Electronic quenching of Al and Ga atoms isolated in rare gas matrices

Abstract

Aluminum and gallium atoms have been trapped in Ne, Ar, Kr, and Xe matrices and studied by optical and ESR spectroscopy at 4.2 °K and slightly higher temperatures. The results indicate that both metal atoms occupy axially distorted substitutional sites in all rare gas lattices. This elongated tetradecahedral MeX₁₂ coordination is particularly stable for rare gas complexes of Group III metal atoms exhibiting a single unpaired electron in their outermost p shell. From the ESR data large splittings of the aluminum and gallium p shells have been derived increasing from [inverted lazy s] 1600 cm⁻¹ in neon to [inverted lazy s] 3200 cm⁻¹ in xenon for both atoms. The corresponding Jahn‐Teller stabilization energies E_JT (increasing from [inverted lazy s] 1.5 kcal/mole for MeNe₁₂ to [inverted lazy s] 3.0 kcal for MeXe₁₂) can be explained by the ``σ‐π'' effect: The van der Waals interatomic correlation energy is maximized, and the repulsive exchange energy is minimized by attraction of the equatorial ligand atoms to the metal center and repulsion of the remaining ligands from the σ antibonding axial positions. The ²S ← ²P[(n + 1)s ← np] electronic transitions are shifted by [inverted lazy s] + 1000 cm⁻¹ (MXe₁₂) to [inverted lazy s] + 6000 cm⁻¹ (MNe₁₂) relative to the free metal atom values. The ESR spectra exhibit axial symmetry, show effects of preferential orientation, and demonstrate almost complete quenching of the free atom angular momentum in each case. The basic features of the g values and the metal hyperfine tensor (and of their strong dependence on the matrix and on temperature) can be understood within a simple crystal field model, but there are significant deviations. The introduction of orbital angular momentum and spin‐orbit reduction factors resulting from orthogonalization of the metal p orbitals to the valence shells of the surrounding rare gas atoms removed a large part of the discrepancies, but quantitative agreement with experiment could be obtained only when the dynamic Jahn‐Teller effect was taken into account. In order to establish the geometries of the rare gas cages surrounding the trapped metal atoms, numerical calculations of orbital and spin‐orbit reduction factors were performed for various sites in the rare gas lattices. For the determination of the vibronic quenching parameters a slight extension of Ham's second order theory of an orbital triplet interacting with an e_2g vibrational mode was required. Our results indicate a remarkable stability of the Al and Ga rare gas complexes. Indeed, from the results of Baylis' semiempirical calculations it can be concluded that atoms with singly occupied p shells form the strongest van der Waals complexes with rare gas atoms among all atoms in the periodic table.