Vacuum-UV fluorescence spectroscopy of PX3 (X = Cl, Br) in the range 9–25 eV

Abstract

The vacuum-UV (VUV) and visible spectroscopy of PCl₃ and PBr₃ using fluorescence excitation and dispersed emission techniques is reported. Non-dispersed fluorescence excitation spectra have been recorded following photoexcitation with monochromatised synchrotron radiation from the Daresbury, UK source in the VUV energy range 9–25 eV, with an average resolution ofca. 0.02 eV. The use of optical filters shows that fluorescence is due to both resonant excitation of Rydberg states of PX₃ (X = Cl,Br) which photodissociate to a fluorescing state of a fragment (e.g. PX₂, X₂) and to non-resonant excitation of excited electronic states of PX₃⁺. Dispersed emission spectra in the UV–VIS have been recorded, with an optical resolution of ca. 4–8 nm, at the BESSY 1, Germany synchrotron source at the energies of the peaks in the excitation spectra. Four different decay channels areobserved. (a) PCl₂ ²B₂–²B₁ fluorescence in the range 350–700 nm for photon energies in the range 10–12 eV; the analogousemission in PBr₂ falls outside the sensitivity range of the detection systems. (b) Cl₂ and Br₂ D′ 2 ³Π_g–A′ 2 ³Π_u emission at ca. 260 and 290 nm, respectively, for photon energies in the range 13–18 eV. (c) Emission from the Ẽ²E state of PCl₃⁺ and PBr₃⁺ for photon energies in excess of the adiabatic ionisation energies of these states of 14.6 and 13.9 eV, respectively. (d) Atomic emission from excited states of the P atom for photon energies in the range 19–25 eV. Action spectra have also been recorded at BESSY 1, in which the energy of the VUV radiation is scanned with detection of the fluorescence at a fixed, dispersive wavelength. Energy thresholds for production of the emitters can then be determined. The ²B₂ state of PCl₂ has a threshold energy which lies well above the thermodynamic energy needed to produce PCl₂ ²B₂ + Cl. We conclude that it forms via a one-step P–Cl bond cleavage of a Rydberg state of PCl₃. From the threshold energy for X₂ D′ 2 ³Π_g production, we conclude that this excited ion-pair state of X₂ forms in combination with the ground electronic state of PX, and not with isolated P + X atoms. Theseproducts form via a single-step photodissociation of a Rydberg state of PX₃ which must now pass through a tightly constrainedtransition state. By contrast, excited states of the phosphorus atom are formed by a sequential, multi-step photodissociation of PX₃^*. Now the thresholds for these emissions correspond to the thermodynamic energies to form the emitting P* atom in combination with three halogen atoms.