Coherent phonons excited by two optical pulses

Theoretical dependences of the amplitude $A$ and phase $\varphi$ of photoinduced coherent oscillations of the crystal lattice on the delay time $\mu$ between two exciting optical pulses have been derived. It has been shown that $\varphi(\mu)$ is a periodic or decreasing function depending on the experimental conditions. Comparison with the experiment on Bi has been carried out.