Excitation enhancement factor

Of the two different types of plasmonic enhancement which were described in section 6.1, the emphasis here is on the excitation enhancement mechanism. There is a linear dependence of the excitation rate of a fluorescent dye on the intensity of the excitation light in the direction of the electric dipole, e, of the molecule. When the dye molecule is located near the NP, the electric field acting on the dipole changes from Ei to Ej + E,. In this case, the excitation enhancement factor, fjoh. for one dye molecule is defined as a ratio of intensities ... 

The use of near-IR-laser excited FT-SERS eliminates the disturbing fluorescence of impurities found with visible excitation, and provides SERS enhancement factors that are about 20 times larger than those found for excitation at 514.5nm [792]. For a strong Raman scatterer (fluorene), a typical detection limit of 500 ng is found for a 3-mm diameter spot. For weak scatterers, the detection limits may be in the high- xg region, which means that some compromise between chromatographic... 

Riehl et al. also characterized the CL system lucigenin-hydrogen peroxide-A-methylacridone in the presence of different cationic surfactants such as HTAC, S-ClV-dodecyl-A lV-dimethylammonio) propane-1-sulfonate, and DODAB [41], Enhancement factors (ratio between CL intensity in the presence of organized medium and CL intensity in the absence of organized medium) of CL intensity were found of 3.4, 2.5, and 1.6, respectively. The alterations in CL intensity are explained in terms of the effect of the different surfactants on the rate of the reaction and on excitation efficiency. 

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. 

A similar increase in the values for the hyperfine constants and parameters of the P,T-odd interactions when the correlations with the core shells (primarily, 5s, bp) are taken into account is also observed for the BaF molecule [93], as one can see in Table 3. Of course, the corrections from the 4/-electron excitations are not required for this molecule. The enhancement factor for the P,T-odd effects in BaF is three times smaller than in YbF mainly because of the smaller nuclear charge of Ba. 

Figure 5 Simulated NMR spectra for a nucleus with spin 5/2 (such as Mg) in a single crystal, in the case of (A) and (B) populations corresponding to thermal equilibrium, with non-selective excitation ( hard pulse) in (A) and CT-selective excitation ( soft pulse) in (B). For (C) and (D) populations achieved after saturation of STs, with non-selective excitation (C) and CT-selective excitation (D). For (E) and (F) Populations achieved after complete inversion of the satellite transitions (in the order first, inversion of STl and ST4 and then inversion of ST2 and ST3), with non-selective excitation (E) and CT-selective excitation (F).The numbers at the right-hand side of the spectra in (B), (D) and (F) indicate the corresponding enhancement factors of the CT resonance.

In conclusion, for most of the molecules discussed here and for others reported in the hterature (and for which full spectral characterization is available), the cross section of multi-branched chromophores is either found to scale linearly with the munber of branches or exhibit a small enhancement when the molecular size is increased (or the evidence for the enhancement depends on the choice of normahzation factors). The enhancement factor can be larger if cross sections at the same wavelength are compared, instead of the 3max values, if there is a change in the band shape or position. These factors could be relevant for applications that have limitations on the operational excitation wavelength. 

A principal obstacle to identification of defects is the difficulty of comparing the results from EPR, luminescence, absorption, and deep state experiments. Probably the least ambiguous is that between EPR and luminescence when, as for transition metal impurities, it is possible for optical Zeeman measurements of a sharp luminescence line to determine the ground state g factor. If the optical and EPR measurements give the same value, then the correlation is made (Watts, 1977). In some cases, when optical excitation enhances or quenches the EPR signal, there may be a similar response in the photoconductivity or luminescence excitation spectrum. 

In requiring less measuring time and producing higher sensitivity in comparison to CW, PFT NMR follows the Fellgett principle [26, 27] The signal noise of any spectroscopic experiment increases if simultaneous multichannel excitation is applied. In the PFT technique, rf pulses simulate multichannel transmitters. If m transmitters stimulate simultaneously, the enhancement factor relative to one channel excitation (m = 1) is the square root of m (eq. (2.17), [26, 27]). 

In the Raman experiments, an excitation wavelength of 785 nm (intensity 1.8 105 W/cm2) was used. The sample, i.e. a drop of Au nanoparticle suspension with soluble pollen content was placed under a (60x) water immersion objective. Raman spectra were recorded with 1 s acquisition time. The control preparations (pollen supernatant with water) did not yield any spectral features. A spectrum of rye pollen supernatant with Au nanoparticles is shown in Fig. 4.9, together with a normal Raman spectrum of a rye pollen grain. The difference in spectral information that can be obtained by both approaches is evident from a comparison of these two spectra. Although an estimate of an enhancement factor is not possible from this experiment, it is clear that... 

M in concentration. This is in the range required for single-molecule detection. These sensitivity levels have been obtained on colloidal clusters at near-infrared excitation. Figure 10.3 is a schematic representation of a single-molecule experiment performed in a gold or silver colloidal solution. The analyte is provided as a solution at concentrations smaller than 10-11 M, Table 10.1 lists the anti-Stokes/Stokes intensity ratios for crystal violet (CY) at 1174 cm-1 using 830-nm near-infrared radiation well away from the resonance absorption of CY with a power of 106 W/cm2 [34]. CV is attached to various colloidal clusters as indicated in the table. Raman cross sections of 10-16 cm2/molecule or an enhancement factor of 1014 can be inferred from the data. 

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