The detection of adsorbed molecules on ice at surface coverage similar to those encountered in environmental conditions requires high surface sensibility that few techniques can afford. An experimental methodology allowing a significant enhancement in the absorption from adsorbed molecules is demonstrated here. It exploits electric field standing wave (EFSW) effects intrinsic to grazing incidence Reflection Absorption Infrared Spectroscopy (RAIRS) where film thickness dependant optical interferences occur between the multiple reflected and transmitted IR beams at the film-vacuum and the substrate-film interfaces. As a case study, CH4 is used as a probe molecule and deposited on a 20 ML thick dense amorphous solid water (ASW) film adsorbed onto underlayers of Ar of various thicknesses. These investigations reveal that an enhancement from 20 to 35 is achieved when destructive interference coincides with the absorption features of the adsorbate, that is the ν3 and ν4 modes of methane, respectively. Simulations using Fresnel transmission and reflection coefficients reproduce the Ar thickness dependant enhancement for CH4 absorption bands and reveal that enhancement in the absorption by adsorbates occurs when the square modulus of the electric field at the film’s surface reaches its minimum. Exploiting the EFSW allows the limit of detection of adsorbed molecules at the surface of ice coverage to be reduced to 0.2 ML using CH4 as a probe molecule which opens interesting perspectives for spectroscopic studies of heterogeneous atmospheric chemistry at coverages that are more representative of those found in the natural environment.

Methodology to enhance absorption of thin films on ice using reflection-absorption infrared spectroscopy (RAIRS)

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.