Forbidden Transitions in Molecular Crystals: The Cameron System of Solid α-CO

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Abstract
A crystal splitting has been observed in the (0, 0) band of the Cameron system (a 3Π←X 1Σ+) of solid α-CO at 20°K, using a well-annealed sample. We believe that this is the first reported example of a triplet exciton splitting in a nonaromatic, the only previously reported examples having been in naphthalene and p-di-iodobenzene. A spin—orbit calculation has been carried out which demonstrates that the spectroscopic activity of this transition is primarily due to spin—orbit mixing of the 3Π1 and 1Π1 levels of the a 3Π and A 1Π states, respectively. The same calculation shows that the mixing of the ground and first triplet states can give intensity to this transition, but such intensity will be many times less than that induced via the first mechanism. The electric-dipole character gained by the transition is insufficient to account for the observed splitting, i.e., as a Davydov splitting. Instead, it is concluded that molecules undergoing this transition are coupled via an intermolecular-exchange interaction. An estimate of the magnitudes of the exchange integrals in solid CO, relative to those in aromatic crystals, results in a prediction of a triplet exciton splitting for CO which is of the order of magnitude of the observed splitting. A vibronic interaction mechanism within the CO crystal may also account for the splitting. Experiments are discussed which can distinguish between the two coupling mechanisms and which can confirm one or the other as that responsible for the observed splitting.