15min:

KEVIN L. CUNNINGHAM, STEPHEN DRUCKER AND ROBERT W. FIELD, Department of Chemistry, Massachusetts Institute of Technology, Cambridge MA 02139-4307; SUSAN J. HUMPHREY, CHRISTOPHER G. MORGAN AND ALEC M. WODTKE, Department of Chemistry, University of California at Santa Barbara, Santa Barbara CA 93106.

Laser Excited Metastable (LEM) spectra of C₂H₂ indicate that a single vibrational level of the T₃ surface couples the 3₃ 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 3₃ S₁ state and one near-degenerate vibrational level of the T₃ state is much larger than the direct interaction between 3₃ S₁ 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 ~ 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 the LEM spectrum, and the strength of the LEM signal for the 3₃ band relative to other bands in the A-X band system. Other experiments in which S₁ ~ (T_3,2,1, S₀) interactions were observed will be discussed in the context of the GMISC model.