THRESHOLD PHOTOELECTRON-SPECTROSCOPY OF BCL3 AND FRAGMENTATION OF THEVALENCE ELECTRONIC STATES OF BCL3+, STUDIED BY COINCIDENCE SPECTROSCOPIES

The valence threshold photoelectron spectrum (TPES) of BCl3 is reporte d for the first time. Threshold photoelectron-photoion coincidence (TP EPICO), photoion-fluorescence coincidence (PIFCO) and threshold photoe lectron-fluorescence coincidence (TPEFCO) spectroscopies have also bee n used to measure for the first time, state selectively, the decay pat hways 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-incidenc e McPherson monochromator ionises the parent molecule, and electrons a nd ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. Undispersed fluorescence from the in teraction region can also be detected, allowing the three different ty pes of coincidence experiment to be performed. There are major differe nces in the intensity distribution of the different bands in the TPES of BCl3 compared with its He I photoelectron spectrum which we attribu te to autoionisation. TPEPICO spectra are recorded continuously as a f unction of photon energy, allowing yields of the fragment ions to be o btained. Over the range 11-19 eV, only the ions BCl2+ and BCl3+ are ob served. The ground electronic state of BCl3+ is stable with respect to dissociation to BCl2+, whereas the five excited electronic states fra gment to BCl2+ + Cl. Kinetic-energy releases into BCl2+ + Cl have been measured at the Franck-Condon maxima of the <(C)over tilde (2)A(2) '' >, <(D)over tilde (2)E'> and <(E)over tilde (2)A(1)'> excited states o f BCl3+ with good time resolution. PIFCO and TPEFCO spectra are record ed 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 fluorescin g state. Radiative decay is confirmed to occur from the fourth excited state of BCl3+(<(D)over tilde (2)(E)'>) 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) ove r tilde state allows us to put an upper limit on the fluorescence quan tum yield of this state of ca. 0.04.