Triplet Exciton Effects in Single Crystals ofp-Diiodobenzene

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Abstract
The singlet-triplet (S→T 1) absorption spectrum of single crystals of p-diiodobenzene was measured in polarized light at 4.2°K. The crystal spectrum provides an example of intermediate to strong vibronic coupling. The three allowed factor group states B1u (a), B2u (b) and B3u (c) are observed to bracket arange of 121 cm−1 with E a-E b=-121 cm−1, E a - E c=-6 cm−1. The forbidden level (A u) was not identified. The polarization ratios do not provide a unique molecular polarization, but it is likely that the transition is polarized in the plane of the long and normal molecular axes. This implies that only two spin-orbital coupling routes are active in the free molecule. A theoretical account of our observations of the triplet exciton band is also described.

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