Observation of the predissociated, quasilinear B(1A') state of CHF by optical-optical double resonance.

We report the first observation of the predissociative B state of a halocarbene molecule. Rovibronic energy levels were measured in the B(1A') state of CHF by fluorescence dip detected optical-optical double resonance spectroscopy via the A state. The origin was found to lie 30 817.4 cm-1 above the zero point level of the X state. Rotational transitions within six purely bending states, and states involving one or two quanta of CF-stretch were observed, including the vibrational angular momentum components. Interpretation of the spectrum, with support of ab initio calculations, shows that CHF is quasilinear in the B state with a small (-200 cm-1) barrier to linearity which lies below the zero-point level. The rotational constant, B=1.04 to 1.09 cm-1, depending on vibrational state, again in good agreement with theory. All observed B state levels were predissociative, as evidenced by Lorentzian line broadening. Linewidths varied with initial state from 0.7-10.8 cm-1, corresponding to excited state lifetimes of 0.5-8 ps.