High resolution infrared spectra of ethylene clusters

Infrared predissociation spectra have been measured for ethylene clusters near the ν7 (950 cm−1) monomer band using CO2 lasers. One of the CO2 lasers was piezoelectrically scanned over a 240 MHz range around each rotational lasing transition, to obtain high resolution spectra of the ν7 band. The resulting spectra showed fine structure with widths of the order of 20 MHz, representing a lower limit on the predissociation lifetime of around 10 ns. The fine structure is shown to be due to ethylene dimers, and the broad background is attributed to a superposition of inhomogeneous contributions from larger clusters.