Second harmonic spectroscopy, described

The rotational reorientation times of the sample in several solvents at room temperature were measured by picosecond time-resolved fluorescence and absorption depolarization spectroscopy. Details of our experimental setups were described elsewhere. For the time-correlated single photon counting measurement of which the response time is a ut 40 ps, the sample solution was excited with a second harmonics of a femtosecond Ti sapphire laser (370 nm) and the fluorescence polarized parallel and perpendicular to the direction of the excitation pulse polarization as well as the magic angle one were monitored. The second harmonics of the rhodamine-640 dye laser (313 nm 10 ps FWHM) was used to raesisure the polarized transient absorption spectra. The synthesis of the sample is given elsewhere. All the solvents of spectro-grade were used without further purification. 

Second harmonic generation (SHG), attenuated total reflection (ATR) and Stark (electroabsorption) spectroscopy were employed to find PAP. The response of the interaction between an electric field (E ) and a material can be described by Equation (10.14). 

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. 

Experimental study of the double layer is not limited to thermodynamics. A variety of spectroscopic methods have been applied to determine the structure and composition of the double layer. Two of these, namely, second-harmonic generation and vibrational sum frequency spectroscopy, have already been described in section 8.11. Other important techniques are based on the absorption of electromagnetic radiation when it is transmitted through or reflected at the interface. Finally, the scattering of X-rays and neutrons at interfaces has proven to be a valuable tool for obtaining atomic level information about the interface. In the following section some of these methods are outlined in more detail. 

Linear effects, e.g. absorption, are described by x and are utilized in spectroscopy. The value of x is only different from zero for a medium lacking a center of inversion, x is responsible for example for the generation of new frequencies (e.g. generation of the second harmonic (SH) hereby the frequency of the laser light is doubled after having passed the medium). x which does not require a non-centrosymmetrical medium can generate new frequencies as well (e.g. generation of the third harmonical, TH). 

The concept of the vibrational force field was initially taken from vibrational spectroscopy where the potential energy of a molecule, upon deformation, could be described by a Taylor series with respect to the potential energy at the equilibrium structure. In this case it turns out that the second derivative gives the force constant, and if the harmonic approximation is assumed, then all higher terms may be neglected. 

In nmr spectroscopy the solute orientation is often described by an orientational probability function P(0,0) [45]. TO,0) is a measure for the probability (per unit solid angle) that the applied magnetic field (or the optic axis of the liquid crystal) assumes the spherical coordinates (dy 0) in the molecule-fixed coordinate system (x, y, z). P(0, 0) can be expressed in terms of spherical harmonics of second order as follows [45] ... 

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