A general strategy for computing the vibrational frequencies of adsorbates on surfaces is presented. Rather than use numerical fits to the surface energy, we advocate diagonalizing a mass weighted energy second-derivative matrix. The computation of energy derivatives has several advantages: It facilitates a more accurate treatment of the adsorbatersubstrate mass ratios. The normal modes of the surface vibrations are given automatically in the diagonalization step. Finally, a major advantage of using energy derivatives is that they provide estimates of the range of the adsorbatersubstrate interactions which can be used to ensure that the frequency values are converged. We illustrate the strategy for cluster calculations which simulate the Ž . Ž . Ž . Ž .
It has been proposed recently that localized excitations and recombinations in the silanone SidO bond is the source for the red photoluminescence (PL) observed for oxygen-exposed porous silicon (PS). One concern with this proposal is the expected low chemical stability of the SidO bond. By using quantum chemistry calculations and molecular clusters to simulate the PS surface, we show that the hydrated silanone bond is less stable than the hydroxylated Si(OH) 2 surface species. However, we also determine there is a low barrier for the conversion of the hydroxylated Si to a hydrated silanone surface species. Our calculations suggest that the origin of the red PL for oxygen-exposed PS first involves the conversion of hydroxylated Si species, presumably by energy provided by the incident photon source, to a surface with a larger concentration of silanone groups. These metastable ground-state silanone groups would then be excited by additional photons to produce the observed PL.
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