Dynamical Test of Davydov-like Solitons in Acetanilide Using a Picosecond Free Electron Laser.

We show that the anomalous 1650 cm sup (-1) band of acetanilide (ACN) which appears on cooling of ACN crystals persists following rapid pulsed heating. This demonstrates that this controversial absorption is associated with a structural transition which can be kinetically suppressed and not with a self-trapped vibrational excitation. The ground state recovery after photoexcitation is measured by time resolved bleaching experiments and occurs in 15 +-5 piconseconds. The unusual behavior of ACN can be satisfactorily accounted for by the occurrence of two slightly nondegenerate crystal structures with a 0.4 eV thermal interconversion barrier.