Electronic structure and dynamics of tunable laser excited p-benzoquinone (-h4 and -d4) in the gas phase

Abstract

Time‐resolved emission from low pressure p‐benzoquinone‐h 4 (and ‐d 4) is observed following single vibronic excitation of the 1 B 1g and 3 Au states with a tunable dye laser. Evidence is presented showing that 1 B 1g levels undergo an intermediate strong coupling with vibronic levels of lower excited electronic states. These hybrid states, which have collision‐free emissive liftimes of 7–15 μsec, are collisisonally relaxed at a near gas kinetic rate, with at least partial conversion to the vibrationally equilibrated 3 Au state. A simple three level relaxation model is investigated and deemed to be inadequate. The 3 Au phosphorescence liftime is 28.4 μsec (37.6 μsec for benzoquinone‐d 4,) independent of pressure for added argon and nitrogen and benzoquinone, itself. Comparison of the 3 Au phosphorescence lifetime with 1 A g → 3 A u f number indicates that the 3 Au lifetime is controlled by a unimolecular process other than 3 A u → A g phosphorescence. We prospose that the unimolecular process competing with the 3 A u → 1 A g phosphorescence could be the symmetry and spin allowed 3 A u → 3 B 1g fluorescence that would occur in the infrared. This process is compared to the other possible deactivating route, a spin‐orbit‐vibronic induced radiationless transition into the 1 Ag ground state.3 Au molecules are mildly deactivated by O2 and C2H2, and deactivated at near the gas kinetic rate by NO.