Optical saturation

Gregor, 1., Patra, D. and Enderlein, J. (2005) Optical saturation in fluorescence correlation spectroscopy under continuous-wave and pulsed excitation. 

From Fig. 4 it can be seen that, for finite bandpass detection, one will obtain different fluorescent intensities per emitting molecule depending on the level pumped. This can produce systematic errors in both the determination of absolute concentrations and the use of excitation scans to obtain ground state rotational temperatures (21) Also, the lack of a thermal distribution imposes restrictions on models of and data analysis in optical saturation techniques. 

ALAN GELB—Physical Sciences Incorporated, Woburn, MA 01801 DAVID R. CROSLEY—SRI International, Menlo Park, CA 94025 Optical Saturation in Laser-Induced Fluorescence... 

The fluorescence technique, like other methods based on scatter (elastic or inelastic), has been shown by us - and others to be a reliable unperturbing method of measuring spatial/ temporal flame temperatures and species concentrations. To avoid the dependency of the fluorescence signal on the environment of the emitting species, it has been shown by several workers that optical saturation of the fluorescence process (i.e., the condition occurring when the photoinduced rates of absorption and emission dominate over the spontaneous emission and colli sional quenching rates) is necessary. Pulsed dye lasers have sufficient spectral irradiances to saturate many transitions. Our work has so far been concerned with atomic transitions of probes (such as In, Pb, or T1) asoirated into combustion flames and plasmas. 

The purpose of this experiment is to determine up to which PNP concentration, the Beer-Lambert-Bouguer law is still linear. In fact, this law cannot be applied when the optical saturation of the photomultiplier is reached and/or when the chromophore aggregates. Each spectrophotometer cannot count more than a precise number of photons. When this number is reached, the OD given by the spectrophotometer will be the highest, and the photomultiplier is saturated. Beyond the saturation concentration, the spectrophotometer will always give the same OD that corresponds to that of saturation. 

As can be seen from this experiment, the spectrophotometric detector is excellent for analytical scale detection of the phthalates which have a good molar absorptivity at 254 nm. However, as was observed in this experiment, the spectrophotometer is much less useful for preparative scale chromatographic separations because it tends to become optically saturated when relatively large amounts of materials are injected, that is, all light entering the detector cell is absorbed and no separation can be seen because of the off-scale readout. Also, spectrophotometric detectors will be blind to molecules that have no chromophore in their structure. The differential refractometer is the preferred detector for preparative LC because generally... 

Since the dissociation of the abundant H2 and CO molecules result from photoabsorption at discrete wavelengths, isotopically selective photodissociation based on self-shielding is possible. Two conditions are required for this (1) dissociation via line absorption for each isotopically substituted molecule, and (2) differential photolysis that depends upon the isotopic abundances. Self-shielding occurs when the spectral lines leading to dissociation of the major isotopic species optically saturate, while the other residual lines relevant for dissociation of the minor isotopes remain transparent. As a consequence, such photolysis depends on nucleic abundance rather than the mass of a molecule see Fig. 4.4 (Langer 1977 Thiemens Heidenreich 1983). 

The effect consisting in a change in magnetic pmneability under the influ ice of an electric field still awaits detection, although considered theoretically for paramagnetics by Van Vleck, as wdl as for diamagnetics. Non-linear Electro- and Magneto-optical Effects, Optical saturation in an electric field. When an optically isotropic dielectric is placed in a very... 

Optical saturation in an optical field. Intense li t, whm traversing an isotropic medium, induces in it an optical non-linearity apparent as a dependence of the refractive index on the li t intensity I. The dynamical electric permittivity and magnetic permeability contributing to the nonlinear change in index are, with regard to equations (21) and (23) ... 

Kielich published a theoretical cUscussion of optical saturation (26) in gases and liquids resulting from non-linear dectron polarizability, optical reorientation, and angular corrdations of molecules, as well as dectrostriction and the dectrocaloric process. 

Optical saturation in a magnetic field. A static magnetic field Hp can also cause optical changes in naturally isotropic bodies by analogy with... 

A treatment for analysing the excitation and fluorescence multiwavelength polarized decay surfaces has been given for the case of a mixture of noninteracting species. An improved model for analysis of fluorescence anisotropy measurements has been presented. Limitations to the use of intense excitation pulses in fluorescence and thermal lens spectrophotometers are discussed in terms of optical saturation. Such artefacts can be eliminated by reference to the fluorescence quantum yield of Rhodamine 6G. A model has been given to describe spectral diffusion in time-resolved hole-burning spectroscopy. ... 

Guo X., Schubert E. F. Current crowding and optical saturation effects in GalnN/... 

Fig. 13. Optical saturation of electrons in alcohols. Formation and decay of e in methanol (a) in the absence and (b) in the presence of a saturating laser pulse at 694 nm. The lower trace (20 ns div is a computer printout of data from e in CHjOH and CH3OD. See text for details.

All of the flicker noises can be effectively eliminated by the use of double-beam optics in conjunction with a background correction system such as Zeeman splitting or a well-aligned (or wavelength-modulated) continuum source. Thus the ultimate limiting noise in atomic absorption is source shot noise, which can be reduced (relative to total source intensity or I, ) by increasing the source intensity, up to the point of optical saturation. 

Laser Fluorescence Noise Sources. Finally, let us examine a technique with very complex noise characteristics, laser excited flame atomic fluorescence spectrometry (LEAFS). In this technique, not only are we dealing with a radiation source as well as an atomic vapor cell, as In atomic absorption, but the source Is pulsed with pulse widths of nanoseconds to microseconds, so that we must deal with very large Incident source photon fluxes which may result in optical saturation, and very small average signals from the atomic vapor cell at the detection limit [22]. Detection schemes involve gated amplifiers, which are synchronized to the laser pulse incident on the flame and which average the analyte fluorescence pulses [23]. 

Ernsting NP, Kaschke M, Weller H, Katsikas L Colloidal Znl-xCdxS — optical saturation of the exciton band and primary photochemistry studied by subpicosecond laser flash-photolysis. Journal of the Optical Society of America B-Optical Physics 1990, 7(8) 1630-1637. 

Detection of UVRR spectra is con tlicated by a number of experimental difficulties the photooxidation and photodestruction of molecules, distortion of spectral information due to optical saturation phenomena and photoinduced transients, and fluctuations of the scattered light intensity in inhomogeneous samples. 

The intensities of lasers exceed by many orders of magnitude those of conventional light sources. At high intensity levels, nonlinear interaction of light with atomic and molecular systems becomes pronounced. New analytical techniques involving multistep and multiphoton excitation and ionization, optical saturation, and excitation of forbidden transitions become possible. High radiation intensity also increases the sensitivity of laser analytical techniques,... 

The interaction of intense laser radiation with matter differs from that of conventional radiation sources. The high incident irradiances available with lasers can lead to a significant depletion of the initial level population. As a result, the optical absorption coefficient decreases as a function of the excitation energy and the absorbed energy tends towards a constant value. This so called optical saturation effect leads to a nonlinear dependence of the absorption signal on the light intensity. Because the amount of energy released by radiative and nonradiative relaxation processes... 

Varsano et al have shown that a many-body derived XC kernel can explain within TDDFT the optical saturation in molecular chains in terms of excitonic confinement. This has been illustrated for both the static and dynamic polarizabilities of H2 and polyacetylene chains of finite or infinite length. 

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