Vacuum-UV fluorescence spectroscopy of BF3 and BCl3 in the range 9-22 eV:: dispersed spectra and radiative lifetimes

The fluorescence processes following vacuum-UV (VUV) photoexcitation of BF3 and BCl3 in the energy range 9-22 eV have been studied using synchrotron radiation from the BESSY 1, Berlin source. The fluorescence is dispersed in a secondary monochromator, whereby information on the nature of the emitting species is obtained. For excitation energies in the range 13-17 eV, photodissociation of Rydberg states of BF3 produces a substantial branching ratio into two, possibly three, excited valence states of the BF2 free radical, which fluoresce to the ground state. The dispersed spectra are compared with predictions from ab initio calculations of the electronic spectroscopy of this radical. Fluorescence from BCl3 photoexcited in the range 9-22 eV is more complicated, with emissions having already been observed and mostly assigned in BCl2, BCl, BCl3+, BCl2+ and B. Using the single-bunch, pulsed mode of the BESSY 1 source, the radiative lifetimes of emitters from BCl3 lying in the range 3-100 ns have been measured. Our results for BCl3 are compared with data from other techniques. Where more than one emitter is excited at a particular VUV energy, the dispersion of the induced fluorescence through a monochromator means that the lifetimes of the different emitters can be differentiated.