Vibrational energy dependence of line broadening in the quinoxaline spectrum

The linewidths as a function of the number of quanta of the totally symmetric progression forming mode, 525 cm^?1 (509 - quinoxaline-d₆) have been measured at high resolution in the vapor phase absorption spectrum of quinoxaline. The line broadening is attributed to efficient intersystem crossing between the lowest single (nπ*) and triplet (ππ*) states. The increased broadening observed with increased energy is consistent with theoretical predictions and with the general results of parallel lifetime studies of individual vibronic levels.