J. Phys. Chem. 1883, 87,2232-2239
2232
Nonradfative Processes in the Channel Three Region of the SI State of Ultracold Benzene C. E.
Otis,+ J. L. Knee,
and P. M. Johnson'
Department of Chemlsby, State University of New York, Stony Brook, New York 17794 (Received: November 9, 1982)
The lifetimes of several vibronic bands in the 'Bzu lAlg transition of benzene have been probed in a two-color photoionization/supersonicexpansion experiment. The singlet and triplet decay rates obtained from the biexponential decay curves are monitored as a function of vibrational energy and found to increase in a smooth fashion into the "channel three" region. More significantly, the quantum yields for internal conversion and intersystem crossing are determined. At vibrational energies greater than =3000 cm-l the quantum yield for intersystem crossing (kc) decreases markedly relative to the lower lying levels. The data suggest that the major channel for S1relaxation is internal conversion, but the internal conversion rate is not a simple function of energy above the S1 origin as demonstrated by the decay behavior of the 7; band. This level, which is almost isoenergetic with the 66 1: band decays with a much larger intersystem crossing quantum yield than the 6;l; level and has a much longer singlet lifetime. +-
Introduction Radiationless decay of molecular excited electronic states has been the subject of exhaustive theoretical and experimental work during the past 10-15 years. Since the pioneering single vibrational level (SVL)work of Parmenter,' and Spears and Rice? numerous experimental techniques have been developed to probe the unimolecular relaxation mechanisms operative in highly vibrationally excited molecular singlet (and sometimes triplet) states. Although nonradiative relaxation is common to almost all molecular excited electronic states and manifests itself via a variety of mechanisms, only a few systems have been well studied, one of those being the first singlet lBzu 'A, excitation of benzene. Due to its sharp vibrational (an$ even rotational) structure, reasonably good assignments and wellknown normal coordinates, this transition is both theoretically and experimentally a good model system to use in testing new theories and in probing for new dynamical information relevant to the phenomenon of radiationless relaxation in intermediate size molecules. Much work, both experimental and theoretical, has been done on the details of benzene's radiationless transitions, but most of the experiments to probe the time evolution of its excited states have been done on samples at relatively torr) and temperature or in conhigh pressure ( p I densed phases. Only recently, with the development of supersonic molecular beams, has it been possible to probe the excited states of molecules in a clean, collision-free, ultracold (
