Photoionization mass spectrometric studies of AsHn (n=1–3)

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Abstract
The photoion yield curves of the free radicals AsH and AsH2, prepared by the reaction of H atoms with AsH3, have been measured. The adiabatic ionization potential of AsH (forming AsH+, X 2Π1/2) is 9.641±0.008 eV. Autoionizing Rydberg states are observed and analyzed to converge to an a 4Σ− state lying 1.94 eV higher in energy. The adiabatic ionization potential of AsH2 (forming AsH+2, X̃ 1A1) is 9.443±0.007 eV. The 3B1 state of AsH+2 is conservatively estimated from the spectrum to lie 0.60–1.46 eV higher in energy, with the lower figure expected to be close to the true value. In addition, the ion yield curves of AsH+3, AsH+2, and AsH+ from photoionization of AsH3 have been measured. From these measurements, the adiabatic ionization potential of AsH3 is 9.82±0.01 eV, the appearance potential of AsH+2 (+H) is 12.69±0.01 eV, and that of AsH+(+H2) is 11.295±0.05 eV. The latter two measurements, when combined with the corresponding ionization potentials, yield D0(H2As–H)=74.9±0.2 kcal/mol and D0(HAs–H)=66.5±0.2 kcal/mol. The value of D0(As–H), as deduced from these measurements, depends upon an accurate heat of atomization of AsH3, which in turn requires an accurate value for ΔH○f0 (As,g). An analysis of alternative values is presented, from which D0(As–H)=64.6±0.7 kcal/mol (2.80±0.03 eV) is obtained. When these stepwise bond energies, and earlier results on PHn and NHn, are compared with the semiempirical model of Goddard and Harding, the largest discrepancy occurs for NHn. An analysis of successive ionization potentials Pn, PnH, PnH2 (Pn=N, P, As), and also Ch, ChH, ChH2 (Ch=O, S, Se) based on the same philosophy again shows a large departure from prediction for the first row elements, but fair agreement for the second and third row hydrides. The deviation of the first row hydrides from the Goddard–Harding model is attributed to the substantial ionic character of these systems.

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