Analysis of the molecular electronic absorption spectra of shock-heated aromatic compounds

Abstract

Advantage has been taken of the rapid and homogeneous heating of a sample in a shock wave to record the electronic absorption spectra of the 1A1g→1B2u transition in benzene and the corresponding 1A1→1B1 transition in toluene as a function of temperature over the range 700–1300 K, prior to thermal decomposition.A model for the band-shape function of electronic absorption spectra of symmetry-allowed and symmetry-forbidden transitions in the context of the Herzberg–Teller theory is proposed based upon the assumption of harmonic oscillators. For symmetry-forbidden transitions, frequency changes in the promoting modes are ignored and only the linear Herzberg–Teller terms are considered to account for the dependence of the electronic transition moment upon nuclear coordinates. Explicit attention is paid to the temperature dependence of the band-shape function and it is shown, in conjunction with a moment analysis of the band-shape function, how important molecular parameters may be deduced in favourable instances, which are determined chiefly by the inherent molecular complexity and symmetry point group of the molecule. It is shown by a moment analysis of the shock data that the induced and allowed transition moments in toluene are of comparable magnitude.