Non-linear optical technique

Non-linear optical techniques, such as second harmonic generation (SHG), have recently been used as surface probes. Bulk materials with inversion symmetry do not generate second harmonic signals, while surfaces and interfaces cannot have inversion symmetry, so the total SHG signal will come from the surface region for many systems. The components of the non-linear polarizability tensor have been used to determine the orientation of chemisorbed molecules. 

Vibrational sum-frequency spectroscopy (VSFS) is a second-order non-linear optical technique that can directly measure the vibrational spectrum of molecules at an interface. Under the dipole approximation, this second-order non-linear optical technique is uniquely suited to the study of surfaces because it is forbidden in media possessing inversion symmetry. At the interface between two centrosymmetric media there is no inversion centre and sum-frequency generation is allowed. Thus the asynunetric nature of the interface allows a selectivity for interfacial properties at a molecular level that is not inherent in other, linear, surface vibrational spectroscopies such as infrared or Raman spectroscopy. VSFS is related to the more common but optically simpler second harmonic generation process in which both beams are of the same fixed frequency and is also surface-specific. 

If one is interested in the orientation of adsorbed molecules one can, as well as using TIRF with polarized light and NMR, also have recourse to non-linear optical techniques, including the analysis of so-called second harmonics. The second harmonic is a wave with twice the frequency of the incident one it is... 

Second-order non-linear optical techniques have been applied for monitoring the adsorption of soluble surfactants at the air/water interface and to characterize the resulting (sub)monolayers ). For Langmuir monolayers SHG allows the observ-... 

Spectroscopic studies of liquid interfaces provide important information about the composition and structure of the interfacial region. Early work was mainly carried out at the solid liquid interface and involved techniques such as neutron and X-ray diffraction, and reflection FTIR spectroscopy. More recently, powerful techniques have been developed to study the liquid liquid and liquid gas interfaces. These studies are especially important because of their relevance to biological systems such as cell membranes. The techniques described here are second-harmonic generation (SHG) and vibrational sum frequency spectroscopy (VSFS). They are both second-order non-linear optical techniques which are specific to the interfacial region. Since the second-order effects involve signals of low intensity, they rely on high-power lasers. 

The instrument employs non-linear optical techniques and photodiode array detection to yield emission spectra at variable delay times after molecular excitation. Examples are given of the utilization of the apparatus in the study of the excited state relaxation of aromatic molecules. Experiments to evaluate the accuracy of the technique are also presented. 

Now, theoretical calculation methods of sufficient accuracy may fill the lack of quantitative information concerning so elusive species. On the other hand, the use of a monocoordinated complex as being the simplest molecular model to simulate a chemisorption phenomenon on a metallic surface, for instance the chimisorption of carbon monoxide on iron or nickel [16,17,18] enables to predict the shifts of the CO stretching vibration of the adsorbed species. Similar effects observed with cyanide anions CN on a cathode of platinum, silver or gold, using non-linear optics techniques can be rationalized by computing the CN vibration mode of the corresponding triatomic systems [19,20,21]. 

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). 

We have demonstrated how electro-optical and non-linear optical techniques can be utilized to characterize the structure/function relationship in biologically as well as in technically important supramolecular systems. In both cases, interaction of the molecules under investigation with a liquid interface can be used to bring about system orientation, which is utilized to enhance the performance of the spectroscopic experiment, or to induce the desired macroscopic properties. 

C. D. Bain, Non-linear optical techniques in Modem Characterization Methods of Surfactant Systems (Eds. B. P. Binks), Marcel Dekker, New York, 1999, pp. 335-373. 

Linear and Non-linear Optical Techniques to Probe Ion Profiles at the Air-Water Interface... 

Petersen PB, Saykally RJ. (2005) Evidence for an enhanced hydrorfium concentration at the liquid water surface. J Phys Chem B 109 7976—7980. Motschmann H, Koelsch P. (2010) Linear and non-linear optical techniques to probe ion profiles at the air-water interface. In Kunz W (ed.), Specific Ion Effects, Chap. 4. World Scientific Publishing, Singapore. 

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