Knowledge Bank

“Many experimental examples now exist where an originally forbidden’’ electronic transition in a molecule becomes strikingly enhanced through weak interactions with a neighboring perturber. The purpose of this paper is to discuss a general mechanism by which both symmetry-forbidden and multiplicity-forbidden transitions of a molecule in a perturbing environment can gain intensity. The theory in its simplest form considers a transition $\Psi_{M}^{O}\Psi_{P}^{O} \longleftrightarrow \Psi_{M}^{a} \Psi_{P}^{O}$ in a molecule (M)-perturber (P)-pair. In symmetry-forbidden but spin-allowed transitions, intensity enhancement occurs if the zero-order system-state $\Psi_{M}^{a} \Psi_{P}^{O}$ mixes by way of au excitation transfer interaction’’ with a state such as $\Psi MO\Psi P\nu$, where the transition $\Psi PO⟷\Psi P\nu$ in the perturber is strongly allowed. In spin forbidden transitions an additional requirement for enhancement is that either P is paramagnetic or that strong spin-orbit coupling in the excited states of P exists. This theory is distinct from those of other workers in that the transition moment is gained from the perturber rather than from the perturbed molecule. A study of these weak interactions allows knowledge to be gained about molecular eigenfunctions in the important but often neglected regions of intermolecular space.”