Tailoring Electric Field Standing Waves in Reflection–Absorption Infrared Spectroscopy to Enhance Absorbance in Buried Molecular Layers

Abstract

Electric field standing waves (EFSW) that establish within thin films during their characterization by reflection− absorption infrared (RAIR) spectroscopy are shown to result in a >25-fold enhancement in the intensity of absorbance features in a molecularly thin methane (Me) layer buried within Argon (Ar) films. Using an all-optical depth profiling method and describing the interference effects responsible for these phenomena using a classical optics model, modulations in the EFSW amplitude at specific sample thicknesses are demonstrated to be responsible for these strong deviations from Beer−Lambert behavior. Tuning sample thickness is shown to enable the EFSW depth profile to be tailored, conferring greatly enhanced sensitivity and increased spatial selectivity to RAIR spectroscopy. These are highly desirable attributes for the characterization of interfacial structures and mesoscopic dynamic processes within complex buried multilayer assemblies and stratified composite films used to model and study a range of materials and life science phenomena.

Preparation and Characterization of Metastable trans-Dinitrogen Tetroxide

“…The absorbance spectra for thin film samples were acquired up to very high dilution (i.e., reaching mixing ratios from 1:200 to 1:5000) by tailoring sample thicknesses to selectively enhance the vibrational spectral features from NO 2 and its dimers, dissolved in solid Ar matrices, using electric field standing waves following a methodology described recently. 45,46 The experimental methods are further described in the Supporting Information (SI). The numbers in the parentheses stand for the computed intensity, in units of km/mole, for the stretching modes; a scaling factor of 0.976 was applied to the frequencies reported in Figure 1F in order to account for vibrational mode anharmonicity.…”

“…We posit that the impact of deposition methods used to manipulate nitrogen dioxide vapors has not, perhaps, been fully appreciated in previous studies. The absorbance spectra for thin film samples were acquired up to very high dilution (i.e., reaching mixing ratios from 1:200 to 1:5000) by tailoring sample thicknesses to selectively enhance the vibrational spectral features from NO 2 and its dimers, dissolved in solid Ar matrices, using electric field standing waves following a methodology described recently. , The experimental methods are further described in the Supporting Information (SI).…”