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

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(o)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. +/- 0.1 D (E-b = 1.6E(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 an the basis of a double minimum potential model for the lower B-1(1g)(n pi*) state. Arguments pro and contra this model are presented and the alternative of two close lying B-1(1g)(n pi*) and (1)A(u)(n pi*) states is explored as well.