Generation of Coherent Zone Boundary Phonons by Impulsive Excitation of Molecules

A coherent zone boundary phonon (ZBP) of solid Kr ($f_m=1.54\mathrm{T}\mathrm{H}\mathrm{z}$) is observed in ultrafast pump-probe spectra of ${\mathrm{I}}_2$ guest molecules in a Kr crystal. Its phase is stable for at least 10 ps. The femtosecond pump pulse induces an electronic transition in ${\mathrm{I}}_2$. 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 Å is estimated.