Absorption spectra of molecules adsorbed within porous media and the effect of molecular orientation

Abstract

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.

Ellipsometry of clean surfaces, submonolayer and monolayer films

“…Watanabe and Hirakuta [59] have calculated the effect of chemisorption on the optical reflection and electrical resistance of thin Ag films by estimating the scattering probabilities of the conduction electrons by chemisorbed atoms with quantum mechanical scattering theory. A model to take surface roughness into account has been developed by Dignam et al [60,61] and used to analyze the transmission and reflectance spectra of gases adsorbed on silver films. With this model and by allowance for the direction of polarization of absorption bands in polarization modulation spectra, Moskovits and McBreen were able to deduce the surface geometry of hydrogen and ethylene adsorbed on copper films [62].…”

Further developments in photoacoustic Fourier-transform infrared spectroscopy: temperature studies

“…In several papers the approach of a highly polarizable inert gas as a coupling medium in the photoacoustic cell was applied to estimate the orientation of small and large molecules on surfaces (6-10). Although we have tried to justify this approach by using Dignam theory (11,12), there are still some experimental issues to be tested. One of these issues is the possibility of a resonance effect in the photoacoustic cell when highly polarizable xenon is used as a coupling gas.…”