Sub-doppler spectroscopy of benzene in the ‘channel-three’ region

Abstract

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 (E_excess= 2492 cm^–1) a drastically reduced number of sharp lines is observed in the 14¹₀1²₀ band (E_excess= 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.