Characterization of the (A)over-tilde(1A") state of HCF by laser induced fluorescence spectroscopy

An extensive experimental exploration of the (A) over tilde ((1)A ") <-- (X) over tilde((1)A') transition of supersonically cooled fluoromethylene has been performed using laser induced fluorescence spectroscopy. Several new bending vibrational bands are reported, which provide the harmonic frequency and anharmonicity constant for this vibration and lead to an estimate of the height of the barrier to linearity as 6400 +/- 500 cm(-1). Assignment of the vibrational hot-band structure leads to the first measurement of the (A) over tilde state CF stretching frequency as nu(3)' = 1260 cm(-1) and tentatively the CH stretching frequency as nu(1)' = 2852 cm(-1). The A' rotational constant increases strongly with increasing quanta of bending vibration, which indicates that the molecular structure is becoming more linear. Consideration of only the average bond angle, calculated from ab initio data for this state, is insufficient to account for the change in A'. The coupling of a-axis rotation with bending vibration must be included. A number of other dynamical effects were observed in the spectra, including lifetime shortening and disappearance of rotational transitions with K' greater than or equal to 1. These were explained in terms of the Renner-Teller interaction between the (X) over tilde and (A) over tilde states. (C) 1999 American Institute of Physics. [S0021-9606(99)01222-2].