Some new aspects in stimulated Raman scattering from hydrogen gas

Stimulated Raman scattering (SRS) from high-pressure H
2
gas was observed when giant-pulse ruby-laser radiation of frequency
\tilde{\nu}_{0} = 14 398
cm
^-1
, and its second harmonic of frequency
2\tilde{\nu}_{0}
, were simultaneously focussed on the sample. The well-known SRS from H
2
was observed when only the radiation of frequency
\tilde{\nu}_{0}
was focussed on the sample. When only the second harmonic power was focussed on the sample, SRS was not observed, and power measurements showed that the threshold in this case was considerably higher than the power available. An SRS spectrum identical to that which would be produced by the second harmonic radiation, that is, the appearance of components at the frequencies
2\tilde{\nu}_{0} + n\tilde{\nu}(Q_{1})
, where
\tilde{\nu}(Q_{1}) = 4155
cm
^-1
and
n = 0, \pm 1, \pm 2
..., was observed when both the fundamental and the second harmonic laser power were simultaneously focussed on the sample. Whenever the SRS threshold for the fundamental component was reached, the SRS of the second harmonic occurred at excitation powers estimated to be two orders of magnitude below the normal threshold power. These results and others previously reported on the generation of SRS vibration-rotation combination lines in H
2
are discussed, and the multiple-photon resonant interaction mechanisms based on the classical or semiclassical theory of molecular motions and light modulation are proposed for the generation of the observed lines.