Interpretation of the infra-red spectra of CO adsorbed on evaporated alkali metal halides

Abstract

Gevirvnan and Kozirovski observed that the G O stretching band for CO adsorbed on evaporated alkali metal halide films at 77 K frequently appears as a doublet, with a splitting of about 10 cm-'. It is shown here that their data may be accounted for quantitatively using the optical equations derived by Dignam, Rao and Roth. On this basis, the splitting of the C-0 band arises predominantly as a result of a local field effect, the planar array of adsorbed CO molecules providing a local unaxiai symmetry.

Concerning the influence of inert gas adsorption on the infrared spectrum of OH groups on powdered silica

“…The difference in the sign of the terms in the denominators of [5] and [6] arises from the fact that for a tangentially oriented external field, the field due to the induced dipole moments augments the external field, while for a normally oriented external field, the opposite is the case.…”

“…Substituting [8] and [9] into [5] and [6] and assuming Ku(4a/3)(N/a)aum and similar terms are small compared to unity, gives the following expressions for the frequency shift on adsorption of the 'inert' gas:…”

“…Substituting [8] into [5] and. [6], and expressing the results as Lorentzian functions gives where L withu = n, t ; Kt = l;andK,, = -2.…”

“…OH groups) occupy only a fraction of the surface area, with subsequently adsorbed species (e.g. Ar) occupying some or all of the remaining area, then on making simplifying approxirnations (i) and (ii): the following substitutions into [5] and [6] may be made. hydroxyl radicals, and the adsorbed 'inert' gas molecules respectively; and N and N" are the corresponding concentrations per unit area.…”

“…These have been tested against data for CO adsorbed on various alkali halides (5) and it was found that local field effects (i.e. phenomena purely optical in nature), could explain most of the spectroscopic features observed and previously explained in terms of special adsorption sites, i.e.…”