lasing spectra obtained from various pressures of a 0.8% F2/99.2% He mix. The relative amplitudes of the various lasing lines are roughly indicated. Note that lines actually appear and extinguish as the pressure is changed. The lines listed in parentheses have been reported by other workers, but did not lase in this experiment at any pressure. In a sense, we see that the laser is pressure tunable over a rather large spectral range. The lasing spectra were detected by a PAR optical multichannel analyzer at the output of a 1/4-m spectrometer with a 148-line/mm grating. This combination results in a reasonably linear amplitude response, allow255
Volume 2 1, number 2
OPTICS COMMUNICATIONS
20 psi0
I IO psia
:
.c Q 5
.62
I
.64
.66
.66
.70 ,I
I,
.?2
I
.74 I,,
.76
70
Air Wavelength, microns Fig. 1. Schematic comparison of the spectral lasing output at various pressures. Line amplitudes are roughly indicated by height.
Table 1 Fluorine lasing lines __~_ h(air) [ll]
ing the rough comparison of line amplitudes of fig. 1. Wavelength accuracy from this detector system was + 5 A, so precise wavelengths were assumed to be the strong lines (within those limits) listed in the tables of Zaidel’ et al. [ 111 . Table 1 is a compilation of all the fluorine lasing lines observed to date (excluding those in ref. [I] , which are too inconclusive) and our assignments for the associated transitions. In most cases these agree with the assignments of previous experimenters. All air wavelengths are taken from Zaidel’. The vacuum wavelengths derived from the air values agree well in all cases with the wavelengths calculated from the the assigned transitions, using the energy level values listed by Moore [ 121 . The transitions are shown more graphically in fig. 2. It is clear that in previous work only doublet states have lased, but that the higher pressures of our experiment induced lasing in the quartet manifold. We conclude that as the pressure is increased, collisions of the excited F atoms transfer population from the doublet to the quartet states. The most likely collision partner for this process is another F atom [ 131 The data are consistent with the conservation of angular momentum in the transfer, as we observe: 1) D States: no lasing is observed in either manifold. 2) S States: the weaking of the 2S (73 11) line is matched by growth of the 4S (6349) line, but
_ _~_~__~~~__._~~~~._ .~
____--
Calc. h (vat) [ 121
Observers
- 3s4p3I2
6350.25
[this work]
- 3s2Ps,2
6968.30
]5,71
7039.39
3pZP&, - 3s2Pa,2
7039.43
~2-5~71
7129.85
3pap;,,
- 3s2P,,2
7129.87
[this work, 2-71
7204.35
3pap;,,
- 3s2Ps,2
7204.35
[2,3,5,71 [this work, 4,5,7]
h (vat)
Transition
6348.50
6350.25
3p 4 s,,, 0
6966.35
6968.27
3pap;,,
1031.45 7127.89 7202.31
____~___~~_
May 1977
_
______
7311.02
7313.03
3p?.~,,
- 3s2Pa,a
7313.04
7398.68
7400.12
3p4Pi,,
- 3PP,,,
7400.72
[this work]
7489.14
7491.20
3p%l~,, - 3s2P,,2
7491.20
]71
7552.24
7554.32
3p4P&, - 3s4Ps,2
7554.30
[this work]
7754.70
1156.83
]71
7802.36
3p2D;,, - 3~‘Ps,~ ’3~2 Dal2 0 - 3~‘Pr,~ ___.
1156.83
1800.22
~.___
256
7802.35 _____-._____
[3,71
--
Volume 21, number 2
OPTICS COMMUNICATIONS
3f 25 a’4
May 1977
at low pressure argues that a different primary excitation mechanism is operative in the low pressure regime [8]. In conclusion, we have observed pressure-tunable lasing in atomic fluorine, with several new quartet manifold lines appearing at high pressures. The obtainable power levels are suitable for infrared dye pumping or photochemical applications. We would like to acknowledge the helpful contributions of SD. Rockwood and C.A. Brau and the invaluable technical support of D.L. Barker.
References F Doublets [II P.K. Cheo and H.G. Cooper, Appl. Phys. Letters 7 (1965) 202.
(21 M.A. Kovacs and C.J. Ultee, Appl. Phys. Letters 17 F Ouortets
Fig. 2. Energy levels of fluorine and all observed lasing transitions. Also shown is the metastable helium 3S, level reduced by the F, dissociation energy.
as the states have nearly the same energy the 2S line is never extinguished. 3) P States: in this case the 4P state is much lower in energy, and the 2P (7 128) line disappears completely as the 4P (7399 and 7552) lines grow. While not conclusive, the high-pressure data suggest that the primary excitation is collisional dissociation of the F2 by He atoms in the metastable 3S, state. This level (reduced by the dissociation energy of F2) is shown in fig. 2. Our observation that vigorous flashboard pre-ionization (and pre-dissociation) is required at high pressure but no flashboard is required
(1970) 39. [31 W.Q. Jeffers and C.E. Wiswall, Appl. Phys. Letters 17 (1970) 444. [41 A.E. Florin and R.J. Jensen, IEEE J. Quant. Electron QE-7, (1971) 472. [51 I.J. Bigio and R.F. Begley, Appl. Phys. Letters 28 (1976) 263. [61 D.G. Sutton, L. Galvan, P.R. Valenzuela and S.N. Suchard, IEEE J. Quant. Electron, QE-11 (1975) 54. [71 L.O. Hacker and T.B. Phi, Appl. Phys. Letters 29 (1976) 493. ISI L.O. Hacker, in press. PI Tachisto Model Tat II, Tachisto, Inc., Needham, MA. 1101 R. Burnham and N. Djeu, Appl. Phys. Letters 29 (1976) 11,707. [Ill A.N. Zaidel’, V.K. Prokofev, S.M. Raiskii, B.A. Slavnyi and E.Ya Schreider, Tables of Spectral Lines (English translation Plenum, New York, 1970). [ 12) C.E. Moore, Atomic Energy Levels, Nat?. Bur. Stds. Circ. 467 (U.S. GPO, Washington, D.C., 1962). [ 131 R.T. Pack, private communication.
257
2 1, number
OPTICS COMMUNICATIONS
2
May 1971
THEATOMICFLUORINELASER:SPECTRALPRESSUREDEPENDENCE* T.R. LOREE and R.C. SZE University of Gdifor??ia, Los Alamos Scientific Laboratory, P.O. Box 1663, Los Alamos, New Mexico 87545, USA
Received
22 February
1971
Atomic fluorine has been lased at pressures up to 30 psia in a double discharge laser with vigorous flashboard pre-ionization. The spectral output of the laser is a function of pressure, with low pressure favoring doublet transitions and high pressure inducing new laser transitions in the quartet manifold.
The lasing of atomic fluorine has been sporadically investigated [l-8] under a variety of lasing conditions, none of which resulted in the same spectral output. We have utilized yet another type of device to lase this species, but our results offer some clues that aid in understanding the spectral behavior of this laser. In the course of a series of experiments on the lasing of ArF in a modified Tachisto double discharge laser ]9] , we required the background fluorescent emission from a standard ArF gas mix with the Ar removed. This mix (0.2% F2/1 .O% Ne/98.8% He) lased superradiantly in the red, although the ArF is not superradiant. Spectral analysis of the lasing, and gas mix variations, convinced us that the lasing was from atomic fluorine, although the initial output (at 30 psia) contained only one one line (of four) in common with the reported fluorine laser wavelengths. The Tachisto device contains 60-cm discharge electrodes separated by 1.5 cm, with preionizing uv provided by a flashboard at one side on the mid-plane. It was designed as a TEA CO2 laser, but has rather fast (-50 ns),discharge risetime, making it useful for lasing some excimers [lo] . For this atomic fluorine experiment the flashboard was fired by the discharge of a 40-nF capacitor, resulting in an optical pulse risetime of about 0.5 ps. After some parametric studies we obtained maximum energy output from a mix of 2.0% F2 in He. * Work performed
under
the auspices
of the U.S. ERDA.
Under normal conditions the main discharge capacitors were charged to 27 kV and the discharge was tired 2 ps after the flashboard. At 30 psia the lasing output was rather insensitive to this delay, and became less sensitive as the pressure decreased until at 3 psia the flashboard was not required. The optimum energy into the flashboard obviously varied with pressure; at 30 psia maximum lasing energy was obtained when the flashboard supply was charged to about 18 kV. Under these conditions 0.60 mJ was obtained on all lasing lines in an 8 ns (FWHM) pulse, giving on the order of 75 kW. The same optical cavity was used at all pressures, consisting of a 5-m radius of curvature broadband high reflector (R > 99%, 635-780 nm) and a flat uncoated quartz output coupler. The remarkable results that emerged from the pressure variation studies are illustrated in fig. 1, in which are shown the lasing spectra obtained from various pressures of a 0.8% F2/99.2% He mix. The relative amplitudes of the various lasing lines are roughly indicated. Note that lines actually appear and extinguish as the pressure is changed. The lines listed in parentheses have been reported by other workers, but did not lase in this experiment at any pressure. In a sense, we see that the laser is pressure tunable over a rather large spectral range. The lasing spectra were detected by a PAR optical multichannel analyzer at the output of a 1/4-m spectrometer with a 148-line/mm grating. This combination results in a reasonably linear amplitude response, allow255
Volume 2 1, number 2
OPTICS COMMUNICATIONS
20 psi0
I IO psia
:
.c Q 5
.62
I
.64
.66
.66
.70 ,I
I,
.?2
I
.74 I,,
.76
70
Air Wavelength, microns Fig. 1. Schematic comparison of the spectral lasing output at various pressures. Line amplitudes are roughly indicated by height.
Table 1 Fluorine lasing lines __~_ h(air) [ll]
ing the rough comparison of line amplitudes of fig. 1. Wavelength accuracy from this detector system was + 5 A, so precise wavelengths were assumed to be the strong lines (within those limits) listed in the tables of Zaidel’ et al. [ 111 . Table 1 is a compilation of all the fluorine lasing lines observed to date (excluding those in ref. [I] , which are too inconclusive) and our assignments for the associated transitions. In most cases these agree with the assignments of previous experimenters. All air wavelengths are taken from Zaidel’. The vacuum wavelengths derived from the air values agree well in all cases with the wavelengths calculated from the the assigned transitions, using the energy level values listed by Moore [ 121 . The transitions are shown more graphically in fig. 2. It is clear that in previous work only doublet states have lased, but that the higher pressures of our experiment induced lasing in the quartet manifold. We conclude that as the pressure is increased, collisions of the excited F atoms transfer population from the doublet to the quartet states. The most likely collision partner for this process is another F atom [ 131 The data are consistent with the conservation of angular momentum in the transfer, as we observe: 1) D States: no lasing is observed in either manifold. 2) S States: the weaking of the 2S (73 11) line is matched by growth of the 4S (6349) line, but
_ _~_~__~~~__._~~~~._ .~
____--
Calc. h (vat) [ 121
Observers
- 3s4p3I2
6350.25
[this work]
- 3s2Ps,2
6968.30
]5,71
7039.39
3pZP&, - 3s2Pa,2
7039.43
~2-5~71
7129.85
3pap;,,
- 3s2P,,2
7129.87
[this work, 2-71
7204.35
3pap;,,
- 3s2Ps,2
7204.35
[2,3,5,71 [this work, 4,5,7]
h (vat)
Transition
6348.50
6350.25
3p 4 s,,, 0
6966.35
6968.27
3pap;,,
1031.45 7127.89 7202.31
____~___~~_
May 1977
_
______
7311.02
7313.03
3p?.~,,
- 3s2Pa,a
7313.04
7398.68
7400.12
3p4Pi,,
- 3PP,,,
7400.72
[this work]
7489.14
7491.20
3p%l~,, - 3s2P,,2
7491.20
]71
7552.24
7554.32
3p4P&, - 3s4Ps,2
7554.30
[this work]
7754.70
1156.83
]71
7802.36
3p2D;,, - 3~‘Ps,~ ’3~2 Dal2 0 - 3~‘Pr,~ ___.
1156.83
1800.22
~.___
256
7802.35 _____-._____
[3,71
--
Volume 21, number 2
OPTICS COMMUNICATIONS
3f 25 a’4
May 1977
at low pressure argues that a different primary excitation mechanism is operative in the low pressure regime [8]. In conclusion, we have observed pressure-tunable lasing in atomic fluorine, with several new quartet manifold lines appearing at high pressures. The obtainable power levels are suitable for infrared dye pumping or photochemical applications. We would like to acknowledge the helpful contributions of SD. Rockwood and C.A. Brau and the invaluable technical support of D.L. Barker.
References F Doublets [II P.K. Cheo and H.G. Cooper, Appl. Phys. Letters 7 (1965) 202.
(21 M.A. Kovacs and C.J. Ultee, Appl. Phys. Letters 17 F Ouortets
Fig. 2. Energy levels of fluorine and all observed lasing transitions. Also shown is the metastable helium 3S, level reduced by the F, dissociation energy.
as the states have nearly the same energy the 2S line is never extinguished. 3) P States: in this case the 4P state is much lower in energy, and the 2P (7 128) line disappears completely as the 4P (7399 and 7552) lines grow. While not conclusive, the high-pressure data suggest that the primary excitation is collisional dissociation of the F2 by He atoms in the metastable 3S, state. This level (reduced by the dissociation energy of F2) is shown in fig. 2. Our observation that vigorous flashboard pre-ionization (and pre-dissociation) is required at high pressure but no flashboard is required
(1970) 39. [31 W.Q. Jeffers and C.E. Wiswall, Appl. Phys. Letters 17 (1970) 444. [41 A.E. Florin and R.J. Jensen, IEEE J. Quant. Electron QE-7, (1971) 472. [51 I.J. Bigio and R.F. Begley, Appl. Phys. Letters 28 (1976) 263. [61 D.G. Sutton, L. Galvan, P.R. Valenzuela and S.N. Suchard, IEEE J. Quant. Electron, QE-11 (1975) 54. [71 L.O. Hacker and T.B. Phi, Appl. Phys. Letters 29 (1976) 493. ISI L.O. Hacker, in press. PI Tachisto Model Tat II, Tachisto, Inc., Needham, MA. 1101 R. Burnham and N. Djeu, Appl. Phys. Letters 29 (1976) 11,707. [Ill A.N. Zaidel’, V.K. Prokofev, S.M. Raiskii, B.A. Slavnyi and E.Ya Schreider, Tables of Spectral Lines (English translation Plenum, New York, 1970). [ 12) C.E. Moore, Atomic Energy Levels, Nat?. Bur. Stds. Circ. 467 (U.S. GPO, Washington, D.C., 1962). [ 131 R.T. Pack, private communication.
257