Laser induced ultrasonic standing waves are observed in molecular beam
deposited ZnSe thin films using the transient grating technique. This
observation is attributed to a very short trapping time for single ph
oton absorption in the band tail. The period of the standing wave is u
sed to determine the acoustic phonon velocity in the ZnSe thin film wh
ich indicates that it is longitudinal acoustic phonons which are excit
ed.