Sub-Doppler High-Resolution Spectra of C6H6

We have measured the Doppler-free rotationally resolved two-photon
spectrum of two vibronic bands of C6H6, 14; 1; and 14; 1:. Compared
to the 14; 1; band (E,,,, = 2492 cm-’) a drastically reduced number
of sharp lines is observed in the 14; 1: band (EexCe=88 3 412
cm-’1. The K = 0 rotational lines are still seen, while K # 0 lines
disappear. This can be understood in terms of Coriolis coupling
between the 14l l2 state and other vibrational states within Sl and
a subsequent nonradiative process. This coupling might be the
explanation for the drastic onset of line broadening in C6H6 at an
excess energy of 3000 cm-l, “channel three”. The line broadening is
then due to a different process than that responsible for the
population lifetime of S1.