Ab initio based study of the ArO- photoelectron spectra: Selectivity of spin-orbit transitions

A combined ab initio atoms-in-molecule approach was implemented to model th e photoelectron spectra of the ArO- anion. The lowest adiabatic states of S igma and Pi symmetry of ArO and ArO- were investigated using the fourth-ord er Moller-Plessett perturbation theory including bond functions. The total energies were dissected into electrostatic, exchange, induction, and disper sion components. The complex of Ar with atomic oxygen is only weakly bound, primarily by dispersion interaction. The Pi state possesses a deeper minim um (R-e = 3.4 Angstrom,D-e = 380 mu E-h) than the Sigma state (R-e = 3.8 An gstrom,D-e = 220 mu E-h). In contrast, the anion complex is fairly strongly bound, primarily by ion-induced dipole induction forces, and the Sigma sta te possesses a deeper minimum at shorter interatomic distances (R-e = 3.02 Angstrom,D-e = 3600 mu E-h) than the Pi state (R-e = 3.35 Angstrom,D-e = 24 00 mu E-h). The Sigma-Pi splittings in both systems are mainly due to diffe rences in the exchange repulsion terms. Atoms-in-molecule models were used to account for the spin-orbit interaction, and to generate adiabatic relati vistic potentials and wave functions. Collisional properties, diffusion, an d mobility coefficients of O and O- in Ar, and absolute total Ar+O scatteri ng cross sections, were calculated and found to agree well with the availab le experimental data. The photoelectron spectra were simulated within vibro nic model, and were found in excellent agreement with the experimental meas urements. The bimodal electron kinetic energy distribution was shown to ste m from the strong selectivity of spin-orbit transitions, which split into t wo dense groups, depending on the initial electronic state of the anion. Th e latter feature cannot be described without explicit consideration of elec tronic intensity factor. (C) 2000 American Institute of Physics. [S0021-960 6(00)31313-7].