Time-resolved X-ray diffraction from coherent phonons during a laser-induced phase transition

Time-resolved x-ray diffraction with picosecond temporal resolution is used to observe scattering from impulsively generated coherent acoustic phonons in laser-excited InSb crystals. The observed frequencies and damping rates are in agreement with a model based on dynamical diffraction theory couple d to analytic solutions for the laser-induced strain profile. The results a re consistent with a 12 ps thermal electron-acoustic phonon coupling time t ogether with an instantaneous component from the deformation-potential inte raction. Above a critical laser fluence, we show that the first step in the transition to a disordered state is the excitation of large amplitude, coh erent atomic motion.