Vacuum UV fluorescence excitation spectroscopy of BF3 in the range 45–125 nm: Observation of new electronic transitions in the BF2 free radical

The fluorescence following excitation of BF3 is studied using two techniques: (a) He* (23S) metastable excitation with dispersed fluorescence detection, (b) photon excitation using tunable vacuum UV radiation from a synchrotron source with undispersed detection. The He* experiment gives an extensive spectrum between 200 and 300 nm with two long progressions, each of separation 525 ± 30 cm-1. They are assigned to transitions to the v 2 bending mode of BF2 [Xtilde] 2A1, probably from the first excited state à2B1. Using photons in the energy range 10–28 eV two different fluorescence decay channels are observed: (1) BF2fluorescence for photon energies below 17eV, (2) BF3 + fluorescence for energies > 21·5eV. The shapes of the excitation functions confirm that (1) is a resonant process via Rydberg states of BF3, whereas (2) is a non-resonant photoionization process. The emitting state in BF3 + is the [Etilde]2A′1 state with a vertical ionization potential of 21·5eV. The two strongest resonant peaks at 13·1 and 14·0eV correspond to excitation of the 3s and 3p Rydberg states of BF3, but they show very different properties. The former is a continuous band, fluorescence being produced at λ > 300 nm, whereas the latter shows vibrational structure in the v 4 bending mode in the excitation spectrum, fluorescence now being predominantly in the range 200–350nm. The emissions are assigned to two different electronic transitions of the BF2 radical. Using the synchrotron in its single-bunch mode, radiative lifetime of the two fluorescing states of BF2 and BF3 + [Etilde] have been measured to be ca. 49, 11 and 10 ns respectively. The results are compared with recent vacuum UV photoabsorption studies of BF3, and the spectroscopy of the BF2 radical is reviewed.