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The He1 photoelectron spectrum of the P2(X'C;) molecule has been recorded and interpreted with the aid of ab initiu and semi-empirical molecular orbital calculations. Three ionic states of the diphosphorus molecule are found to be accessible with He1 radiation. Comparison is made between the properties of these states and the corresponding well established ionic states of nitrogen.The main objective of this work was to use photoelectron spectroscopy (pees.) to identify and characterize the low lying electronic states of P2f obtained from photoionization of P2 by He1 radiation. This was done in order to make a direct comparison between the low-lying ionic states of P2 and N2 and to compare the characteristics of these P i states with information derived from other techniques.A number of states of P2 and P2f have already been characterized by optical or electrical discharge 3 9 5 9 of P4 vapour. Some theoretical studies have also been made on the P, molecule ' 9 but no similar calculations appear to have been carried out on the ground or any of the low-lying states of P i .The p.e. spectrum of P, produced by pyrolysis of P4 has been studied previously in the 9.0-1 1.0 eV regio11.~ In this work, because the pyrolysis of P, was incomplete, the bands attributable to P, were roughly the same intensity as those from the parent P4. However, two bands were observed with vertical ionization potentials equal to 10.65 and 10.84 eV which were associated with ionization from the 2nu and 5ag P2 molecular orbitals respectively.

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The He(1) photoelectron spectrum of the C10(X211i) radical has been recorded and interpreted with the aid of ab initio ASCF calculations. Of the expected eleven ionic states, five have been positively identified. The remaining six ionic states have been shown to occur within a 1.5 eV energy range and contribute to a broad band centred at 14.68 eV.

The vacuum ultraviolet photoelectron spectrum of the PN(X'Z:f) molecule has been recorded and interpreted with the aid of ab initio molecular orbital calculations. Three bands corresponding to ionization of the PN molecule have been observed with vertical ionization potentials of 11.88, 12.30 and 15.74 eV respectively. The properties of the observed states are compared with those of the low-lying ionic states of Nz and P2.As part of our interest in the electronic structure of small molecules produced by high temperature methods,l9 we have investigated the PN(XIC+) molecule using photoelectron spectroscopy (p.e.s.). The main aim of this work was to identify and characterize the low-lying states of PNf with a view to comparison of these states with those already known for N2+ and PZ. Because the nitrogen molecule is the classic case where Koopmans' theorem fails to predict the correct order of the low-lying ionic it is anticipated that Pz and PN will provide good tests of both the qualitative and quantitative applicability of Koopmans' theorem to small closed shell molecules. and a preliminary account of the He1 spectrum of PN has appeared in the l i t e r a t~e .~ In that study, although the signal-to-noise ratio was rather poor, two bands with vertical ionization potentials of 11.85 and 12.52 eV could be associated with PN. Although a number of other spectroscopic studies of the PN molecule have been performed,*-' only one other experimental estimate of the first ionization potential has appeared. This was obtained via an electron impact mass spectrometric study and gave a value of (I 1.8 0.1) eV. The He1 p.e. spectra of N2(X1E8+) and P2(X1XP+) have already been published 1D $ * EXPERIMENTAL AND COMPUTATIONAL D E T A I L SThe photoelectron spectrometer used in this study was of the electrostatic analyser type and has been described previously.2 Under the operating conditions, the usual resolution was 20-25 meV as measured (full width at half maximum) for argon ionised by He1 radiation. Unless otherwise stated the i.p. values are quoted to an estimated accuracy of kO.01 eV and the vibrational spacings to 30 cm-l.In this study the PN molecule was produced using the following methods.(a) P Y R O L Y S I S O F P3N5 P3N5 (Pfaltz and Bauer) was pyrolysed using an inductively heated furnace into flowing helium approximately 4 c m away from the ionization region of the spectrometer. As the temperature was increased, no new bands were observed in the spectrum, apart from a small water signal, until a temperature of -1100 K was reached, when a spectrum attributed to PN and N2 was seen. The water signal arose from a very small leak in the water cooling system of the induction furnace. This method of PN production was very useful in that it 983