  We have applied femtosecond pump-probe technique with variable pump
wavelength to study coherent lattice dynamics in Bi single crystal. Comparison
of the coherent amplitude as a function of pump photon energy for two different
in symmetry Eg and A1g phonon modes with respective spontaneous resonance Raman
profiles reveals that their generation mechanisms are quite distinct. We show
that displacive excitation, which is the main mechanism for the generation of
coherent A1g phonons, cannot be reduced to the Raman scattering responsible for
the generation of lower symmetry coherent lattice modes
