Self-trapping of the amide I band in a peptide model crystal

A femtosecond pump-probe study of the peculiar amide I band of acetanilide, a molecular crystal with hydrogen bonded chains of peptide units, is presented. The almost perfect harmonicity of the 1666 cm−1 subpeak is related to significant delocalization of this state at low enough temperatures (93 K). The “anomalous” peak (1650 cm−1), on the other hand, is strongly anharmonic, and hence assigned to a self-trapped state. This assignment is in agreement with a more indirect previous work. With increasing temperature, thermal disorder localizes the 1666 cm−1 band (Anderson localization) and at the same time destroys the self-trapping mechanism. Both the self-trapped state and the delocalized state decay on a 2 ps time scale into states outside the spectral window of this study. The excitation energy reappears on a much slower 35 ps time scale in the form of an increased lattice temperature.