Vibrational Spectroscopy of Surface Adsorbates on Metal Surfaces. Experiments and Calculations

Abstract: This work deals with a detailed analysis of the vibrational spectra of methoxy (CH₃O-) and ethoxy (CH₃CH₂O-) on W(110) and Cu(100) single crystal surfaces. By using theoretical ab initio quantum mechanical modeling, it is demonstrated that an unprecedented quantitative understanding of the vibrational frequencies of a surface adsorbate can be obtained. A qualitative understanding of the intensities is also obtained. This was achieved using a simple model in the calculations, where the metal surface was presented with one molybdenum metal atom only. The model was shown to be able to determine small isotopic shifts on both metal surfaces studied, dispite both material and structural differences in the metal substrates. The isotopic shifts were obtained through ¹³C isotopic labeling in ethoxy, using ¹³CH₃CH₂O-, CH₃¹³CH₂O- and ¹³CH₃¹³CH₂O-. Once the accuracy of the calculated shifts had been established, it was possible to assign previously undetected vibrational modes in ethoxy-Cu(100) in the C-H stretch region, where the spectra are complicated due to bend overtone interactions with the stretch fundamentals. In addition, the interaction between fundamentals and overtone modes in methoxy, Fermi resonances, is treated using a phenomenological model. Fermi resonances occur when a near accidental degeneracy exists between a fundamental and overtones having the same symmetry, causing the modes to shift away from each other and share intensity. An effective Hamiltonian was used to calculate the Fermi resonance coupling constants through a least-square fitting procedure. This latter treatment reproduces qualitatively the observed intensity redistribution caused by the coupling. The technique used to record the vibrational spectra was Fourier transform reflection-absorption infrared spectroscopy in the mid-infrared spectral region. This technique does not disturb the surface chemistry and gives the resolution required to be able to resolve the small isotopic vibrational shifts induced by the ¹³C labeling. All experiments were performed in a stainless steel ultra high vacuum chamber with a base pressure below 7'10^-11 Torr.