Molecular beam infrared spectra of dimers formed from acetylene, methyl acetylene, and ethene as a function of source pressure and concentration

Infrared spectra have been obtained for dimers formed from acetylene, methyl acetylene, and ethylene using a wide variety of free jet source conditions. An F-center laser was used to excite various C–H stretch fundamentals in the range 3000–3300 cm−1. Rotational structure was resolved in the spectra of both acetylene and methyl acetylene dimers when dilute mixtures of either gas in helium were expanded from high pressures. The spectrum of the ethylene dimer showed no such structure even when formed in very dilute mixtures. These results contradict previous suggestions that the vibrational relaxation lifetime is uniformly short for all polyatomic dimers.