DIPOLE-DIPOLE COUPLING EFFECTS UPON INFRARED-SPECTROSCOPY OF COMPRESSED ELECTROCHEMICAL ADLAYERS - APPLICATION TO THE PT(111) CO SYSTEM/

Experimental infrared spectra for CO adlayers on Pt(111) electrodes ha ving known real-space structures as deduced by scanning tunneling micr oscopy are compared with predictions extracted from conventional dipol e-dipole coupling models in order to test the validity of such treatme nts for compressed electrochemical adlayers, especially with regard to band-intensity transfer effects. The specific structures considered a re (2X2)-3CO and (root 19X root 19)R23.4 degrees-13CO hexagonal adlaye rs; the former is especially close packed (theta(CO)=75) with a pair o f threefold hollow and one atop CO per unit cell, while the latter has a lower coverage (theta(CO)=13/19) and involves largely asymmetric bi nding sites. The comparisons between dipole-coupling theory and experi ment include infrared spectra for various (CO)-C-13/(CO)-C-12 mixtures , thereby exploiting the well-known systematic alterations which are i nduced in the degree of coupling for a given adlayer. Consistent with an earlier assessment (Ref. 6) the conventional dipole-dipole treatmen t can account semiquantitatively for the marked higher intensity of th e atop relative to the threefold hollow C-O stretching band in the obs erved infrared spectra even though the occupancy on the latter site is twofold greater and the singleton frequencies are substantially (simi lar to 280 cm(-1)) different. This coupling-induced intensify transfer toward the higher-frequency band component is likely to be a widespre ad phenomenon for densely packed adlayers. For the (2X2) adlayer, howe ver, the isotope composition-dependent spectral band frequencies and r elative intensities deviate markedly from the experiment. While the in clusion of stochastic broadening effects associated with adlayer disor der improves the situation, a satisfactory fit between theory and expe riment requires the incorporation of vibrational coupling associated w ith short-range intermolecular interactions. For the (root 19X root 19 ) adlayer, on the other hand, dipole-dipole coupling with stochastic b roadening accounts well for the observed spectral behavior. The more p ronounced limitation of the conventional theory for the (2X2) structur e may well be due to the abnormally high adsorbate packing density enh ancing the importance of short-range interactions. (C) 1995 American I nstitute of Physics.