Vibrational Relaxation and Dynamical Transitions in Atactic Polystyrene

Abstract

Infrared bands and Raman lines recorded in the frequency domain have a counterpart in the time domain in the form of time-correlation functions, which are sensitive to molecular dynamics on the picosecond time scale. Time correlation functions and their variation with temperature have been calculated for the modes observed near 1601 and 1583 cm−1 in the infrared spectrum of atactic polystyrene. The correlation functions can be modeled by assuming that there is a fast relaxation process characterized by a single relaxation time that is inhomogeneously broadened by a slower process, also characterized by a single relaxation time. The fast modulation is of the order of 0.014 ps for both modes. Librational motions of the phenyl ring occur on the 0.01 ps time scale, suggesting that this mode of relaxation involves a rapid dephasing process. One cannot draw firm conclusions, however, because dephasing and vibrational energy relaxation mechanisms cannot be separated by studies of simple band shapes alone. A slow...