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.

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' 7C35A71 CODES 'S SUBjECT TERMS (Continue on roverse if necessary an'd identify by biock number) ELD ;ROL' 5,8-GROuP * ~ Vibrational analysis, adsorption, platinum metal .9 A&SItRACT XCantinu* on reverse if necessary iand identify by block number) (L7~i c >___The vibrationail frequencies of carbon monoxide adsor ed on a platinum metal cx.electrode are obtained in terms of discrete pairwise mt ratomic interactions. 0~ On the one hand, we show that there is a linear dependence of the shift in * C..l frequency with uniform linear changes In the binding energy of carbon and oxygen to the surface. Experimental evidence suggests that there is a relationship LLJbetween the binding energy and the applied electrostatic potential; based on our-model, we suggest that the relationship is linear. As a result, we predict a ILlinear dependence of the frequency on electrochemical potential in accord with C73experiment. On the other hand, we demonstrate that the applicpat 6on of even a ~elatively large external electric field (of the order of-1-l 0 V/rn) has only a ~Inimus effect on the vibrational frequency shift. We conclude that the direct Uin~ffect of the electrostatic field is unlikely to account for the observed spectral shifts. 20. O$TRIISUTON0 AVAILAII ITYOP ABSTRACT 21 ABSTRACT SECI-RiTY CLASS.FiCATiON 63UNCLASSIFIEOUNt.'TEO 10 SAME AS RPT C3orIC USERS Unclassified 138 14AMI Of 46SPONSIBSLE NOIVIOUAL

Articles you may be interested inOn the ground state calculation of a many-body system using a self-consistent basis and quasi-Monte Carlo: An application to water hexamer A new ab initio intermolecular potential energy surface and predicted rotational spectra of the Kr−H2O complex J. Chem. Phys. 137, 224314 (2012); 10.1063/1.4770263 Communication: Quasiclassical trajectory calculations of correlated product-state distributions for the dissociation of (H2O)2 and (D2O)2 J. Chem. Phys. 135, 151102 (2011); 10.1063/1.3655564Elucidating the role of many-body forces in liquid water. I. Simulations of water clusters on the VRT(ASP-W) potential surfaces Rigid-body diffusion Monte Carlo simulations of the ground state and ten low-lying intermolecular excited vibrational states for the cage form of (H 2 O) 6 are reported. The excited states are found by a nodal optimization procedure in which the fundamental excited-state nodes are constructed from the harmonic normal coordinates. The anharmonic effects in the excited states are found to be large. One of the states with relatively large transition intensity involves primarily flipping motions of the free OH bonds on the doubly bound monomers, and is assigned to the vibration-rotation-tunnelling band observed experimentally by Liu et al. ͓Nature 301, 501-503 ͑1996͔͒.

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