Non-linear optical methods

Mineral-liquid or mineral-gas interfaces under reactive conditions cannot be studied easily using standard UHV surface science methods. To overcome the pressure gap between ex situ UHV measurements and the in situ reactivity of surfaces under atmospheric pressure or in contact with a liquid, new approaches are required, some of which have only been introduced in the last 20 years, including scanning tunneling microscopy [28,29], atomic force microscopy [30,31], non-linear optical methods [32,33], synchrotron-based surface scattering [34—38], synchrotron-based X-ray absorption fine structure spectroscopy [39,40], X-ray standing wave... 

An excellent alternative to the streak camera approach is fluorescence time resolution by the "up-conversion" method, which we describe in detail below. In the simplest form of this technique, non-linear optical methods are employed to essentially construct a picosecond "shutter" or "gate". In most applications a single emission datum is acquired for each laser pulse, i.e. I(t=constant, X=constant). By repeating the experiment at different delay times, kinetic traces can be acquired that are of comparable quality to those obtained by streak camera methods (16-17). Alternatively, if delay time is held constant but X is scanned, high quality emission spectra can be obtained (3). 

A key requirement for in-situ spectroscopic methods in these systems is surface specificity. At Uquid/Uquid junctions, separating interfacial signals from the overwhelmingly large bulk responses in linear spectroscopy is not a trivial issue. On the other hand, non-Unear spectroscopy is a powerful tool for investigating the properties of adsorbed species, but the success of this approach is closely linked to the choice of appropriate probe molecules (besides the remarkably sensitivity of sum frequency generation on vibrational modes of water at interfaces). This chapter presents an overview of linear and non-linear optical methods recently employed in the study of electrified liquid/liquid interfaces. Most of the discussion will be concentrated on the junctions between two bulk liquids under potentio-static control, although many of these approaches are commonly employed to study liquid/air, phospholipid bilayers, and molecular soft interfaces. 

In Situ Non-linear Optical Methods and Second Harmonic Generation [40]... 

This non-linear optical method of generating frequency tunable infrared radiation was initially operated by Boyd and Ashkin,72 and developed by Pine73 into a very powerful spectroscopic tool. Radiation from a dye laser (v ) and radiation from a single-mode Ar ion laser (v ) are mixed in a LiNbO crystal. Their planes of... 

Abstract Plasmonic nanostructures exhibit unique optical properties, and fundamental studies of these structures are relevant to wide range of research areas, both fundamental and applied. Potential applications of the plasmonic nanostructures originate from their ability to confine (and sometimes propagate as well) optical fields in nanometer scales, and are closely related to the static and dynamic properties of plasmonic waves. In this chapter, visualization of wavefunctions and optical fields in plasmonic nanostructures using near-field linear and non-linear optical methods is described. 

Non-linear optical methods have various advantages over linear methods [91]. Spahal resoluhon, optical contrast, and, occasionally, signal-to-noise raho can be improved in the non-linear measurements with respect to those of the linear measurements. In addition, the non-linear methods often enable us to obtain informahon that is not accessible by linear methods. Since high peak power is essenhal to excite the non-linear processes, a pulse laser with a short pulse width is frequently used. For excitation of the non-linear processes, spatial conhnement and focusing of light is helpful. Combination of the near-held method and the short pulse laser source enables effechve excitation of non-linear processes, such as two-photon induced photoluminescence (PL), second harmonic generation (SHG), and so forth in nanomaterials. 

Polythiophenes (PTs) have received a great deal of attention due to their electrical properties, environmental stability in doped and undoped states, non-linear optical properties, and highly reversible redox switching [1]. Thiophene possesses a rich synthetic flexibility, allowing for the use of several polymerization methods and the incorporation of various side chain functionalities. Thus, it is of no great surprise that PTs have become the most widely studied of all conjugated polyheterocycles [184]. 

Non-linear optical susceptibilities and experimental methods to evaluate x and p. 153... 

Two-photon fluorescent (TPF) detection, which was initiated by a non-linear optical absorption process, has been performed on a quartz chip. Since the fluorescent efficiency in TPF is inversely proportional to the excitation beam area, the path length dependence problem in fluorescence is significantly reduced. This method is used for analysis of P-naphthylamine (excitation at 580 nm), which is the enzymatic product of leucine aminopeptidase (LAP) acting on the fluorogenic substrate leucine P-naphthylamide [675],... 

Spectroscopic methods are very useful for determining molecular properties. Time-resolved spectroscopic methods are useful for monitoring the evolution of the molecular properties in real time. Moreover, time-resolved spectroscopic techniques have the best time resolution available among all kinds of time-resolved experimental techniques. Thus, very often time-resolved spectroscopic methods reveal the dynamics of a molecular system in the non-equilibrium regime. In this section, the density matrix method is applied to calculate the spectroscopic properties of molecular systems. These include the linear and non-linear optical processes, in equilibrium or non-equilibrium cases. The approach is based on the susceptibility theory. 

Syntheses and X-ray structures have been reported for (53) [94T11205] and (54) [95MI645], The preparation and non-linear optical properties of 2-imino-l,3-dithioles (55) have been described [95MI35] and the dimeric derivatives (R3 = p-phenylene) have also been reported [94BSF774]. The product (56), obtained from reaction of 4-methyl-5-phenyl-l,2-dithiol-3-thione with DMAD, undergoes cycloaddition with a further molecule of DMAD to afford the spiro compound (57) whose structure is confirmed by X-ray methods [94KGS908]. 

Four frequently used conventions exist for the definition of non-linear optical polarizabilities, leading to confusion in the realm of NLO. This has been largely clarified by Willets et al. (1992) and in their nomenclature we have used the Taylor series expansion (T convention), originally introduced by Buckingham (1967), where the factorials n are explicitly written in the expansion. Here the polarizabilities of one order all extrapolate to the same value for the static limit w— 0. /3 values in the second convention, the perturbation series (B), have to be multiplied by a factor of 2 to be converted into T values. This is the convention used most in computations following the sum-over-states method (see p. 136). The third convention (B ) is used by some authors in EFISHG experiments and is converted into the T convention by multiplication by a factor of 6. The fourth phenomenological convention (X) is converted to the T convention by multiplication by a factor of 4. 

These equations are used in semiempirical quantum chemical calculations of non-linear optical polarizabilities by applying perturbation theoretical expressions [the so-called sum-over-states (SOS) method]. Here we use them to derive some qualitative and very general trends in a few simple model systems. To this end we concentrate on the electronic structure, i.e. on the LCAO coefficients. We do not explicitly calculate the transition frequencies. This is justified for the qualitative discussion below since typical transition energies... 

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