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

摘要

Aluminum and gallium atoms have been trapped in Ne, Ar, Kr, and Xe matrices and studied by optical and ESR spectroscopy at 4.2 deg;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 MeX12coordination is particularly stable for rare gas complexes of Group III metal atoms exhibiting a single unpaired electron in their outermostpshell. From the ESR data large splittings of the aluminum and galliumpshells have been derived increasing from inverted lazy s 1600 cmminus;1in neon to inverted lazy s 3200 cmminus;1in xenon for both atoms. The corresponding Jahnhyphen;Teller stabilization energiesEJT(increasing from inverted lazy s 1.5 kcalsol;mole for MeNe12to inverted lazy s 3.0 kcal for MeXe12) can be explained by the ``sgr;hyphen;pgr;'' 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 sgr; antibonding axial positions. The2Slarr;2P(nplus; 1)slarr;np electronic transitions are shifted by inverted lazy s plus; 1000 cmminus;1(MXe12) to inverted lazy s plus; 6000 cmminus;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 thegvalues 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 spinhyphen;orbit reduction factors resulting from orthogonalization of the metalporbitals 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 Jahnhyphen;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 spinhyphen;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 ane2gvibrational 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 occupiedpshells form the strongest van der Waals complexes with rare gas atoms among all atoms in the periodic table.