The problem of optical absorption due to oriented molecular films adsorbed within porous media has been treated theoretically in the approximation that the adsorbent behaves as a collection of spherical particles or cavities of small size compared with the wavelength of light. Expressions for the contribution to the spectrum arising from optical modes involving charge displacement paralleI to the local substrate surface, relative to those involving normal displacements, are derived from which a number of conclusions are reached. Thus for example it is shown that in general the relative intensities of different absorption bands are dependent upon the detailed geometry of the porous adsorbent. The equations presented offer the first opportunity for quantitative interpretation of infra-red absorption spectra for porous adsorbent-adsorbate systems.

. Can. J. Chem. 54, 1388 (1976).McDonald's study of the absorption band at 3749 cm-1, due to the 0-H stretching vibration of hydroxyl groups on outgassed powdered silica, revealed substantial changes in center frequency and band area upon adsorption of Ar. The main features of these changes are accounted for here using an optical model for porous adsorbent-adsorbate systems due t o Dignam, Rao, and Roth. The possibility of using the measured change in band intensity on adsorbing an inert gas, as a general indicator of the orientational distribution of the adsorbed species responsible for the band, is noted. [Traduit par le journal]