  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.
