Coherent THz Phonons Driven by Light Pulses and the Sb Problem: What is the Mechanism?

Femtosecond laser pulses generate in Sb coherent $E_g$ phonons at $\approx 3.4\mathrm{THz}$, in addition to oscillations of $A_{1g}$ symmetry accounted for by the phenomenological displacive-excitation model. Experiments agree with theoretical calculations showing that the coherent driving force in absorbing materials like Sb is determined by Raman processes, as in transparent media. The Raman formalism provides a unifying approach for describing light-induced motion of atoms of both impulsive and displacive character.