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

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 3Πg–A′ 2 3Πu 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 3Πg 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.