Phosphorescence excitation spectroscopy in supersonic jets. The lowest triplet state of pyrazine

Abstract

Motivated principally by dynamic issues, we have observed and assigned the laser-induced phosphorescence excitation spectrum of pyrazine in the collision-free environment of a supersonic jet. The origin of the lowest triplet state (T1) lies at 26 820 cm−1 and exhibits a symmetric parallel-type rotational contour, confirming that this state is 3B3u (nπ*) with an equilibrium geometry that is similar to those of the S0 (1Ag) and S1 (1B3u , nπ*) states. Thirty vibrational bands were also observed in the ∼4000 cm−1 interval between the T1 and S1 origins. Of these, the 13 lower energy bands all exhibit parallel-type contours and may be assigned as T1←S0 transitions, principally involving totally symmetric modes. The 17 higher energy bands exhibit both parallel and perpendicular contours and may be assigned as S1←S0 hot band transitions, some involving nontotally symmetric modes. No evidence for a second, ππ* triplet state lying below the S1 origin is found, nor is there any evidence for rapid relaxation of any of the zero-order T1 levels at a resolution of ∼1 cm−1. We conclude from these and other available spectroscopic data on the isolated molecule that the intersystem crossing dynamics of S1 pyrazine is governed by the interaction of the two largely nested potential surfaces, S1 and T1, zero–order nπ* states that appear to differ primarily in the extent to which they interact vibronically with other zero-order states in manifolds of the corresponding multiplicity.