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A doorway-mediated intersystem crossing model is well accepted for the
acetylene molecule, but its impact on the spectrum is not well
characterized for most vibrational levels of $S_1$.  Connections
between the doorway model and spectroscopic measurements are developed
and applied to the LIF/SEELEM spectra of several $S_1$ vibrational
sublevels.  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 time delay is developed as an
analysis tool for LIF spectroscopy.  Triplet perturbations in the
spectra of four $S_1$ vibrational sublevels are explained 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 modes in promoting $S_1 \sim T_3$
vibrational overlap.  A collisional method for the population of
metastable molecules in a pulsed supersonic expansion is briefly
presented and discussed.
 
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