The spatial structure of compressed carbon monoxide adlayers on Pt(111) in aqueous acidic solution has been explored by means of in-situ scanning tunneling microscopy (STM) along with infrared reflection–absorption spectroscopy (IRAS). Besides offering a detailed structural picture of this electrochemical interface in comparison with the well-studied Pt(111)/CO system in ultrahigh vacuum (uhv) environments, the real-space structural information provided by STM allows an assessment of the obfuscating influence of dynamic dipole coupling upon IRAS binding-site assignments. In turn, the latter data provide an important crosscheck on the validity of binding-site assignments deduced from the STM images. Emphasis is placed on the structures formed from near-saturated CO solutions, encouraged by the electrode potential-induced adlayer phase transition at ca. 0 V vs SCE observed previously under these conditions by IRAS. At potentials below 0 V, a hexagonal close-packed (2×2)–3CO adlayer is observed, with a CO coverage, θCO, of 0.75. The z-corrugation pattern evident in the STM images indicates the presence of two threefold hollow and one atop CO per unit cell. This binding-site assignment is supported by the corresponding IRAS data which yield C–O vibrational bands at ca. 2065 and 1775 cm−1. The relative intensities of these two νCO bands, ca. 2:1, differs markedly from the 1:2 binding site occupancy deduced from STM. This apparent disparity, however, can be accounted for by dynamic dipole coupling effects between the atop and multifold CO oscillators. At potentials above 0 V (up to the onset of CO electrooxidation at ca. 0.25 V), a markedly different adlayer arrangement is formed, having a (√19×√19)R23.4°–13CO unit cell, with θCO=13/19. This hexagonal structure features CO binding in predominantly asymmetric sites inbetween atop and bridging geometries. A distinction between several alternate adlayer arrangements sharing (√19×√19) symmetry was achieved on the basis of the z-corrugation pattern along with the corresponding IRAS data upon consideration of dipole-coupling effects. Another CO adlayer structure, having a (√7×√7)R19.1°-4CO unit cell (θCO=4/7), was commonly observed at potentials below 0.2 V after the removal of solution-phase CO. These adlayer arrangements are distinctly different to the compressed Pt(111)/CO structures found in uhv. The increased accommodation of CO in multifold sites observed for the former can be understood chiefly from the markedly (ca. 1 V) lower surface potentials (and excess electronic surface charges) characterizing the electrochemical interface.

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Experimental infrared spectra for CO adlayers on Pt(111) electrodes having known real-space structures as deduced by scanning tunneling microscopy are compared with predictions extracted from conventional dipole–dipole coupling models in order to test the validity of such treatments for compressed electrochemical adlayers, especially with regard to band-intensity transfer effects. The specific structures considered are (2×2)–3CO and (√19×√19)R23.4°–13CO hexagonal adlayers; the former is especially close packed (θCO=0.75) with a pair of threefold hollow and one atop CO per unit cell, while the latter has a lower coverage (θCO=13/19) and involves largely asymmetric binding sites. The comparisons between dipole-coupling theory and experiment include infrared spectra for various 13CO/12CO mixtures, thereby exploiting the well-known systematic alterations which are induced in the degree of coupling for a given adlayer. Consistent with an earlier assessment (Ref. ) the conventional dipole–dipole treatment can account semiquantitatively for the marked higher intensity of the atop relative to the threefold hollow C–O stretching band in the observed infrared spectra even though the occupancy on the latter site is twofold greater and the singleton frequencies are substantially (∼280 cm−1) different. This coupling-induced intensity transfer toward the higher-frequency band component is likely to be a widespread phenomenon for densely packed adlayers. For the (2×2) adlayer, however, the isotope composition-dependent spectral band frequencies and relative intensities deviate markedly from the experiment. While the inclusion of stochastic broadening effects associated with adlayer disorder improves the situation, a satisfactory fit between theory and experiment requires the incorporation of vibrational coupling associated with short-range intermolecular interactions. For the (√19×√19) adlayer, on the other hand, dipole–dipole coupling with stochastic broadening accounts well for the observed spectral behavior. The more pronounced limitation of the conventional theory for the (2×2) structure may well be due to the abnormally high adsorbate packing density enhancing the importance of short-range interactions.

Some virtues of modeling electrochemical systems by dosing interfacial components onto clean metal surfaces in ultrahigh vacuum (UHV) are discussed, with an emphasis on elucidating the nature of double-layer solvation and how solvent molecules influence the intermolecular interactions. This “non situ” strategy (as distinct from ex situ approaches involving electrode transfer to/and from UHV) allows each interfacial component (solutes, ions, solvent) to be added sequentially and in controlled amounts, enabling the various molecular (and hence intermolecular) ingredients that constitute the double layer to be accessed in incremental fashion. The approach also provides an invaluable means of understanding the differences in structure and bonding between analogous electrochemical interfaces and the constituent metal−UHV systems. Such issues are particularly germane with the recent advent of microscopic-level structural information for in situ electrochemical systems. Described specifically here is the UHV-based vibrational characterization of solvent and chemisorbate modes by employing infrared reflection−absorption spectroscopy (IRAS), together with work-function measurements, as a function of interfacial composition. The former provides a sensitive monitor of intermolecular interactions as well as being applicable (albeit with more restrictions) to in situ systems, whereas the latter yields insight into surface-potential profiles and also links the potential scales of metal−UHV and electrochemical interfaces. Several distinct examples aimed at elucidating double-layer solvation effects on Pt(111), recently scrutinized in our laboratory, are discussed. These include examining the progressive solvation of cations, adsorbed anions, and combinations thereof, by water and methanol. Comparisons with vibrational spectra for the solvation of gas-phase (i.e., isolated) ions enables the substantial influence of the metal surface upon double-layer solvation to be explored in detail. The converse role of double-layer charge upon the inner-layer solvent orientation (as exemplified for acetone and acetonitrile) is also found to be considerable and involves long-range forces. The combined influences of solvent and double-layer charge upon chemisorbate structure and bonding are also considered for the archetypical example of carbon monoxide. Marked electrostatic effects of solvation upon CO structure and bonding are seen even in the absence of net charge. The complex short-range influences of added cationic (K+) charge upon the CO adlayer are quenched upon partial K+ solvation. The longer-range electrostatic effects are progressively modified as the chemisorbate layer as well as the ionic charges become fully solvated, so to reveal a simple “Stark-tuning” frequency−potential behavior identical with that familiar in electrochemistry. Some more general implications and applications of such “UHV double-layer modeling” tactics are also briefly considered.

The effects of water coadsorption on nitric oxide adlayers on Pt(lll) in ultrahigh vacuum (uhv) are examined with infrared reflection-absorption spectroscopy (IRAS) along with work-function measurements with the objective of relating the uhv-based system to NO chemisorption at the Pt(lll)-aqueous electrochemical interface as studied recently by in-situ IRAS. In contrast to the corresponding (and extensively studied) Pt(lll)/CO system, solvent coadsorption apparently yields little or no change in the NO surface binding geometry at low as well as saturated chemisorbate coverages, the solvent-induced downshifts (ca. 35-70 cm-1) in the N-O stretching (vso) frequencies being consistent with the occurrence of only an electrostatic Stark effect. This behavior, along with the stability of the electrochemical NO adlayer at relatively high electrode potentials (£), facilitates intercomparison of the surface potentials for the aquated uhv and in-situ interfaces by matching the vNo spectrum for the former with the Vno frequency-E data for the latter interface. This procedure yields an estimate of the "absolute" electrode potential, Ek, of the normal hydrogen electrode equal to 4.9 ± 0.1 V. The approximate consistency of this value with some previous estimates of Ek supports the essential validity of the low-temperature uhv-based approach for exploring chemisorbate solvation effects.