Threshold photoelectron spectroscopy of BCl3 and fragmentation of the valence electronic states of BCl3+, studied by coincidence spectroscopies

The valence threshold photoelectron spectrum (TPES) of BCl3 is reported for the first time. Threshold photoelectron–photoion coincidence (TPEPICO), photoion-fluorescence coincidence (PIFCO) and threshold photoelectron-fluorescence coincidence (TPEFCO) spectroscopies have also been used to measure for the first time, state selectively, the decay pathways of the valence states of BCl3+ in the range 11–19 eV. Vacuum UV radiation from a synchrotron source dispersed by a 5 m normal-incidence McPherson monochromator ionises the parent molecule, and electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. Undispersed fluorescence from the interaction region can also be detected, allowing the three different types of coincidence experiment to be performed. There are major differences in the intensity distribution of the different bands in the TPES of BCl3 compared with its He I photoelectron spectrum which we attribute to autoionisation. TPEPICO spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Over the range 11–19 eV, only the ions BCl2+ and BCl3+ are observed. The ground electronic state of BCl3+ is stable with respect to dissociation to BCl2+, whereas the five excited electronic states fragment to BCl2++ Cl. Kinetic-energy releases into BCl2++ Cl have been measured at the Franck–Condon maxima of the C 2A″2, D 2E′ and E 2A′1 excited states of BCl3+ with good time resolution. PIFCO and TPEFCO spectra are recorded at fixed photon energies. The former experiment can yield the fate of the lower electronic state of the parent ion to which fluorescence occurs. The latter experiment can yield the lifetime of the fluorescing state. Radiative decay is confirmed to occur from the fourth excited state of BCl3+(D 2E′) at 15.5 eV, and the lifetime of this state is measured to be 5.5 ± 0.5 ns. The absence of parent ion signal in the energy-scanning TPEPICO spectrum through the D state allows us to put an upper limit on the fluorescence quantum yield of this state of ca. 0.04.