Photoionization of As2 and As4: Implications for group V clusters

The vacuum ultraviolet photoionization mass spectrum of As, is presented, from the ionization threshold to 600 A. 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 3 ‘A, state of As:. The first fragment, Ass+, has an appearance potential (0 K) of 11.23 + 0.05 eV, from which we extract Mi (As: ) ~228.7 f 1.3 kcal/mol. The photoion yield curve of As,+ (As, ) is obtained under conditions where As, is dominant in the vapor. The adiabatic ionization potential is 9.69 & 0.02 e-V. Two prominent autoionizing Rydberg series are observed, converging to the A ‘Z: state of As:, with an ionization potential of 10.238 f 0.002 eV. At higher energy, three members of a window resonance series can be seen, converging to the B 22 + state of As 2+, with an ionization potential of 15.37 eV. From an upper limit to the partiil pressure of As,, equilibrium conditions, and assuming a triangular As,, we deduce AH;. (As, ) 260.0 kcal/mol; other criteria suggest AHy&(As, ) ~63 kcal/mol. Consequently, the adiabatic ionization potential of As, is < 7.32 eV, and probably ~7.19 eV. Several implications are drawn, relevant to recent studies of antimony and bismuth clusters.