On the interrelation between nuclear dynamics and spectral line shapes in clusters

We analyze spectral absorption line shapes simulated using the molecular dynamics spectral density method. We explore three classes of line shapes: (1) the region of the 0-0 S-0-->S-1(pi pi*) transition of perylene . Ar-N clusters, (2) the (XeS0)-S-1-->P-3(1) transition of XeArN clusters, and (3) the photoelectron spectrum of the Li4F4 cluster in the valence region. These spectra represent examples for weak, unresolved, and extensive vibrational progressions, which have been analyzed and assigned. Employing a simplified model for the energy gap autocorrelation function allows for an understanding of the different behaviors and for a classification of the interrelation between nuclear dynamics and spectral line shapes. With decreasing the characteristic decay time of the transition dipole autocorrelation function, the line shape passes the limiting cases of the model in the order fast modulation limit-->vibrational progression limit-->slow modulation limit, with the vibrational progression limit extending the limiting cases of the Kubo stochastic model of line shapes. Some simple qualitative rules have been extracted to predict the overall character of a line shape. (C) 1996 American Institute of Physics.