GATEWAY MEDIATED INTERSYSTEM CROSSING IN $C_{2}H_{2}$
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Date
1997
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Publisher
Ohio State University
Abstract
Laser Excited Metastable (LEM) Spectra of $C_{2}H_{2}$ indicate that a single vibrational level of the $T_{3}$ surface couples the $3\nu_{3}$ level of the $S_{1}$ surface to a dense manifold of $T_{1}$ and $T_{2}$ states. In the proposed coupling mechanism, Gateway Mediated Intersystem Crossing (GMISC), the spin-orbit matrix element between the $3\nu_{3}\;S_{1}$ state and one near-degenerate vibrational level of the $T_{3}$ state is much larger than the direct interaction between $3\nu_{3}\;S_{1}$ and any other $T_{1}$ or $T_{2}$ level. This is due to a near degeneracy between the $S_{1}$ and $T_{3}$ vibrational states and a nearby $S_{1}-T_{3}$ surface crossing. The $S_{1}$ state interacts with the background triplet states indirectly through $T_{3}$ via $T_{3} \sim T_{2,1}$ 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 $\bar{A}-\bar{X}$ band system. Other experiments in which $S_{1} \sim (T_{3,2,1,} S_{0})$ interactions were observed will be discussed in the context of the GMISC model.
Description
Author Institution: Department of Chemistry, Massachusetts Institute of Technology; Department of Chemistry, University of California at Santa Barbara