Stark Effects on the B1g and Au nπ* Triplet States of p-Benzoquinone

The results of polarized Stark effect absorption experiments on the lower triplet states of p-benzoquinone (PBQ)-h4 and -d4 in single crystals at 1.8°K are reported. These measurements give strong support for the assignment of the lowest triplet state as triplet 3B1g (nπ*). The origin of this state in PBQ-h4 is observed as an electric field induced transition at 18602.8 cm-1 and is interpreted as the g-inversion level of a double-minimum potential well. The line observed at 18619.7 cm-1 in the PBQ-h4 crystal is therefore assigned as the u-inversion level of this double well with Au (B1g ⊗ b1u) total symmetry. Its intensity manifests the strong vibronic coupling that occurs among the B1g and Au nπ* triplet states along b1u-type nuclear motions