  We investigated the coherent phonon generation mechanism by irradiation of an
ultrashort pulse with a simple two-level model. Our derived formulation shows
that both impulsive stimulated Raman scattering (ISRS) and impulsive absorption
(IA) simultaneously occur, and phonon wave packets are generated in the
electronic ground and excited states by ISRS and IA, respectively. We identify
the dominant process from the amplitude of the phonon oscillation. For short
pulse widths, ISRS is very small and becomes larger as the pulse width
increases. We also show that the initial phase is dependent on the pulse width
and the detuning.
