Second harmonic generation, surface structure

Surface-extended X-ray absorption fine structure (analysis) spectroscopy Sum frequency generation Second harmonic generation Surface ionization Secondary ion mass spectroscopy Subtractively normalized interfacial Fourier transform infrared spectroscopy Single potential alteration infrared (spectroscopy)... 

According to the data obtained with SXRS in salt solutions,519 520 at a < 0 the surface of Au(lll) forms a ( 3 x 22) structure as in a vacuum. At a > 0 the reconstruction disappears and the (1 x 1) structure is observed. On the reconstructed Au(l 11) surface there are 4.4% more atoms than on the (1 x 1) structure and on the reconstructed Au( 100) there are 24% more atoms than on the (1 x 1) structure.506,519 This phase transition shifts in the negative direction with the adsorbability of the anion. The adsorption-induced surface reconstruction of Au(l 11) electrodes has been studied in situ by second harmonic generation by Pettinger et al.521... 

In addition to the indirect experimental evidence coming from work function measurements, information about water orientation at metal surfaces is beginning to emerge from recent applications of a number of in situ vibrational spectroscopic techniques. Infrared reflection-absorption spectroscopy, surface-enhanced Raman scattering, and second harmonic generation have been used to investigate the structure of water at different metal surfaces, but the pictures emerging from all these studies are not always consistent, partially because of surface modification and chemical adsorption, which complicate the analysis. 

As an alternate way to measure the gradients of structural and chemical composition, we intend to use the method of SH microscopy. In an isotropic environment, which has a center of symmetry, the second harmonic generation is not allowed in a dipole approximation, and is predominantly governed by weak quadrupole and surface dipole terms, which are orders of magnitude weaker. However, as we have already pointed... 

Nonlinear optical spectroscopies such as second harmonic generation (SHG) and sum frequency generation (SFG) are finding increasing use in probing species at interfaces (e.g., Eisenthal, 1996). For example, SHG was used by Donaldson et al. (1995) to detect a surface-bound S02 species, and SFG has been applied to elucidate the structure of dimethyl sulfoxide at liquid interfaces (Allen et al., 1999). These techiques are... 

In the first study of its kind, second harmonic generation has been used to study potential induced reconstruction on Au(lll) and Au(100) by Kolb and coworkers [156]. These surfaces have been known to reconstruct in UHY when they are clean [153, 157], Surface reconstruction occurs when the surface atoms of a solid rearrange themselves in a structure different from that expected from simple termination of the bulk lattice. Various studies by cyclic voltammetry, electroreflectance spectroscopy and ex situ electron diffraction have suggested that flame-treated crystals form stable reconstructions in solution. Unfortunately, due to the lack of in situ probes, very little direct evidence for this reconstruction has been available. 

G. L. Richmond contributes an authoritative summary of the theory and applications of second harmonic generation by a laser beam reflected from an electrode surface. This powerful new in-situ technique yields information on the structural and electronic properties of electrode surfaces. 

Refs. [i] Hoke R (2008) Surface and interface analysis an electrochemists toolbox. Springer, Berlin [ii] Tadjeddine A, Peremans A (1998) Non-linear optical spectroscopy of the electrochemical interface. In Clark RJH, Hester RE (eds) Advances in spectroscopy (spectroscopy for surface science), vol. 26. Wiley, Chichester, p 159 [iii] Shen YR (1990) In Gutierrez C, Melendres C (eds) Spectroscopic and diffraction techniques in interfacial electrochemistry (NATO ASI series C, vol. 320). Kluwer, Dordrecht, p 281 [iv] Shen YR (1986) Applications of optical second-harmonic generation to surface science. In Hall RB, Ellis AB (eds) Chemistry and structure at interfaces. VCH, Deerfield Beach, p 151 [v] Williams CT, Beattie DA (2002) SurfSci 500 545... 

A. A. Fedyanin A. A. Nikulin A. N. Rubtsov, Second-harmonic generation in metal and semiconductor lowdimensional structures. Surface Science 1995, 325, 343-355. 

Among the ex situ methods that can be employed in surface analysis, low-energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) can give the crystal structure and the nature of the surface ad-layers after the electrochemical and adsorption experiments as explained in this chapter [31,32]. Among the in situ non-electrochemical techniques, the radiotracer method [33] gives information about the adsorbed quantities however, infrared spectroscopy in FTIR mode [34] allows the identity of the bonding of the adsorbed molecules, and finally ellipsometry [35] makes possible the study of extremely thin films. Recently, some optical methods such as reflectance, x-ray diffraction, and second harmonic generation (SHG) [36] have been added to this list. 

Recent decades have witnessed spectacular developments in in-situ diffraction and spectroscopic methods in electrochemistry. The synchrotron-based X-ray diffraction technique unraveled the structure of the electrode surface and the structure of adsorbed layers with unprecedented precision. In-situ IR spectroscopy became a powerfiil tool to study the orientation and conformation of adsorbed ions and molecules, to identify products and intermediates of electrode processes, and to investigate the kinetics of fast electrode reactions. UV-visible reflectance spectroscopy and epifluorescence measurements have provided a mass of new molecular-level information about thin organic films at electrode surfaces. Finally, new non-hnear spectroscopies such as second harmonics generation, sum frequency generation, and surface-enhanced Raman spectroscopy introduced unique surface specificity to electrochemical studies. 

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