Threshold photoelectron spectroscopy of PX3 (X=Cl, Br), and fragmentation of the valence electronic states of PX3+ studied by TPEPICO spectroscopy

The valence threshold photoelectron spectra (TPES) of phosphorous trichloride (PCl3) and phosphorous tribromide (PBr3) are presented for the first time. Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has also been used to measure, state selectively, the decay pathways of the valence states of PX3+(X=Cl, Br) in the range 10–25 eV. Vacuum-UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules, electrons and ions being detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing coincidence ion yields of the fragment ions to be obtained. The ground and first excited states of both molecular parent ions are stable with respect to dissociation to PX2+, whereas the B2E, C2E, D2A1 and E2E states dissociate exclusively to PX2+. The first experimental observation of the F2A1 state of PBr3+ is presented, which we deduce to have a very small cross-section to photoionisation. Fragmentation of the F2A1 states of both parent ions leads to exclusive production of PX+. Kinetic-energy (KE) releases into PX2++X have been measured at the Franck–Condon maxima of the B2E, C2E, D2A1 and E2E states of PCl3+ and the B2E, C2E and E2E states of PBr3+. For the first time, the method used to calculate the KE releases allows for the full range of daughter ion isotopes to be accommodated.