A combined ab initio atoms-in-molecule approach was implemented to model the photoelectron spectra of the ArO- anion. The lowest adiabatic states of Sigma and Pi symmetry of ArO and ArO- were investigated using the fourth-order Moller-Plessett perturbation theory including bond functions. The total energies were dissected into electrostatic, exchange, induction, and dispersion components. The complex of Ar with atomic oxygen is only weakly bound, primarily by dispersion interaction. The Pi state possesses a deeper minimum (R-e = 3.4 Angstrom,D-e = 380 mu E-h) than the Sigma state (R-e = 3.8 Angstrom,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 state 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 = 2400 mu E-h). The Sigma-Pi splittings in both systems are mainly due to differences in the exchange repulsion terms. Atoms-in-molecule models were used to account for the spin-orbit interaction, and to generate adiabatic relativistic potentials and wave functions. Collisional properties, diffusion, and mobility coefficients of O and O- in Ar, and absolute total Ar+O scattering cross sections, were calculated and found to agree well with the available experimental data. The photoelectron spectra were simulated within vibronic model, and were found in excellent agreement with the experimental measurements. The bimodal electron kinetic energy distribution was shown to stem from the strong selectivity of spin-orbit transitions, which split into two dense groups, depending on the initial electronic state of the anion. The latter feature cannot be described without explicit consideration of electronic intensity factor. (C) 2000 American Institute of Physics. [S0021-9606(00)31313-7].