Doppler-free two-photon spectrum of the 000 band of the A1B1X1A1 transition in difluorodiazirine, F2CN2
The Doppler-free two-photon excitation spectrum of the vibrationless \xc31B1\u2190Image 1A1 transition of difluorodiazirine (F2CN2) has been recorded with a resolution of 15 MHz using a cw single-mode dye laser coupled to an external concentric resonator. The asymmetric rotor spectrum has been analysed and more than 350 lines randomly selected from all five branches were assigned in order to fit the ground- and excited-state rotational and quartic centrifugal distortion constants. From the rotational constants the rNN and rFF distances in the ground Image 1A1 and excited \xc31B1 state were determined. The geometry change upon excitation is found to be \u0394rNN = 3.89(2) pm and \u0394rFF = \u22124.09(2) pm. No perturbation in the rotational structure of the 000 band has been found. This points to a small singlet-triplet coupling matrix element in the small molecule limit.