Stark effects on the lowest 1B 1g(nπ*) state of p-benzoquinone

Abstract

Stark effects on the optical absorption spectrum of the lowest singlet state in p-benzoquinone (PBQ)- h 4 and − d 4. single crystals at 1.8°K are reported. The singlet origin components in PBQ- h 4 at 20057.8 cm −1 and 20061.3 cm −1 are shown to be states of opposite parity. The Stark experiments yield a transition moment between these levels of 2.2. ±0.1D ( E b= 1.6 E eff) for both PBQ- h 4 and − d 4. Polarization experiments on the lower energy origin expose its magnetic dipole character and this state is assigned as a state of B 1g-total symmetry. A sizeable deuteration effect on the oscillator strength of this B 1g origin is also reported and discussed. Vibronic lines up to 1000 cm −1 from the origin lines also have been studied in external electric fields and these results are reported as well. A qualitative discussion of these results is given on the basis of a double minimum potential model for the lower 1B 1g(nπ*) state. Arguments pro and contra this model are presented and the alternative of two close lying 1B 1g(nπ*) and 1A u(nπ*) states is explored as well.