Pathways for Intramolecular Relaxation in S1 Benzene

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.