Excited States of Solid Ar:H. II. Line Shapes and Interstitial Sites

Abstract

The line shape of the $1s-2p$ transitions of substitutional H and D atoms in solid argon has been analyzed on the basis of a "four-mode model." The impurity is treated as interacting (semiclassically) with a linearly coupled "breathing mode" and with three localized quadratically coupled modes. A full quantum-mechanical treatment of the latter is essential. The experimentally observed isotope shift of the peak is fitted and part of the isotopic width change accounted for by this model. Local-mode infrared absorptions at 0.033 and 0.023 eV are predicted for Ar:H and Ar:D, respectively. It is shown that the relatively high concentration of H (or D) impurities used (∼2%) simultaneously accounts for the additional width change and the low-energy tail of the spectrum. Excitation energies for the $1s-2p$ transitions of H atoms trapped at octahedral or tetrahedral interstitial sites are computed using a tight-binding model which was previously used to predict the excitation energy for the substitutional site. Within computational accuracy the transitions are found to take place at the same energy as that for substitutional atoms. Although it seems unlikely that interstitials are present in Baldini's work (here analyzed), experiments to verify their absence are suggested. Experimental measurement of the concentration dependence of the linewidths and the intensity of the low-energy tail will provide useful checks of the model.