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) 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−1. The Ka = odd lines of the qqQ5(J)
subbranch show small and quite regular perturbations of 60 ± 5 MHz
which are probably due to a coupling to another vibrational state
of the S1 manifold.