Excitation Processes and Relaxation Rates in the Pulsed Water Vapor Laser

Abstract

This paper presents an experimental study of time-resolved gain in H(2)O, H(2)O-He, and H(2)O-H(2) mixtures as a function of gas composition and excitation current. Utilizing the fast rising (~70 nsec) pulse from H(2)O-He laser as a probe, the amplifier gain was measured with a time resolution of about 100 nsec. The gain was observed to follow the excitation current pulse rather closely indicating that population inversion was established in times less than 100 nsec. This suggested that excitation was most likely by means of rapid cascading from higher levels and/or by direct electron impact. The gain was found to be describable by a two-level rate equation model containing one dominant relaxation rate and assuming immediate excitation of the levels involved by inelastic collisions with electrons. With pure H(2)O, the relaxation rate was proportional to pressure to within 10%, indicating that the upper level was de-excited primarily by c llisions with other H(2)O molecules. At a pressure of 1 Torr the relaxation rate in pure H(2)O was 0.35 +/- 0.05 for the 28-microm transition. The addition of small amounts of foreign gases was observed to increase this relaxation rate, consistent with the measured decrease in the amplifier gain. By subsequently increasing the water vapor pressure it was found possible to optimize the gain at an enhanced level over the pure H(2)O case. The peak gain obtained in water vapor at 1000 A was 0.34 m(-1). Under foreign gas addition this increased to 0.68 m(-l) for the same peak current. In this case the relaxation rate, as a function of the foreign gas (He or H(2)) pressure, remained constant to within 10%, suggesting that these gases at higher concentrations may enhance the system gain by altering the discharge conditions without appreciably collisionally de-exciting the upper laser level.

Potential for a hydrogen water-plasma laser

“…12 In addition, helium-water plasmas as well as water-plasma lasers were explored for a source of submillimeter wavelengths. 13 More recently, emission from OH* radicals in water and heliumhydrogen water plasmas has been investigated as an efficient source of radiation in the region Ͻ2000 Å for the replacement of expensive working media based on krypton and xenon in microelectronics, photochemistry, and medical applications. 14 These prior water-plasma light sources were based on high-voltage glow discharges.…”

Vibrational energy transfer in H2O-H2-O2 mixtures for gas cooled rapidly in supersonic nozzles

“…As the experimental results of [3,8] showed, it is necessary to take into account the rapid but finite nonresonance transfer between the modes v I and ~3" This is also indicated by the results of measurements of the probabilities of Ul § 2~2 and ~3 § 2u 2 collisional transitions [3].…”

“…The high radiative probabilities characteristic of such molecules 42] make it possible to have acceptable values of the gain even at a low density of the particles with population inversion [3]. The most powerful stimulated emission is observed in electrodischarge systems based on vibrational transitions for the R branch in the H20 molecule.…”

“…Model of Vibrational Energy Transfer in H20-H2~_2 Mixtures It was shown earlier [3,7,8] in an investigation of the mechanism of vibrational energy transfer in the H20 molecule that the most probable relaxation channels for the energy stored in the symmetric and asymmetric modes (~i and v 3) is through transition of quanta to the deformation mode (u § 2u ) and (~ + 2u ), and then to translational degrees I 2 . Model of Vibrational Energy Transfer in H20-H2~_2 Mixtures It was shown earlier [3,7,8] in an investigation of the mechanism of vibrational energy transfer in the H20 molecule that the most probable relaxation channels for the energy stored in the symmetric and asymmetric modes (~i and v 3) is through transition of quanta to the deformation mode (u § 2u ) and (~ + 2u ), and then to translational degrees I 2 .…”

Vibrational relaxation of H2O by H2, HCl, and H2O at 295 K

“…11 LIF measurements have provided detailed information on relaxation of and among the I, 2, 2,v2, and P 3 vibrational levels. An unusual picture has emerged of very efficient V-*T,R relaxation of the bending vibrational levels and less efficient V-.V relaxation between bending and stretching modes.…”