Highly Excited Systems

Abstract

The structure and dynamics of weakly bound molecular systems are often characterized by low potential barriers and large tunelling probabilities. Similar effects can be observed for molecules with internal motions, especially if these molecules are excited to energies near to or over above the barriers. We discuss this similarity with examples drawn from Raman overtone spectroscopy (e.g. C₂H₆) and from dimer predissociation spectra (e.g. (NH₃)₂). IR overtone spectroscopy often leads to vibrational transitions which are heavily perturbed by resonances but still show well defined rotational structure and sharp lines. For dimer predissociation spectra similarly sharp transitions have been observed in many cases, notwithstanding the high density of states in the neighbourhood of the observed transitions. In the singular case of (C₂H₄)₂ this leads to a predissociative lifetime of about 10 ns, since the minimum value derived from the linewidth practically coincides with the maximum value derived from time resolved pump-probe laser experiments.