Vacuum-ultraviolet stimulated emission from two-photon-excited molecular hydrogen

Abstract

Intense vacuum-ultraviolet stimulated emission in molecular hydrogen, on both the Lyman and Werner bands, following excitation by two-quantum absorption at 193 nm on the X /sup 1/..sigma../sub g//sup +/..-->..E,F /sup 1/..sigma../sub g//sup +/ transition, has been observed. The shortest wavelength seen in the stimulated-emission spectrum was 117.6 nm corresponding to the C /sup 1/Pi/sub u/..-->..X /sup 1/..sigma../sub g//sup +/ (2-5) Q(2) transition. The C /sup 1/Pi/sub u/ state appears to be populated with a mechanism involving electron collisions. The radiative cascade mechanism found to lead to the vacuum-ultraviolet emissions on the Lyman band also causes strong infrared stimulated emission to occur on the E,F /sup 1/..sigma../sub g//sup +/..-->..B /sup 1/..sigma../sub u//sup +/ band. Two entirely separate radiative excitation channels are observed to play important roles in the state-selective molecular population of the E, F /sup 1/..sigma../sub g//sup +/ level. One involves two 193-nm quanta in the Xreverse arrowE,F amplitude while the other process combines a 193-nm quantum with a first Stokes-shifted photon in H/sub 2/. The optical Stark effect was seen to play a significant role in the excitation process with shifts of molecular resonances as large as approx.45 cm/sup -1/. Substantial deviations from Born-Oppenheimer behavior, resulting in a dramaticmore » shift of the stimulated spectrum depending upon the excited-state rotational quantum number, were clearly observed for molecular levels close to the potential maximum separating the inner and outer wells of the E,F /sup 1/..sigma../sub g//sup +/ state. The maximum energy observed in the strongest stimulated line was approx.100 ..mu..J, a value corresponding to an energy conversion efficiency of approx.0.5%. The pulse duration of the stimulated emission is estimated from collisional data to be approx.10 ps, a figure indicating a maximum converted vacuum-ultraviolet power of approx.10 MW.« less