Resonant generation of coherent phonons in Sb and GaSe

Summary form only given. The generation of coherent optical phonons in molecules and solids has attracted much attention since practical and reliable femtosecond lasers became available. While there is consensus that impulsive stimulated Raman scattering (ISRS) is the mechanism responsible for the generation of coherent phonons in transparent materials, the relevant processes in opaque substances have not yet been conclusively elucidated. Mechanisms other than ISRS have been previously suggested. The leading non-Raman proposal is the so-called displacive excitation of coherent phonons, which was motivated by observations in Sb and Bi that only fully symmetric A/sub 1/-modes were excited. However, these findings have been challenged by recent measurements. Insight about the generation process can be gained by studying the dependence of the phonon amplitude on excitation energy near optical critical points. Contributions are then limited primarily to a small set of states associated with the resonance and, thus, predictions made by theories can be more readily tested.