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

Time-resolved emission from low pressure p-benzoquinone-h4 (and -d4) is observed following single vibronic excitation of the 1B1g and 3Au states with a tunable dye laser. Evidence is presented showing that 1B1g 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 3Au state. A simple three level relaxation model is investigated and deemed to be inadequate. The 3Au phosphorescence liftime is 28.4 μsec (37.6 μsec for benzoquinone-d4,) independent of pressure for added argon and nitrogen and benzoquinone, itself. Comparison of the 3Au phosphorescence lifetime with Ag1→Au3 f number indicates that the 3Au lifetime is controlled by a unimolecular process other than Au3→Ag1 phosphorescence. We prospose that the unimolecular process competing with the Au3→Ag1 phosphorescence could be the symmetry and spin allowed Au3→B1g3 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 1Ag ground state. 3Au molecules are mildly deactivated by O2 and C2H2, and deactivated at near the gas kinetic rate by NO.