Accommodation and migration of the ground-state (2s22p4 3P) oxygen atom in the ideal Ar, Kr, and Xe rare gas crystals are investigated using the classical model. The model accounts for anisotropy of interaction between guest and host atoms, spin–orbit coupling, and lattice relaxation. Interstitial and substitutional accommodations are found to be the only thermodynamically stable sites for trapping atomic oxygen. Mixing of electronic states coupled to lattice distortions justifies that its long-range thermal migration follows the adiabatic ground-state potential energy surface. Search for the migration paths reveals a common direct mechanism for interstitial diffusion. Substitutional atoms are activated by the point lattice defects, whereas the direct guest–host exchange meets a higher activation barrier. These three low-energy migration mechanisms provide plausible interpretation for multiple migration activation thresholds observed in Kr and Xe free-standing crystals, confirmed by reasonable agreement between calculated and measured activation energies. An important effect of interaction anisotropy and a minor role of spin–orbit coupling are emphasized.

The complexes of the Ba atom and Ba+ cation with the rare gas atoms Ar, Kr, and Xe in the states associated with the 6s → 5d, 6p excitations are investigated by means of the multireference configuration interaction techniques. Scalar relativistic potentials are obtained by the complete basis limit extrapolation through the sequence of aug-cc-pwCVnZ basis sets with the cardinal numbers n = Q, T, 5, combined with the suitable effective core potentials and benchmarked against the coupled cluster with singles, doubles, and non-iterative triples calculations and the literature data available for selected electronic states. Spin-orbit coupling is taken into account by means of the state-interacting multireference configuration interaction calculations performed for the Breit-Pauli spin-orbit Hamiltonian. The results show weak spin-orbit coupling between the states belonging to distinct atomic multiplets. General trends in the interaction strength and long-range anisotropy along the rare gas series are discussed. Vibronic spectra of the Ba and Ba+ complexes in the vicinity of the 1S → 1P° and 2S → 2P° atomic transitions and diffusion cross sections of the Ba(1S0, 3DJ) atom in high-temperature rare gases are calculated. Comparison with available experimental data shows that multireference calculations tend to underestimate the interaction strength for excited complexes.

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The electronic transitions of ytterbium atoms in a solid Xe matrix grown at 4.8 K are investigated. Absorption bands are detected in the regions of the gas-phase 6s2 1S0 → 4f135d6s2 and 6s2 1S0 → 6s6p 1P1 transitions. Both bands indicate that Yb atoms occupy multiple trapping sites, of which three are identified. Emission induced by the 6s2 1S0 → 6s6p 1P1 excitation is found to be concentrated entirely in the region of the 6s6p 3PJ → 6s2 1S0 decay, whereas the singlet emission is completely quenched. Multiple emission peaks are observed and the effects of annealing and prolonged irradiation on their amplitudes are found to be significant and are interpreted as a consequence of Yb population transfer from one type of site to another. Modeling of the ground-state site structure and stability predicts three Yb/Xe occupation types, substitutional (ss), tetravacancy (tv) and hexavacancy (hv), in order of decreasing stability. Their tentative associations with observed absorption and emission features are discussed. Time correlated single photon spectroscopy is used to determine the lifetimes of the individual emission bands. They are found to be different from each other with indications of a mixture of short- and long- lived 6s6p 3PJ fine-structure components and demonstrate distinct temperature dependencies. A dramatic decrease in the lifetime of the emission peak tentatively assigned to the most stable site with temperature is explained by a competition between the radiative and non-radiative decay paths of the 6s6p 3P1 state. The mechanism of the latter can be attributed to electron–phonon coupling as confirmed by a model of the temperature-dependence of the lifetime.

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