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

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 Xg+1E,Fg+1 transition, has been observed. The shortest wavelength seen in the stimulated-emission spectrum was 117.6 nm corresponding to the Cu1Xg+1(2-5) Q(2) transition. The Cu1 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,Fg+1B u+1 band. Two entirely separate radiative excitation channels are observed to play important roles in the state-selective molecular population of the E,Fg+1 level. One involves two 193-nm quanta in the XE,F amplitude while the other process combines a 193-nm quantum with a first Stokes-shifted photon in H2. The optical Stark effect was seen to play a significant role in the excitation process with shifts of molecular resonances as large as 45 cm-1. Substantial deviations from Born-Oppenheimer behavior, resulting in a dramatic 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,Fg+1 state. The maximum energy observed in the strongest stimulated line was 100 J, a value corresponding to an energy conversion efficiency of 0.5%. The pulse duration of the stimulated emission is estimated from collisional data to be 10 ps, a figure indicating a maximum converted vacuum-ultraviolet power of 10 MW. © 1983 The American Physical Society.