Doppler-free two-photon spectrum of the 000 band of the Ã1B1←X1A1 transition in difluorodiazirine, F2CN2

The Doppler-free two-photon excitation spectrum of the
vibrationless Ã1B1Image 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 Ã1B1
state were determined. The geometry change upon excitation is found
to be ΔrNN = 3.89(2) pm and ΔrFF = −4.09(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.