Vacuum UV fluorescence excitation spectroscopy of BCl3. Electronic spectroscopy of BCl2, BCl2+ and BCl3+

Abstract

The fluorescence processes following vacuum-UV excitation of BCl₃ in the photon range 9–25 eV have been studied using synchrotron radiation. Excitation spectra have been recorded at the UK Daresbury source with no dispersion of the fluorescence. Such spectra give information on the primary excitation process, i.e. the formation of Rydberg states of BCl₃ and electronic states of the parent molecular ion which show radiative decay. The use of optical filters gives a limited degree of information on the nature of the emitting species. Using the radiation source in its pulsed, single-bunch mode, lifetimes of the emitting states have been measured. Dispersed fluorescence spectra have been recorded at the German BESSY 1 source in Berlin where, by dispersing the vacuum-UV-induced fluorescence through a secondary monochromator, low-resolution information has been obtained on the nature of the emitting species. For photon energies below 12 eV, photodissociation of Rydberg states of BCl₃ produces a substantial branching ratio into two, or possibly three, excited valence states of the BCl₂ radical which fluoresce to its ground state. For energies between 13 and 18 eV, BCl A ¹Π is produced, leading to emission to its ground state at 272 nm. Emission is observed from the D ²E′ excited state of BCl₃⁺ with a threshold energy of 15.32 eV, the adiabatic ionisation potential of this state. A weak emission between 280 and 350 nm, having a threshold for production of 16.75 eV, is assigned to a vibronically resolved electronic transition in the BCl₂⁺ ion, the first observation of a spectrum of any kind in this ion. Emission from two excited states of the boron atom is observed at 209 and 250 nm; the thresholds for these emissions occur at the thermochemical threshold for production of that state of B* with three chlorine atoms.