Second harmonic generation, in situ

Com, R. M., In situ second harmonic generation studies of molecular orientation at electrode surfaces, in Adsorption of Molecules at Metal Electrodes, J. Lipkowski and P. N. Ross, Eds., VCH, New York, 1992, p. 391. 

Pozniak B, Scherson DA. 2003. Dynamics of a surface phase transition as monitored by in situ second harmonic generation. J Am Chem Soc 125 7488-7489. 

Nakano. T.. Yamada. Y., Matsuo, T., Yamada, S. In-situ second-harmonic generation and luminescence measurements for structural characterization of ruthenium-polypyridine complex monolayers with two and four aliphatic tails at the air/water interface. J. Phys. Chem. B 102, 8569-8573 (1998)... 

Schiirer B, ElserMJ, Stenrig A, Peukert W, Diwald O Delamination and dissolution oftita-nate nanowires a combined structure and in situ second harmonic generation study, J Phys Chem C 115 12381-12387, 2011. 

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

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... 

Yagi I, Lantz JM, Nakabayashi S, Corn RM, Uosaki K (1996) In situ optical second harmonic generation studies of electrochemical deposition of tellurium on polycrystalline gold electrodes. J Electroanal Chem 401 95-10... 

Pozniak B, Mo YB, Stefan IC, Mantey K, Hartmaim M, Scherson DA. 2001. In situ time-resolved second-harmonic generation from Pt(lll) microfacetted single-crystal platinum microspheres. J Phys Chem B 105 7874-7877. 

Optical second harmonic generation (SHG), which is the conversion of two photons of frequency u to a single photon of frequency 2co, is known to be an inherently surface-sensitive technique, because it requires a noncentrosymmetrical medium. At the interface between two centrosymmetrical media, such as the interface between two liquids, only the molecules which participate in the asymmetry of the interface will contribute to the SHG [18]. SHG has been used as an in-situ probe of chemisorption, molecular orientation, and... 

Optical second harmonic generation from electrode surfaces is employed in situ to study the... 

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. 

A general objective in any in situ spectroscopic technique is to maximize the signal that arises specifically from the electrode surface. Nonlinear optical techniques such as second-harmonic generation (SHG) and sum frequency generation (SFG) are of interest because they involve optical signals that by definition can only arise at the electrode-solution interface [65],... 

Second Harmonic Generation as an In-situ Probe of Single Crystal Electrode Surfaces... 

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. 

In situ Nonlinear Optical Methods and Second Harmonic Generation [xi]... 

Refs. [i] Richmond GL, Robinson JM, Shannon VL (1988) Progr Surf Sci 28 1 [ii] Richmond GL (1991) Optical second harmonic generation as an in situ probe of electrochemical interfaces. In Bard A] (ed) Electroanalytical chemistry, vol. 17. Marcel Dekker, New York, p 87 [iii] Corn RM, Romagnoli M, Levenson MD, PhilpottMR (1984) / Chem Phys 81 4127 [iv] Georgiadis R, Richmond GL (1991) / Phys Chem 95 2895 [v] Holze R (2008) Surface and interface analysis an electrochemists toolbox. Springer, Berlin... 

Second harmonic is a convenient in situ probe of the dynamics of photoisomers in a polymer environment if the photoisomers have a second-order optical nonlinearity. A corona discharge is used to apply a large electrostatic field to align the molecules. When a change in the orientation of the photoisomers is induced through photoisomerization, second harmonic generation is used to follow the changes in the state and orientation of the photoisomers. 

A prerequisite for the development indicated above to occur, is a parallel development in instrumentation to facilitate both physical and chemical characterization. TEM and SPM based methods will continue to play a central role in this development, since they possess the required nanometer (and subnanometer) spatial resolution. Optical spectroscopy using reflection adsorption infrared spectroscopy (RAIRS), polarization modulation infrared adsorption reflection spectroscopy (PM-IRRAS), second harmonic generation (SFIG), sum frequency generation (SFG), various in situ X-ray absorption (XAFS) and X-ray diffraction spectroscopies (XRD), and maybe also surface enhanced Raman scattering (SERS), etc., will play an important role when characterizing adsorbates on catalyst surfaces under reaction conditions. Few other methods fulfill the requirements of being able to operate over a wide pressure gap (to several atmospheres) and to be nondestructive. 

As already mentioned, the only techniques sensitive to the polar order are even order nonlinear optical techniques such as the already-described second harmonic generation and linear electro-optic effect (cf. Chapter 2). The hrst technique offers a high sensitivity to the fast electronic contributions to susceptibility and is widely used. As already mentioned, it also gives the opportunity to study the kinetics of the poling by in situ measurements [152]. 

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. 

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