Coherent phonon generation and the two stimulated Raman tensors

We show that stimulated Raman scattering by phonons is described by two separate tensors, one of which accounts for the phonon-induced modulation of the susceptibility, and the other one for the dependence of the coherent mode amplitude on the light intensity. These tensors have the same real component, associated with impulsive generation of phonons, but different imaginary parts. If the imaginary term dominates, the mechanism for two-band processes is displacive in nature. Femtosecond pump-probe experiments on antimony, using laser energies in the range of the $E_2^{\prime }$ gap, support the two-tensor model.