Spectroscopic signatures of doorway-mediated intersystem crossing

Abstract

Intersystem crossing in acetylene, C₂H₂, is described by a doorway-mediated model, where singlet-triplet mixing between the first excited singlet~ state, S₁, and the manifold of low-lying T₁,₂ states is controlled by special vibrational levels of the third excited triplet state, T₃. However, the influence of T₃ doorway states on the ~A¹Au <-- ~X¹[sigma]+g spectrum of acetylene is not well characterized for most S₁ vibrational levels. The doorway model is formulated to account for the LIF/SEELEM spectra of S₁ vibrational sublevels that are weakly perturbed by T₃ doorway states. The structure and properties of a doorway-mediated effective Hamiltonian are discussed, and a method for spectral deconvolution is presented for the doorway model. The dependence of fluorescence center-of-gravity on the time delay of a gated integration region is developed as an analysis tool for LIF spectroscopy. Triplet perturbations in the spectra of four S₁ vibrational sublevels are discussed in terms of the doorway model, using this new tool. IR-UV double resonance LIF/SEELEM spectroscopy is used to investigate the role of the torsional and "cis"-bending vibrational modes of S₁ in the promotion of vibrational overlap between levels of S₁ and T₃. A collisional method for the population of metastable molecules in a pulsed supersonic expansion is presented and discussed.