FARINFRARED LASING IN H2S, OCS, AND SO2J. C. Hassler and P. D. Coleman Citation: Applied Physics Letters 14, 135 (1969); doi: 10.1063/1.1652747 View online: http://dx.doi.org/10.1063/1.1652747 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/14/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Farinfrared ellipsometer Rev. Sci. Instrum. 64, 870 (1993); 10.1063/1.1144135 Tunable farinfrared laser spectroscopy of deuterated isotopomers of Ar–H2O J. Chem. Phys. 94, 824 (1991); 10.1063/1.460308 New interpretation of the farinfrared SO2 laser spectrum J. Appl. Phys. 46, 2620 (1975); 10.1063/1.321939 ASSIGNMENTS OF THE FARINFRARED SO2 LASER LINES Appl. Phys. Lett. 18, 511 (1971); 10.1063/1.1653517 Farinfrared spectroscopy Phys. Today 23, 44 (1970); 10.1063/1.3022333

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Volume 14, Number 4 APPLIED PHYSICS LETTERS IS February 1969

FAR-INFRARED LASING IN "2 S, OCS, AND S02 * J. C. Hassler and P. D. Coleman Electro- Physics Laboratory, Department of Electrical Engineering University of illinois Urbana, Illinois 61801 (Received 10 January 1969)

Twenty-four lasing lines have been found in an H2S laser in the range 33 to 225 /l, three lines in S02 at 141, 151, and 215 /l, and two lines in OCS at 123 and 132 /l. Comparing H2S to H20, the pulse power outputs are about equal and the lines about as numerous. However, while the H20 lines es­sentially lase during the exciting 5-lJ.Sec current pulse, the H2S lines turn on about 20-30 lJ.Sec after the 5-lJ.Sec current pulse and last for 20 to 100 lJ.Sec.

Since the first papers on H20l and HCN2 in 1964, the list of far-infrared lasing molecules has grown very little. Also, it has taken some 3 to 4 years to find a satisfactory explanation3

,4 for the major las­ing lines in these moiecules. In both of these mole­cules, an irregular Fermo-Dennison perturbations occurs between rotational levels of the stretch and o,vertone bending modes which have about the same energy and the same J and symmetry. This pertur­bation results in a mixing of the wavefunctions to strongly increase the vibrational-rotational trans­ition probabilities.

Necessary conditions for laSing in these mole­cules include: (1) A population inversion between VI or V3 and 2V2 caused by the rapid relaxation of 2V2' (2) A perturbational mixing of a vibration­rotatiOn level to increase the transition probability between VI or V3 and 2V2'

Both H20 and H2 S are asymmetric rotors of about the same asymmetry (K ~ - 0.4 for H20 and K ~ 0.5 for H2S), with roughly the same relation­ship between the lower vibrational levels 5

,6 as in­dicated in Table I.

Table I. Lower vibrational states in H20 and H2S.

3651. 7 3755.8 3151.4

2614.56 2627.48 2353.93

One difficulty in trying to make H2S lase, as contrasted to H20, is in the chemistry of the two molecules. Given a certain pulse current and flow rate of H20 in a discharge, HP will quickly form a quasi-steady state of breakdown and recombina­tion products (H20, H2 , O2 , H20 2 , etc.) all of which are gases. However, H2S forms solid sulfur which no longer partiCipates in the resultant steady state with the result that H2S is rapidly removed from the system leaving only H2 •

*This research was supported by the Electronics Division of the U.S. Air Force Office of Scientific Research.

If these assertions are correct, one must adjust the experimental conditions so that H2S is not de­stroyed too rapidly in the discharge. Favorable conditions might be expected to include: a low pulse repetition frequency, low discharge power, high gas flow rate and/or pumping speed.

The laser equipment used has been described previously,7 the only exception being that two 250-liter /min vacuum pumps were used in parallel to increase the pumping speed. A 0.5-m B and L grat­ing spectrometer and a Ga:Ge-cooled detector were used to observe the lasing signals in the range 30 to 200 /l.

Lasing was observed in H2S using the conditions: prf = 2 Hz,P = 0.15 Torr, and V= 3.6 KV. If the prf was raised much above 2 Hz, no lasing could be ob­served. Also if V was increased, the lasing power output dropped, although it was not a sharp fUnction of V. The power output was fairly insensitive to pressure. However, if N2 , He, or H2 were added, the power output decreased.

In the same laSing system, H20 by comparison ran best for prf > 50 Hz, P ~ 0.15 Torr, and V ~ 6 KV.

Table II lists the H2S lines that have been ob­served to date. The air wavelengths are accurate to slightly less than 1 %. An adequate pulse stretcher was not available at the time of the experiment which made it difficult to drive a recorder because of the low average power resulting from 2-Hz prf. Hence, the wavelengths were determined by observ­ing the peak signal pulse height on an osciiloscope while scanning the monchromator by hand.

The relative lasing line power outputs are only approximate. They have been corrected for detec­tor sensitivity changes with wavelength. 8

The time behavior of the H2S signals is quite different from that of H20.9 In H20, driven by a 5-lJ.Sec current pulse, lasing generally starts 2 to 5 lJ.Sec after the leading edge of the current pulse, and lasts from 2 to 6 lJ.Sec. In H2S, as seen from Table II, lasing begins from 20 to 40 lJ.Sec after the trailing edge of the current pulse, and lasts floom 20 to as much as 100 lJ.Sec. This would indicate either a different excitation mechanism, radically different relaxation times in the molecule, or a metastable level feeding the laSing levels.

135

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Volume 14, Number 4 APPLIED PHYSICS LETTERS 15 February 1969

Table II. Time behavior and relative strengths of tional levels. Unfortunately, the V3 levels have never been observed in absorption. 6 Hence, experimental data do not exist. However, good matches have

H2S lasing lines (V = 3.6 KV, P = 0.15 Torr, prf = 2 Hz).

Air wavelength T Delay T Duration Relative been made on 21 lines using only the (VI - 2V2) transitions. Work is in progress to verify these aSSignments. (Microns) (fJ.Sec) (fJ.Sec) strength

33.47 30 30 0.6 33.64 18 40 480 49.62 20 40 1 52.40 30 40 60 56.84 25 10 60 60.29 25 40 200 61.50 25 100 1000 73.52 18 40 0.2 80.50 25 40 220 83.43 30 30 0.1 87.47 25 100 1000 92.00 20 40 0.1 96.38 25 40 3

103.3 30 50 125 108.8 30 100 7 116.8 20 60 4 126.2 30 20 2 129.1 20 40 4 130.8 20 20 1 135.5 20 60 2 140.6 25 30 10 162.4 30 30 560 192.9 20 60 20 225.3 25 60 1000

By analogy with H20, one might expect that the

Preliminary experiments with S02 10 and OCS have yielded lines at 141,151, and 215 11 for S02 and lines at 123 and 132 11 in OCS in the presence of a buffer gas. (He, N2, O2) and low prfs (~2 Hz).

The authors wish to thank the members of the Electro-Physics Laboratory for their help, in par­ticular, Curt Wittig, Don Akitt, and Tom Newkirk.

1 A Crocker, M. F. Kimmitt, H. A. Gebbie, and L. E. S. Mathias, Nature 201, 250 (1964).

2 H. A. Gebbie, N. W. B. Stone, and F. D. Finlay, Nature 202, 685 (1964).

3 D. R. Lide, Jr., and A. G. Maki, Appl. Phys. Letters 11, 62 (1967).

4W. S. Benedict, M. A. Pollack, and W. J. Tom­linson, IEEE. J. Quantum Electron. (to be published).

5 G. Herzberg, Infrared and Raman SPectra of Polyatomic Molecules (D. Van Nostrand Book Co., Inc., Princeton, N.J., 1945).

6 H. C. Allen, Jr., and E. K. Plyler, J. Chern. Phys. 25, 1132 (1956).

7 D. P. Akitt, W. Q. Jeffers, and P. D. Coleman, Proc. IEEE 54, 547 (1966).

BW. Q. Jeffers and C. J. Johnson, Appl. Opt. 7, 1859 (1968).

9W. Q. Jeffers and P. D. Coleman, Appl. Phys. Letters 10, 7 (1967).

assignment of the 24 lasing lines of H2S could be lOS. F. Dyubbo, V. A. Svich, and R. A. Valitov, JETP Letters 7, 320 (1968). made between the (VI - 2V2) and (V3 - 2V2) vibra-

TRANSIENT AND STEADY STATE THERMAL SELF-FOCUSING

R. L. Carman* Gordon McKay Research Laboratory, Harvard University Cambridge, Massachusetts 02138 and Lincoln Laboratory, t Massachusetts Institute of Technology Lexington, Massachusetts 02173 and A. Mooradian, P. L. Kelley,t and A. Tufts Lincoln Laboratory, t Massachusetts Institute of Technology, Lexington, Massachusetts 02173 (Received 13 December 1968)

Self-focusing of a cw argon laser beam due to absorptive heating in glass is reported here, with particular emphasis on the time dependence.

Beam self-trapping1 has been observed in liquids2

and in glasses and crystals3 using high-power Q-switched lasers. In liquids, the largest trapping ef­fects were found to occur for anisotropically polar­izable molecules such as CS2 ; while in the glasses and crystals the trapping (after external focusing) has been attributed to electrostriction. In other ex­periments,4 absorptive heating of liquids produced by low-power cw beams was seen to give rise to

*Work partially supported by a NASA grant. tOperated with support from the U.S. Air Force. tTemporary address: Department of Physics,

University of California, Berkeley, Calif. 94720.

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