INFRARED-SPECTROSCOPY OF MIXED NITRIC-OXIDE-CARBON-MONOXIDE ADLAYERS ON ORDERED IRIDIUM(111) IN AQUEOUS-SOLUTION - A MODEL STUDY OF COADSORBATE VIBRATIONAL INTERACTIONS

In-situ infrared reflection-absorption spectra are reported for mixed nitric-oxide-carbon-monoxide adlayers along with the constituent chemi sorbates separately as a function of coverage on ordered Ir(111) at 0. 4-0.45 V vs standard hydrogen electrode in aqueous 0.1 M HClO4, with t he objective of assessing the composition-dependent nature of the coad sorbate vibrational interactions. This substrate-coadsorbate combinati on provides an informative model system since both chemisorbates appea r to bind exclusively in atop (or near-atop) surface sites on the basi s of their simple N-O (nu(NO)) and C-O (nu(CO)) vibrational fingerprin ts, and composition-dependent mixed adlayers can readily be formed via partial replacement of saturated irreversible adsorbed NO layers by e xposure to dilute CO solutions. Increasing the CO coverage, theta(CO), both in the absence and presence of coadsorbed NO, yields marked prog ressive blueshifts in the vco band frequency (ca. 2020-2075 cm(-1)), a ttributable chiefly to enhanced dipole-dipole coupling. While adsorpti on of NO alone exhibited virtually coverage (theta(NO))-independent nu (NO) frequencies, ca. 1835 cm(-1), indicative of chemisorbate island f ormation, dilution within mixed CO/NO adlayers yields progressive nu(N O) redshifts (down to ca. 1790 cm(-1)). The composition-dependent nu(C O) and nu(NO) frequencies within the CO/NO adlayers are consistent wit h molecular intermixing, as supported by comparison with numerical sim ulations extracted from conventional dipole-coupling theory, although the observed nonlinear nu(CO)-theta(CO) dependence suggests the format ion of locally enriched CO regions at intermediate compositions. Evide nce supporting coadsorbate intermixing is obtained by comparing the co mposition-dependent CO and NO band absorbances with the dipole-couplin g predictions. In particular, the presence of coabsorbed CO yields mar ked (up to 3-fold) decreases in the NO band absorbance, especially tow ard lower nu(NO) values, which arise from band-intensity transfer to n eighboring higher-frequency (CO) oscillators. Despite the large (ca. 2 50 cm(-1)) difference in nu(NO) and nu(CO) singleton frequencies,this striking effect is in approximate agreement with dipole-coupling theor y, again presuming molecular CO/NO intermixing. The observed marked in creases in the vco bandwidth toward lower theta(CO) values, along with pronounced asymmetric band shapes, are in good agreement with theoret ical predictions that include stochastic fluctuations of the local ads orbate population density, Moreover, the larger intermediate-theta(CO) nu(CO) bandwidths observed in the presence of coadsorbed NO are also quantitatively accounted for on this basis in terms of coadsorbate int ensity transfer. The more broad-based utility of such dipole-coupling analyses for elucidating local interactions within mixed adlayers is c onsidered in light of these findings.