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

Abstract

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.

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