DOPPLER-FREE TWO-PHOTON SPECTRUM OF THE 01 BAND OF THE ii 'B,cz 'A, TRANSITION IN DIFLUORODIAZIRINE, FICNz

The Doppler-free two-photon excitation spectrum of the vibrationless A ‘Btc jt ‘A, transition of difluorodiaxirine (F&N,) 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 r,.,,., and rvv distances in the ground a ‘A, and excited A ‘B, state were determined. The geometry change upon excitation is found to be Ar,=3.89 (2) pm and Ar,= -4.09(2) pm. No perturbation in the rotational structure of the 0: band has been found. This points to a small singlet-triplet coupling matrix element in the small molecule limit.