Abstract
An equation-of-motion procedure to study intramolecular vibrational relaxation in polyatomic molecules is developed and applied to study the transient structural response of a polyene to photoexcitation. The lattice response at short times (∼20 fs) following excitation is characterized by an adiabatic relaxation which is highly specific to the exciting frequency. For longer times (20-1000 fs) this relaxation is strongly modified by the nonadiabatic relaxation of the excited carrier and takes the form of a "splash" in the bond-alternation profile, which reflects the transient distribution of the excited carrier in energy.