Photoionization of As2 and As4: Implications for group V clusters

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Abstract
The vacuum ultraviolet photoionization mass spectrum of As4 is presented, from the ionization threshold to 600 Å. The apparent adiabatic ionization potential is ≤8.49 eV, but the true value may be significantly lower. Three broad autoionization features are observed, probably comprising members of a Rydberg series converging to the B̃ 2 A 1 state of As+ 4. The first fragment, As+ 3, has an appearance potential (0 K) of 11.23±0.05 eV, from which we extract ΔH 0 f 0 (As+ 3)≤228.7±1.3 kcal/mol. The photoion yield curve of As+ 2(As2) is obtained under conditions where As2 is dominant in the vapor. The adiabatic ionization potential is 9.69±0.02 eV. Two prominent autoionizing Rydberg series are observed, converging to the A 2Σ+ g state of As+ 2, with an ionization potential of 10.238±0.002 eV. At higher energy, three members of a window resonance series can be seen, converging to the B 2Σ+ u state of As+ 2, with an ionization potential of 15.37 eV. From an upper limit to the partial pressure of As3, equilibrium conditions, and assuming a triangular As3, we deduce ΔH 0 f 0 (As3)≥60.0 kcal/mol; other criteria suggest ΔH 0 f 0 (As3)≂63 kcal/mol. Consequently, the adiabatic ionization potential of As3 is <7.32 eV, and probably ≤7.19 eV. Several implications are drawn, relevant to recent studies of antimony and bismuth clusters.

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