High-resolution IR spectroscopy of ethylene dimers

A differentially pumped free-jet expansion, together with a waveguide CO2 laser, has been used to measure the infrared predissociation spectrum of ethylene clusters near 950 cm-1. The CO2 laser has a pressure broadened tuning range of 240 MHz. We observe sharp transitions in association with many of the laser lines which activate the monomer C2H2 molecules in their v7 mode. By varying the concentration of C2H4 in a carrier gas, by changing the carrier gas, and by using both bolometric and mass spectrometric detection systems, we are able to confirm the findings of Snels et al.1 that there is fine structure in the ethylene dimer predissociation spectrum near 950 cm-1. The linewidths of all the observed features lie in the 15-20-MHz range, giving a lower limit on the pre-dissociation lifetime of ~ 10ns.