Analysis and deconvolution of some J′≠0 rovibronic transitions in the high resolution S1←S fluorescence excitation spectrum of pyrazine

Abstract

Fluorescence excitation spectra are reported for several J′≠0 rotational members of the 000 band of the S1(1B3u)←S0(1A1g) electronic transition of pyrazine at a resolution of about 10 MHz. The transitions studied, namely R(0)–R(2) and P(2)–P(4), are each split into a large number of sharp lines ascribed, as in the case of the previously studied P(1) (J′=0) transition, to coupling with the lowest triplet state T1(3B3u). Despite this complexity, we show in this paper that it is possible to separate the lines into clusters of transitions that terminate in the same K′ component of the electronically excited, mixed S1–T1 state. This demonstrates that K′ is a good quantum number, at least at low J′ in the zero-order S1 state. From this analysis, we determine the rotational constants of the S0 and S1 states. We also determine: (i) the relative cluster intensities; (ii) the coupled T1 level densities; and (iii) by using standard deconvolution techniques, the S1–T1 coupling matrix elements, each as a function of J′,K′. Cluster intensities decrease with increasing J′, but K′=0 clusters are significantly less intense than K′≠0 clusters in the fluorescence excitation spectra. Observed triplet level densities in each cluster exceed by an order of magnitude the calculated density of rovibronic states if selection rules appropriate to the D2h point group are taken into account. Neither the observed level densities nor the coupling matrix elements (which vary from less than 5 MHz to more than 500 MHz) show a clear-cut systematic dependence on J′ or K′, although K′=0 levels appear to be more strongly coupled than K′≠0 levels. Possible explanations for these results and their implications for intersystem crossing dynamics in the isolated molecule are discussed.