Spectral Signatures of Inter-System Crossing Mediated by Energetically Distant Doorway Levels: Examples from the Acetylene S1 State

We review recent research on the acetylene S-1 state that illustrates how mechanistic rather than phenomenological information about intersystem crossing (ISC) may be obtained directly from frequency-domain spectra. The focus is on the dynamically rich doorway-mediated ISC domain that lies between isolated spectroscopic spin-orbit perturbations and statistical-limit interactions between one singlet bright state and a quasi-continuum of triplet dark states. New and improved experimental and data processing techniques permit the statistical-model curtain to be drawn back to reveal mechanistically explicit pathways via one or more identifiable, hence, manipulatable, doorway states, between a user-selected bright state and the undifferentiated bath of dark states.