In addition to an increased accuracy in the determination of structural parameters sub-

Doppler spectroscopy of large molecules is shown to render valuable information about intramolecular

processes and relaxation mechanisms. As an example we have measured the

Doppler-free rotationally resolved two-photon spectrum of two vibronic bands of CsHs,

1461; and 14i1;. Compared with the 14i1: band (EcxE :,:=css 2492 cm-') a drastically reduced

number of sharp lines is observed in the 14i1; band (Eence=ss 3 412 cm-I). The X = 0

rotational lines are still seen, while K # 0 lines disappear. This can be understood in terms

of Coriolis coupling between the 14'1' state and other vibrational states within S1 and a

subsequent om-radiative process. This coupling might be the explanation for the drastic

onset of line broadening in C6H6 at an excess energy of 3000 cm-', " channel three." The

line broadening is then due to a different process from that responsible for the population

lifetime of s1.