Probes adsorbed, vibrational spectra

Sum frequency generation is a second-order non-linear optical technique that has unique advantages for probing the vibrational spectrum of molecules adsorbed at a surface. The vibrational SFG process occurs when two laser beams, one in the visible spectral region and one in the infrared spectral region, are incident on the sample so that a third beam at the sum frequency of the incident beams is emitted, as shown in (1). 

To acquire an SFG vibrational spectrum of adsorbate molecules on a metal catalyst, two (picosecond) laser pulses are spatially and temporally overlapped on the sample (Fig. 5). One input beam is in the visible range at fixed frequency (covis), and the second one is tunable in the mid-IR region (giir) to probe the vibrational... 

Figure 6.6 Vibrational coherence on a pNB-adsorbed TiO2(110) surface, (a) The raw SH intensity, (b) the modulated component, (c) the Fourier-transformed spectrum, the gray lines show the transformed spectrum. The spectrum simulated with Lorentzian functions is overlaid with broken lines. The pNB-adsorbed surface was irradiated in air with p-polarized pump (8mjcm ) and p-polarized probe (8mjcm ) pulses of a 550-nm wavelength.

The studies presented here illuminate just a few of the exciting possibilities for the use of VSFS to study chemistry at liquid/liquid surfaces. Solvents and adsorbates can be probed and orientations and conformations obtained. Molecular dynamics has recently been employed to gain additional information using the constraints provided by the spectroscopy. The future of this technique lies in expanding the spectrum to longer wavelengths so that more vibrations can be probed in each molecule and more complicated molecules can be studied. The study of interfacial dynamics will also offer exciting opportunities for the future. 

Adsorbed moleeules and intermediates at high pressures can be detected by vibrational speetroseopies provided they can differentiate between gas phase and surfaee signals. For example, Fig. 4 shows a (conventional) IRAS spectrum of CO at 50mbar on Pd(l 11) at 300 K (113,114). The signal of adsorbed on-top CO at approximately 2060 cm is nearly obscured by the rovibrational absorption spee-trum of the CO gas phase. In contrast, as shown below, SFG and PM-IRAS selectively probe the adsorbed surface species and thus provide surface-sensitive information, even in the presence of a gas phase. Investigations of the catalyst structure and composition under working conditions can be earried out by high-pressure (HP-) STM and (HP-) XPS, provided that the instruments are properly modified (9,115). 

In the field of catalysts characterization the use of small unreactive probe molecules to identify coordinatively unsaturated sites is well established [89]. Not always, however, a direct correlation between the CO vibrational frequency, the strength of the interaction, and the surface electric field exists. Recent DPT cluster calculations [90] have shown that CO adsorbed on step sites gives rise to a relatively strong interaction but to a negligible CO vibrational shift this is due to the inhomogeneity in the electric field above a MgO(lOO) step. This study [90] has permitted the complete attribution of the IR spectrum of CO adsorbed on MgO [81,83,91], Table 2. 

The infrared spectrum of adsorbed nitrogen can also be used to probe cation sites in zeolites. Zecchina et al [34] compared vibrational frequencies of CO and N2 adsorbed at low temperatures in mordenite containing different alkali metal cations. In both cases the vibrational frequencies could be correlated with (Rx + Rm) > where Rx is the cation radius and Rm the radius of the adsorbed molecule, suggesting a simple electrostatic field explanation for the frequency shifts between different cations. The appearance of a band due to N2 interacting with a particular zeolite cation will also mean that that particular cation is located in sites accessible to the N2 molecule. 

FT-IR Surface Spectrometry. Fourier transform infiared spectrometry is one of the most convenient techniques for the surfece characterization of ceramic nanopowders. Indeed, the vibrational spectra bring information on the nature of the bond formed between the surfece and the adsorbed molecules and consequently on the nature of the adsorption centres. Moreover, chemical species irreversibly grafted on the surfece have specific absorption bands in the powder spectrum, and are considered as intrinsic probes since they may be perturbed by molecular adsorption. 

The IR spectrum of NiCO isolated in solid argon gave assignments to Vi, V3 and V5 modes, with isotopic shifts.The IR spectrum of CO adsorbed on Nin clusters shows the presence of 4 (vibrationally-coupled) CO molecules per cluster. FTIR spectra (vCO) were used to probe the effects of co-adsorption of on-top CO on bridge CO on a Ni(lll) surface.The FTIR spectrum of CO on an anodic nickel oxide surface had a band at 2112 cm assigned to CO adsorbed to Ni(II) or Ni(0) sites perturbed by oxidation of neighbouring nickel atoms. The geometry of CO or NO coordination on NiO(lll) thin films was deduced from the vCO and vNO values. 

A commonly used way of probing the surface Lewis acidity of aluminas, as for other oxides, is to use the Lewis base, p)mdine (py), monitoring the IR spectrum of the chemically adsorbed species. Coordination results in observable shift in some vibrational modes, in particular the 8a and 19b modes [58]. Because of its industrial importance, y-alumina has been studied most widely as a result, the py IR probe method is well developed [7a,7b]. 

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