Quantum beat spectroscopy is presently the only direct probe of intramolecular couplings and spacings between individual highly exicted states of polyatomic molecules with state densities of the order of 1000 states per wave number. We have applied this method to methylglyoxal in an attempt to discover if there are preferred pathways of intramolecular energy flow and if it is possible, in this and similar molecules, to localize energy in particular bonds. The method has enough resolution to allow observation of hyperfine splittings of excited triplet states as well as states which are different vibrationally excited triplet states. Consequently, the vibrational coupling and spacing trends are slightly obscured. However, our data indicate that there are no selection rules for intersystem crossing other than rigorous conservation laws and a requirement for conserving nuclear spin states. The dependence on the initial rotational angular momentumNis evidently not important at all at lowNbut might be useful at highN.
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