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Sub-Doppler Spectroscopy of Benzene in the "Channel-three" Region

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 C6H6, 1410110 and 1410120. Compared with the 1410110 band (Eexcess= 2492 cm–1) a drastically reduced number of sharp lines is observed in the 1410120 band (Eexcess= 3412 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 14112 state and other vibrational states within S1 and a subsequent non-radiative process. This coupling might be the explanation for the drastic onset of line broadening in C6H6 at an excess energy of 3000 cm–1, “channel three.” The line broadening is then due to a different process from that responsible for the population lifetime of S1.