Generation of Coherent Zone Boundary Phonons by Impulsive Excitation of Molecules

A coherent zone boundary phonon (ZBP) of solid Kr (f(m)=1.54 THz) is observed in ultrafast pump-probe spectra of I2 guest molecules in a Kr crystal. Its phase is stable for at least 10 ps. The femtosecond pump pulse induces an electronic transition in I2. The resulting expansion of the guest's electronic cloud impulsively excites phonons in the host. Detection at the impurity after some picoseconds selects the ZBP due to its low group velocity. A ZBP amplitude of 0.02 A is estimated.