At high vibrational energies the redistribution of vibrational energy is very rapid in accord with the broad linewidths measured. At low energies, no such consistent picture has emerged. For many systems the line-broadening threshold is at about 1500cm1, yet for others no broadening is observed for thousands of cm−1. Infrared spectra of ethylene dimers are marked by large linewidths whereas lines narrower by several orders of magnitude are observed for the larger molecular dimers benzene and methylacetylene.