DETERMINATION OF CONSECUTIVE BOND-ENERGIES BY PHOTOIONIZATION OF SBHN(N=1-3)

A photoionization mass spectrometric study of SbH3 is presented. The a diabatic ionization potential (IP) of SbH3 is less-than-or-equal-to 9. 40+/-0.02 eV. The lowest energy fragment ion, SbH+ (+H-2), has an appe arance potential (0 K) of 9.73(0)+/-0.08 eV, while SbH2+ has an AP of 11.66+/-0.02 eV. The transient species SbH2 and SbH are generated in s itu by reacting F atoms with SbH3. The IP of SbH2, forming SbH2+ (X1A1 ), is 8.731+/-0.012 eV. The IP of SbH (X3SIGMA-,0+) to form SbH+ (X2PI 1/2) is probably 8.753+/-0.009 eV, but certainly < 8.79 eV. Autoionizi ng structure in the photoion yield curve of SbH + (SbH) is interpreted as Rydberg series converging to SbH+ (a 4SIGMA-), which appears to be split into 1/2 and 3/2 components, with IP's of 10.843+/-0.011 eV and 10.866+/-0.01 1 eV. The difference in IP's (Sb-SbH, SbH-SbH2) appears to conform to the extended Goddard-Harding model, when adjusted for s pin-orbit splittings. The derived heats of formation are DELTAH(f0)0(S bH) = 59.1+/-0.3 kcal/mol and DELTAH(f0)0(SbH2) = 52.5+/-0.6 kcal/mol. These values lead to D0(SbH) = 56.4+/-1.0, D0(HSb-H) = 58.3+/-0.6, D0 (H2Sb-H) = 67.5+/-0.5 (in kcal/mol). The differences in successive bon d energies, 1.9+/-1.2 and 9.2+/-0.8 kcal/mol, depart significantly fro m the constant value (4.44 kcal/mol) predicted by the Goddard-Harding model. A rationalization is presented, that incorporates relativistic effects. This relativistic picture implies that for BiH(n), D0(BiH) > D0(HBi-H), a conclusion for which some experimental evidence exists. H owever, relativistic ab initio calculations, which agree rather well i n their calculated differences in successive bond energies for SbH(n), do not predict this reversal in BiH(n).