Absorption enhancing principles

As noted above, observations of large enhancements of the photoluminescence are insufficient to guarantee utility for application of plasmon-enhanced emission in OLEDs where the excited state is not photogenerated. In principle, increases in photoluminescence observed exfierimentally could be completely due to absorption enhancement. Even observation of reduced excited state lifetimes in conjunction with increased emission is insufficient to prove radiative rate enhancement since the lifetime reduction could be due to excited state quenching by the metallic surface and compensated by large absorption enhancements. 

Surface-enhanced infrared absorption spectroscopy principles and applications... 

Theory. Conventional anion and cation exchange resins appear to be of limited use for concentrating trace metals from saline solutions such as sea water. The introduction of chelating resins, particularly those based on iminodiacetic acid, makes it possible to concentrate trace metals from brine solutions and separate them from the major components of the solution. Thus the elements cadmium, copper, cobalt, nickel and zinc are selectively retained by the resin Chelex-100 and can be recovered subsequently for determination by atomic absorption spectrophotometry.45 To enhance the sensitivity of the AAS procedure the eluate is evaporated to dryness and the residue dissolved in 90 per cent aqueous acetone. The use of the chelating resin offers the advantage over concentration by solvent extraction that, in principle, there is no limit to the volume of sample which can be used. 

The LUMO-HOMO gap is approximately given by the spectroscopic properties of the SO group. Since DMSO absorbs in the region of 2380 and 2560 A, we obtain96 1.24 x 104/2560 = 4.8eV. This measures the drastic enhancement in redox properties anticipated for this compound. Indeed the 4.8 V value for the ground-state potential covers the whole range of usual redox potentials. The same principle applies to the sulfones because their electrode potential and absorption properties differ little from their sulfoxide analogues. 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

In contrast to Raman scattering, the absorption of infrared (IR) radiation is a first-order process, and in principle a surface or an interface can generate a sufficiently strong signal to yield good IR spectra [6]. However, most solvents, in particular water, absorb strongly in the infrared. There is no special surface enhancement effect, and the signal from the interface must be separated from that of the bulk of the solution. 

Quantitative concentration data are often required from XRF analyses. In principle (for both WD and ED) the intensity of the fluorescent X-ray peak is proportional to the amount of the element present. This is complicated, however, by absorption and enhancement processes. Absorption can cause both attenuation of the input (primary) radiation and the fluorescent (secondary) radiation, as discussed above. Enhancement is the result of the observed element absorbing secondary radiation from other elements present in the sample, thus giving more fluorescent radiation than would otherwise... 

Savio D, Harrasser PC, Basso G. Softgel capsule technology as an enhancer device for the absorption of natural principles in humans. Arzneim Forsch/ Drug Res 1998 48 1104-1106. 

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