Coherent microwave radiation emitted during the process of stimulated Raman scattering in weakly compressed hydrogen : experimental studies, together with an attempt to interpret this microwave emission

Abstract

A coherent microwave radiation, concomitant with experiments of stimulated Raman scattering (SRS) in weakly compressed hydrogen, had been observed; qualitative and quantitative results had been obtained, and are given in this article. .An attempt to interpret this emission on a phenomenological basis is presented, on the basis of a model in which the influence of hydrogen molecule anharmonicity is taken into account, leading to two main effects:1) possible existence of an intermediate state involving a vibrationally excited quasiparticle system, the lifetime of which should be very short (0.1-1 ps); 2) nonlinear coupling between the Stokes light wave and the longitudinal propagating waves associated to the quasiparticle system. Moreover, several processes are also to be taken into account; existence of a macroscopic longitudinal electric field, due to coulombian interactions; electrostriction effect giving rise to a "piezoelectric" character for the quasiparticle system; Cherenkov-type and anomalous relativistic Doppler effects. Then, the source of the coherent microwave radiation might be the in phase quadrature part of the longitudinal polarisation associated to the quasiparticle waves, and its emission should appear like parametric instabilities. Concerning the SRS process itself, an additional contribution to the Raman gain should arise from the laser induced electro-optic effect.