Sub-doppler two-photon spectrum of asymmetric rotor molecules
The Doppler-free two-photon excitation spectrum of the qqQ branch of the 1410 vibrational band of the S1(1B2u) \u2190 S0(1A1g) transition of benzene-d1 has been recorded using a cw single-mode dye laser coupled to an external concentric resonator. The spectrum has been analysed using a non-rigid Watson Hamiltonian. More than 200 lines with J up to 20 have been assigned and the rotational constants which best reproduce the spectrum are A1v = 0.181435, B1v = 0.169990, C1v = 0.089055 cm\u22121. The Ka = odd lines of the qqQ5(J) subbranch show small and quite regular perturbations of 60 \xb1 5 MHz which are probably due to a coupling to another vibrational state of the S1 manifold.