Intensity enhancement

Intensity enhancement takes place on rough silver surfaces. Under such conditions, Raman scattering can be measured from monolayers of molecular substances adsorbed on the silver (pyridine was the original test case), a technique known as surface-enhanced Raman spectroscopy. More recendy it has been found that sur-fiice enhancement also occurs when a thin layer of silver is sputtered onto a solid sample and the Raman scattering is observed through the silver. 

A feature of interest is the possibility of obtaining the condition of total internal reflection (TIR). Under this condition, the fundamental beam impinges on the interface from the liquid with the highest index of refraction yielding a SH intensity enhancement of more than a hundred times. Two TIR angles exist, given by the following relationships ... 

Gordeychuk TV, Kazachek MV (2009) Experimental observation of the intense enhancement of metal sonoluminescence under pressure and temperature. Opt Spectra 106 238-241... 

Capomacchia et al. [120] utilized the background emission present when DNPO and hydrogen peroxide are mixed to detect ouabain and urea. In the presence of these analytes, an intensity enhancement was observed and detection limits were in the picomole range. 

The same authors studied the CL of 4,4,-[oxalylbis(trifluoromethylsulfo-nyl)imino]to[4-methylmorphilinium trifluoromethane sulfonate] (METQ) with hydrogen peroxide and a fluorophor in the presence of a, p, y, and heptakis 2,6-di-O-methyl P-cyclodextrin [66], The fluorophors studied were rhodamine B (RH B), 8-aniline-l-naphthalene sulfonic acid (ANS), potassium 2-p-toluidinylnaph-thalene-6-sulfonate (TNS), and fluorescein. It was found that TNS, ANS, and fluorescein show CL intensity enhancement in all cyclodextrins, while the CL of rhodamine B is enhanced in a- and y-cyclodextrin and reduced in P-cyclodextrin medium. The enhancement factors were found in the range of 1.4 for rhodamine B in a-cyclodextrin and 300 for TNS in heptakis 2,6-di-O-methyl P-cyclodextrin. The authors conclude that this enhancement could be attributed to increases in reaction rate, excitation efficiency, and fluorescence efficiency of the emitting species. Inclusion of a reaction intermediate and fluorophore in the cyclodextrin cavity is proposed as one possible mechanism for the observed enhancement. 

Figure 3. The increase in the incident UV light intensity enhances the cross-linking rate, as indicated by the larger slopes of the elastic modulus (G ) plotted as a function of UV exposure time. The use of either the pulse or continuous modes of irradiation do not affect the rheological properties for the NOA 61 sample. Here, the frequency of oscillation is 10 rad/s and the intensities have units of mW cm 2.

Coherent optical phonons can couple with localized excitations such as excitons and defect centers. For example, strong exciton-phonon coupling was demonstrated for lead phtalocyanine (PbPc) [79] and Cul [80] as an intense enhancement of the coherent phonon amplitude at the excitonic resonances. In alkali halides [81-83], nuclear wave-packets localized near F centers were observed as periodic modulations of the luminescence spectra. 

In the normal-incident transmission measurements of LB films deposited on transparent substrates, the electric vector of the infrared beam is parallel to the film surface (Figure 5A). Therefore, only absorption bands which have the transition moments parallel to the film surface can be detected by this method. On the other hand, in the above-mentioned RA measurements, in which the p-polarized infrared beam is incident upon the LB film prepared on Ag-evaporated substrates at a large angle of incidence, we have a strong electric field perpendicular to the film surface as shown in Figure 5B. Therefore, in this case, only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement. Thus, if the molecules are highly oriented in the LB films, the peak intensities of particular bands should be different between the transmission and RA spectra. 

Here, Kx and Kz are the X and Z components of the absorption coefficient, and mx and mz are those of the intensity enhancement factor in the RA measurements due to the presence of the Ag film. The second term in the denominator of the right hand side of Eq. (1) was added to take into account the slight contribution of the electric field parallel to the film surface in the RA measurements. After a simple calculation under the condition of the uniaxial orientation, we have... 

Surface-enhanced Raman scattering (SERS) has emerged as a powerful technique for studying species adsorbed on metal films, colloidal dispersions, and working electrodes. SERS occurs when molecules are adsorbed on certain metal surfaces, where Raman intensity enhancements of ca. 105-106 may be observed. The enhancement is primarily due to plasmon excitation at the metal surface, thus the effect is limited to Cu, Ag, and Au, and a few other metals for which surface plasmons are excited by visible radiation. 

The spectra in this figure represent one of the strongest cases for the ring current mechanism of VCD intensity enhancement. In this case, currents increase the magnetic dipole transition moment without altering the relative arrangement of electric dipole transition moments and the relative intensity pattern is preserved. In the structures 42 and 43, we show that in the A complex, 42,... 

For an octapeptide sequence taken from the C-terminal residues of the Alzheimer s Ap-peptide Lansbury et al. identified a large intensity enhancement for 13C-labeled modes that was sequence-dependent and assessed as largely due to interstrand dipole-coupling.1207,2351 Mendelsohn et al. found a similar effect by double labeling on alternate sites a peptide that formed a P-sheet-like structure in methanol.12501 Subsequent theoretical modeling showed this latter intensity enhancement to be a function of forming extended, flat, anti-parallel P-sheet structures,12511 and the overall effect to be highly position sensitive.12451... 

On the other hand, the Intensity enhancement of the peak at m/e=56 probably means that the prolonged polymerization time causes the formation of butene from the remaining BuLi, although the mechanism is not clear at present. 

Finally, it should be pointed out that methods used to study short-lived chemical intermediates in fast thermal reactions may be applicable also to photochemical studies. Radical intermediates, however generated, can be studied by CIDNP ichemically induced dynamic nuclear spin polarization), in which the n.m.r. spectrum of the reaction mixture is recorded during the reaction period. II a substrate is continuously irradiated with ultraviolet/visible light in the cavity of an n.m.r. spectrometer, the resulting n.m.r. spectrum of the substrate/product mixture exhibits intensity variations as compared with the normal spectrum—intensity enhancement, reduction or even reversal (i.e. emissionl. Note that the spectrum involved is not... 

Enhancement of fluorescence due to the complexation of metal ions with fluoroionophores has been used as a well-precedented technique to analyze for the presence of metal ions [189-191], A number of studies have reported chelating fluorophores whose emission spectra change upon the addition of metal ions [192-198]. One remarkable result of this emission intensity enhancement is shown in Scheme 23, where the chelation of zinc chloride to 9,10-bis(((2-(dimethylamino)ethyl)methylamino)methyl)anthracene drastically enhances the observed fluorescence by a factor greater than 1000-fold [199], In the absence of Zn2+, the singlet excited state of anthracene moiety is strongly quenched by intramolecular photoinduced electron transfer from the amine to the anthracene moiety. The complex formation of Zn2+ with the amine moiety may result in the largely positive shift of the one-electron oxidation potential. Thus, intramolecular photoinduced electron transfer is strongly suppressed by the complexation of the amine moiety with Zn2+,... 

---