Low temperature measurements of the rate of association to benzene dimers in helium

The association of benzene molecules to form dimers has been studied at tem peratures between 15 and 123 K in a CRESU (Cinetique de Reactions en Ecoule ment Supersonique Uniforme) apparatus with helium as the buffer gas. Second -order rate coefficients (k(2)) for the formation of dimers have been deter mined for temperatures between 15 and 88 K. The effective third-order rate coefficients (no study of fall off behavior has been undertaken) obtained b y dividing the values of k(2) by [He] show a strong negative temperature de pendence. Further evidence for the formation of dimers at critical concentr ations of benzene has been obtained from a study of the CH + C6H6 reaction and from LIF spectra. Modelling calculations indicate that any systematic e rrors in the results arising from the formation of higher oligomers are sma ll. Based on a model in which it is assumed that the benzene molecules in t he dimer are free to rotate, third-order rate coefficients have been calcul ated for dimerization and are in good agreement with the experimental value s for T greater than or equal to 30 K, where effects of falloff should not be too great. (C) 2000 American Institute of Physics. [S0021-9606(00)01710- 4].