Doppler-free two-photon spectroscopy of benzene in a cold supersonic beam

Doppler-free two-photon absorption is demonstrated for cold molecules in a supersonic beam with a spectral resolution of 10 MHz. It was realized using a frequency stabilized tunable external concentric resonator placed within the vacuum chamber and crossed by the molecular beam. The achieved high resolution allows us to resolve single rotational lines in the S-1 <-- S-o, 14(o)(1) two photon vibronic band of benzene molecules. The theoretical analysis of the symmetric top rotational line structure of the isotropic Q-branch in a totally symmetric two-photon band leads to accurate rotational constants and a value for the rotational temperature achieved at various distances from the nozzle orifice in the cold molecular beam. The smallest linewidth of the rotational lines is 8.5 MHz and given by the flight time of the molecules in the supersonic beam through the laser beam waist in the resonator and the laser bandwidth.