Knowledge Bank

Laser Excited Metastable (LEM) Spectra of $C2H2$ indicate that a single vibrational level of the $T3$ surface couples the $3\nu 3$ level of the $S1$ surface to a dense manifold of $T1$ and $T2$ states. In the proposed coupling mechanism, Gateway Mediated Intersystem Crossing (GMISC), the spin-orbit matrix element between the $3\nu 3S1$ state and one near-degenerate vibrational level of the $T3$ state is much larger than the direct interaction between $3\nu 3S1$ and any other $T1$ or $T2$ level. This is due to a near degeneracy between the $S1$ and $T3$ vibrational states and a nearby $S1-T3$ surface crossing. The $S1$ state interacts with the background triplet states indirectly through $T3$ via $T3\sim T2,1$ matrix elements. Evidence for this mechanism includes a shift between the LIF and LEM spectra, an interference effect near $J=5$ in LEM spectrum, and the strength of the LEM signal for the $3\nu 3$ band relative to other bands in the $A¯-X¯$ band system. Other experiments in which $S1\sim (T3,2,1,S0)$ interactions were observed will be discussed in the context of the GMISC model.