Frequency shifts in the spectra of molecules adsorbed on metals, with emphasis on the infrared spectrum of adsorbed CO

Abstract

Two processes are considered which cause the frequency of the band maximum in the infrared absorption spectrum of molecules adsorbed on metals to shift with increasing coverage. The first arises from the coverage-dependent modification of the local field acting on an adsorbed molecule while the second originates from vibrational coupling arising from a through-metal interaction between molecules adsorbed on different metal atoms. Infrared absorption strengths are discussed and the method used heretofore to obtain the extinction coefficients of adsorbed molecules is questioned. In particular, a factor of 2 which results from the constructive superposition of the incident and reflected electric fields is shown to be often ignored. We show that dipolar (through-space) coupling is too weak to account for the magnitudes of the shifts recently observed for CO on single-crystal Pt and Pd as coverage progressed from low to high values. Vibrational coupling is also shown to account for the disparity in the intensities of the two bands observed when 12CO and 13CO are coadsorbed on supported metals. Similar experiments on single-crystal surfaces are suggested to help determine to what extent the observed spectral frequency shifts are derived from adsorbate—adsorbate coupling as opposed to other mechanisms.