Metal-adsorbate stretch modes

One of the classic examples of an area in which vibrational spectroscopy has contributed to the understanding of the surface chemistry of an adsorbate is that of the molecular adsorption of CO on metallic surfaces. Adsorbed CO usually gives rise to strong absorptions in both the IR and HREELS spectra at the (C-O) stretching frequency. The metal-carbon stretching mode ( 400 cm-1) is usually also accessible to HREELS. 

Fig. 6. Schematic representation of the normal modes of an adsorbed diatomic molecule neglecting the surface structure, after Richardson and Bradshaw . In parentheses the experimentally measured values for CO in the ontop position on Pt(lll). (a) A frustrated translation (60 cm (b) A frustrated rotation (not yet detected), (c) The metal-molecule stretch (460cm ) . (d) The intramolecular stretch model (2100cm" ) . ...

Figure 2. Frequencies of SER C-N stretching mode, VCN> plotted against electrode potential for Fe(CNg3-/4- adsorbed at gold electrode in supporting electrolytes containing various alkali metal cations. Solutions were 1 mM Fe(CN)g - or Fe(CN)g with 0.1 M MCI + 0.01 M HCIO4, where M = Na+, K, or Cs+ as indicated. Laser excitation wavelength was 647.1 nm.

The HREEL spectrum of water adsorbed on Pt(100) in multilayer amounts resembles that of ice. In submonolayer quantities, three vqh peaks were observed at 2850 (353 meV), 3380 (419meV), and 3670 cm (455 meV) respectively, these were assigned to O-H stretch modes in metal-bonded hydroxyls, hydrogen-bonded water, and water devoid of intermolecnlar interactions. It is known that hydrogen bonding leads to a decrease in the O-H stretch freqnency (voh)- When the oxygen is coordinated to the metal, the decrease in vqh is more profonnd. In fact, the presence of the lowest freqnency vqh peak snggests that a fraction of... 

In 2002, our group extended this approach to a metal surface covered with adsorbates [24]. Time-resolved SHG was applied to a cesium-covered Pt(lll) crystal surface under ultrahigh vacuum. The intensity variation of the SHG shows oscillatory components as a function of the delay time, due to coherent nuclear wavepack-et dynamics of the Cs-Pt stretching mode. The nonlinear susceptibility of the system is considered to depend on, among other things, nuclear displacements of surface normal modes, that is,... 

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