Absorption nonlinear effects

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Nonlinear effects are characterized by new components of the -field generated from the acceleration of charges as the nonlinear polarization P [second, third, and higher terms in Equation (4.1)] drives the electric field. When linear absorption... 

Two-photon excitation fluorescence is currently the most widely nsed nonlinear contrast mechanism for microscopic investigations. The first experimental demonstration of two-photon excitation fluorescence was provided in 1961 (Kaiser and Garrett 1961), even though the first theoretical description of two-photon excitation flnorescence stems back to 1931 (Goppert-Mayer 1931). Three-photon absorption was demonstrated a few years later by Singh and Bradley (1964). Two-photon absorption is a third-order nonlinear effect, whereas three-photon absorption is a fifth-order nonlinear effect. The transition rate for two-photon absorption, R, depends on the square of the intensity, /, as follows (see Boyd 1992) ... 

Nonlinear optics is the interaction of laser radiation with a substance to produce new radiation which is altered in phase, frequency, and amplitude from the incident radiation. There are several types of nonlinear effects but the most important are second-order frequency doubling and reverse saturable absorption. 

Although the peak power of the pump laser must be high, the power of the probe laser should be kept low to avoid nonlinear effects (multiphoton absorption, stimulated Raman scattering see Section 3.9.2) and dielectric breakdown (ionization of molecules) that damage the sample. Thus, signal averaging of many pulses (high repetition rate) is made to obtain acceptable S/N ratios. Multichannel detectors such as an intensified silicon photoiode... 

Overall, the LMCT absorptions conform to expectation based on Eq. 9, but the effective values of Xp are smaller than those for IPCT absorptions. Nonlinear covalent effects in appear to be relatively small. 

Unlike to usual photosources lasers differ from them at their use to stimulate photochemical reactions by their photons beam high density. This circumstance provokes extreme necessity to determine the boundaries of lasers application. This means to study lasers intinsity regions for which either classical photochemistry laws are applicable or distortions may be observed as result of nonlinear effects superposition due to nonusual light emission absorption by the system. 

From the stated it follows that the use of highintensivity lasers (I 1024 28 photons/(cm2.s) results in the change of radical chain processes initiation mechanism as result of nonlinear effects rised dy AIBN emission absorption. It is to be noted that according to literature data [14,16,19,32,37] usually lasers with I<1020 photons/ (cm2.s) are being used in research work to initiate radical chain reactions. The laser intensity is changed in narrow interval. 

Variable absorption Nonlinear kinetics Induction of cytochrome P450s Zero-order kinetic of elimination ° Side effects ... 

The nonlinear material is critical to the operation of a limiter. Several different nonlinear effects can be used in the nonlinear element to provide a limiting function. Nonlinear absorption, where the material absorbance increases with the intensity of the incident light, is the most useful mechanism to remove light from the transmitted beam. A large number of organic and organometallic materials have been reported to exhibit an intensity dependent increase in absorption, sometimes called a reverse saturable absorption. The most effective are phthalocyanines (2, 3,4, 5) and porphyrins. (6,7)... 

Instead of using optical frequency doubling other nonlinear effects can also be used, such as two-photon absorption in liquids or solids, which can be monitored by the emitted fluorescence. If the optical pulse is again split into two pulses traveling in the opposite -directions through the sample cell (Fig. 11.46), the spatial intensity profile /fl( ) can be imaged by... 

Laser-microwave spectroscopy based on nonlinear phenomena developed from the type of experiments on molecules already discussed in Section 3.2 which make use of optical pumping or double resonance. Occasionally, the laser and the rf power were high enough to create the nonlinear phenomena mentioned above, i.e., to saturate the transitions involved and/or to induce multiphoton transitions. The intermediate level in, e.g., two-photon transitions did not have to be a real state but could be virtual as well. Therefore, a drawback often encountered in earlier infared laser-microwave experiments could be avoided if the laser transition frequency did not exactly coincide with the molecular absorption line the Stark or Zeeman effect had to be used for tuning. This results in an undesired line splitting. With laser-microwave multiphoton processes, however, the laser can be operated at its inherent transition frequency. Exact resonance with molecular lines is then achieved by using a nonlinear effect, i.e., a radiofrequency quantum is added to or subtracted from the laser frequency (see Figure 28). 

Although the two-color Z-scan can separate the refractive and absorptive nonlinear contributions, it is not capable of distinguishing between ultrafast (i.e., electronic) and cumulative (i.e., thermal effects) nonlinearities. However, with the introduction of a time delay between the probe and excitation beams of a two-color Z-scan apparatus it is possible to discern nonlinear effects having different temporal responses [25]. 

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