Riedle, Eberhard; Neusser, H. J.; Schlag, E. W.
Pathways for Intramolecular Relaxation in S1 Benzene.
In: Philosophical Transactions of the Royal Society of London. A, Vol. 332: pp. 189-201
Sub-Doppler spectra of various one- and two-photon vibronic bands of benzene are discussed and analysed to determine the pathways of intramolecular relaxation for S1 benzene. New results are presented for the 14011011622 band of C6H6 and the 1401102 band of 13C6H6. The decay behaviour depends strongly on the excess energy and the rotational quantum numbers rather than on the vibrational character and symmetry of the excited state. At low vibrational excess energy the pathway for intramolecular relaxation is a coupling in the strong limit between pairs of states in S1 leading to shifts of lines, whereas at intermediate excess energy coupling in the weak limit to background states in S1 is present. These background states are strongly broadened due to a fast electronic non-radiative process. The intramolecular relaxation is found to be initiated by the coupling to the broadened S1 background states and energy can flow via these states to the T1 or S0 state. The rotationally selective disappearance of lines is believed to be due to an intricate interplay of the rotational dependence of the coupling matrix elements and accidental resonances, which lead to interference of possible decay channels.