Quantitative laser-induced fluorescence spectroscopy of the CF A²Σ⁺-X²Π transition: Electronic transition dipole moment function and predissociation

Several previously unmeasured aspects of the quantitative spectroscopy of the CF A²Σ⁺-X²Π transition have been investigated experimentally. Ground state CF radicals were prepared in an rf discharge chamber and laser-induced fluorescence (LIF) was excited on the A-X transition. Relative vibrational transition probabilities were deduced from dispersed fluorescence spectra emitted by CF A²Σ⁺ (ν′ = 0 and 1), from which the form of the electronic transition dipole moment function, R_e(r), could be assessed. Reasonable agreement was found with a previous ab initio prediction of a moderate decline in R_e(r) over the relevant, fairly limited range of internuclear distances sampled by the experiment. Weak fluorescence has also been detected for the first time from the heavily predissociated ν′ = 2 level. A rotationally-resolved LIF excitation spectrum was obtained. Lower and upper limits on the ν′ = 2 fluorescence lifetime were estimated to be 0.4 and 2 ns, respectively. There is a weak, if any, decline in the ν′ = 2 fluorescence quantum yield with rotational quantum number up to ca. 20.5. These observations are shown to be consistent with a predissociation mechanism involving tunneling through the predicted barrier to dissociation which results from an avoided crossing at longer internuclear distances.