SFG spectroscopy

In SFG vibrational spectroscopy, cOi is usually fixed in the visible region and CO2 is scanned in the infrared region. In the most widely used geometry, the two laser 

As shown in this equation, the angle of emission changes as cOir is scanned. 

The intensity of emitted SF light (Isfg) is expressed by the following equation [38] 

If neither cOyis nor cOsfg is in resonance with an electric dipole transition in the material and only electric dipole transitions are considered, the hyperpolarizability. 

From the above equation, it is clear that PijR takes a maximum value when the IR frequency is in resonance with the molecular vibration, that is, cOir = (Oq, and only the molecular vibration, which is both IR- and Raman-active is SFG-active. 

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SFG spectroscopy is an ideal technique to investigate the mechanism of interfacial processes at solid/liquid interfaces [5, 6, 10-16]. 

Here, we describe the basic principles and detailed experimental arrangement of SFG spectroscopy and present several examples of SFG study at solid/liquid interfaces. HRS is also described briefly. 

In addition to the surface/interface selectivity, IR-Visible SFG spectroscopy provides a number of attractive features since it is a coherent process (i) Detection efficiency is very high because the angle of emission of SFG light is strictly determined by the momentum conservation of the two incident beams, together with the fact that SFG can be detected by a photomultiplier (PMT) or CCD, which are the most efficient light detectors, because the SFG beam is in the visible region, (ii) The polarization feature that NLO intrinsically provides enables us to obtain information about a conformational and lateral order of adsorbed molecules on a flat surface, which cannot be obtained by traditional vibrational spectroscopy [29-32]. (iii) A pump and SFG probe measurement can be used for an ultra-fast dynamics study with a time-resolution determined by the incident laser pulses [33-37]. (iv) As a photon-in/photon-out method, SFG is applicable to essentially any system as long as one side of the interface is optically transparent. 

Here, we demonstrate the usefulness of SFG spectroscopy in the study of water structure at electrode/electrolyte solution interfaces by showing the potential dependent SFG spectra in the OH-stretching vibration region at a Pt/thin film electrode/0.1 M HGIO4 solution interface in internal reflection mode. 

Here, the structures of interfacial water at a fused quartz surface with and without contact of polyvinyl alcohol (PVA) were investigated by in situ SFG spectroscopy and their role in low friction between PVA and a fused quartz surface is discussed. 

The structure of water at the PVA/quartz interface was investigated by SFG spectroscopy. Two broad peaks were observed in the OH-stretching region at 3200 and 3400 cm , due to ice-like and liquid-like water, respectively, in both cases. The relative intensity of the SFG signal due to liquid-like water increased when the PVA gel was pressed against the quartz surface. No such increase of the liquid-like water was observed when the PVA gel was contacted to the hydro-phobic OTS-modified quartz surface where friction was high. These results suggest the important role of water structure for low friction at the polymer gel/solid interfaces. 

Figure 12.1 Schematic of the spectroelectrochemistry apparatus at the University of Dlinois. The thin-layer spectroelectrochemical cell (TLE cell) has a 25 p.m thick spacer between the electrode and window to control the electrolyte layer thickness and allow for reproducible refilbng of the gap. The broadband infrared (BBIR) and narrowband visible (NBVIS) pulses used for BB-SFG spectroscopy are generated by a femtosecond laser (see Fig. 12.3). Voltammetric and spectrometric data are acquired simultaneously.

Vidal F, Busson B, Tadjeddine A. 2005. Probing electronic and vibrational properties at the electrochemical interface using SFG spectroscopy Methanol electro-oxidation on Pt(llO). Chem Phys Lett 403 324-328. 

Sum-frequency generation (SEG) is also widely recognized as a powerful tool for intrinsically surface-specific spectroscopy (Shen 2000). As shown in Figure 10.16a, SFG spectroscopy, which requires two color input beams, is technically rather complicated, but one can expect additional features. For example, SFG is now expected to be a new probe for molecular chirality (Belkin et al. 2000 Champagne et al. 2000 Ji et al. 2006). From the viewpoint of biomolecular imaging, this feature is more attractive. Therefore, we hereinafter focus on sum-frequency (SF) chiral... 

Another relatively new technique is the infrared visible sum frequency generation (SFG) spectroscopy and the difference frequency generation (DFG) spectroscopy [13], both of which represent vibrational spectroscopic techniques sensitive only to the material located at interfaces. 

As discussed in Chapter 8, enhanced reactions of S02 at the interface have also been observed (Jayne et al., 1990). Surface second harmonic generation (SHG) experiments (Donaldson et al., 1995) subsequently identified a unique adsorbed S02 species at the air-water interface that may be involved in this enhanced reaction. Such SHG work on the uptake and reaction of N02 on water would clearly also be of value in understanding the kinetic anomalies. In addition, the use of sum frequency generation (SFG) spectroscopy, which in effect allows one to obtain the infrared spectrum of species present at interfaces, may shed some light on such reactions. 

Quayum, M. E., Kondo, T., Nihonyanagi, S., Miyamoto, D. and Uosaki, K. Formation of organic monolayer on a hydrogen terminated Si(l 11) surface via silicon-carbon bond monitored by ATR FT-IR and SFG spectroscopy Effect of orientational order on the reaction rate. Chemistry Letters, 208 (2002). 

Thus, there is a strong motivation to implement techniques to characterize catalytic surfaces during the reaction and to elucidate their active phases. The first such reaction investigations were carried out with optical absorption, diffraction, and structural techniques that work at atmospheric pressures the techniques included are IR spectroscopy (Beitel et al., 1996 Szanyi et al., 1994), SFG spectroscopy (Dellwig et al., 2000 Su et al., 1996), XAS (Knop-Gericke et al., 1998), SXRD (Peters et al., 2001), and STM (Hendriksen and Frenken, 2002 McIntyre et al., 1994). 

To combine optical SFG spectroscopy with the more traditional surface analysis methods (e.g., LEED, AES, TPD, XPS), the basic requirement is to simply add IR-transparent windows (e.g., CaF2 or BaF2) to a UHV chamber. However, if SFG spectroscopy is to be carried out at high pressure or during catalytic reactions, instruments combining a EIHV surface analysis system with an SFG-compatible... 

C.2.2. SFG Spectroscopy of CO on Palladium Nanoparticles. The first SFG spectra of CO on supported nanoparticles were obtained by using a Ti Sa-based laser... 

SFG spectroscopy of CO adsorbed on Si02-supported platinum nanoparticle arrays (with 30-, 40-, 45-, 200-, and 1000-nm mean particle diameters), evaporated platinum nanoparticles (6-nm mean diameter), and a thin platinum film (all at approximately 1 bar) was reported by Baldelli et al (132). Unfortunately, the EBL samples had to be prepared under nonUFlV conditions and were cleaned by a rather unconventional method (treated with concentrated FtN03/Fl2S04 solution. 

Ethene adsorption on Pd(l 11) was investigated by SFG spectroscopy (68,83,84,98,120). Figure 43 shows SFG spectra after adsorption of ethene at various temperatures. At 100 200 K, ethene adsorbed in a di-a configuration with a characteristic peak at 2910 cm (vs(CH2) Fig. 43a). The second, weak peak at... 

SHG spectroscopy SFG spectroscopy Electronic transitions. Vibrational transitions identification of interfacial molecules. Quantitative analysis complicated knowledge of hyperpolarizabilities of each vibrational mode required. 

A fmther step is to spectroscopically monitor adsorbed and reacting molecules on the surface of the nanoparticles under technically relevant conditions [59]. A few techniques with high-pressme capability are illustrated in Fig. 15.9b. Ambient pressure vibrational spectra of adsorbed molecules can be obtained by IR-vis SFG or PM-IRAS [51, 55, 60, 61]. Both methods can be applied from UHV to ambient pressure. Nonlinear optical SFG spectroscopy is inherently interface specific (i.e., there is no gas-phase SFG signal), and PM-IRAS allows for an accurate... 

Morkel M, Unterhalt H, Kliiner T, Rupprechter G, Freund H-J (2005) Interpreting intensities in vibrational sum frequency generation (SFG) spectroscopy CO adsorption on Pd surfaces. Surf Sci 586 146... 

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