Radiation-scattering processes

The laser control of atoms and molecules usually uses the processes of resonant absorption and emission of photons. But the processes of scattering of photons by atoms, molecules, macroparticles, and free electrons (Chapter 13) also prove very useful in a number of cases. These processes are fairly diverse, and concrete information about them can be found elsewhere in the text. Here we shall restrict ourselves to a general description only. 

A scattering processes possess two general properties. First, as the frequency of the scattered photon approaches a resonance frequency, the scattering cross section 

Secondly, each type of spontaneous scattering process has a stimulated counterpart of its own. In other words, if there is a strong external field that has the same direction and frequency as the scattered photon, with a mode occupation number nscatt) 2 1, the scattering probability grows higher in accordance with Dirac s law (eqn 2.9). The process we have now is a two-quantum transition of the atom from one state into another in a two-frequency laser field. In this process, both energy and momentum, are imparted to the atom, which is also of interest from the standpoint of laser control. 

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The mechanism for Stokes and anti-Stokes vibrational Raman transitions is analogous to that for rotational transitions, illustrated in Figure 5.16. As shown in Figure 6.3, intense monochromatic radiation may take the molecule from the u = 0 state to a virtual state Vq. Then it may return to u = 0 in a Rayleigh scattering process or to u = 1 in a Stokes Raman transition. Alternatively, it may go from the v = state to the virtual state Fj and return to V = (Rayleigh) or to u = 0 (Raman anti-Stokes). Flowever, in many molecules at normal... 

Light-scattering processes involve the interaction of light with gases or particles in such a manner that the direction or frequency of the light is altered. Absorption processes occur when the electromagnetic radiation interacts with gases or particles and is transferred internally to the gas or particle. 

The third common level is often invoked in simplified interpretations of the quantum mechanical theory. In this simplified interpretation, the Raman spectrum is seen as a photon absorption-photon emission process. A molecule in a lower level k absorbs a photon of incident radiation and undergoes a transition to the third common level r. The molecules in r return instantaneously to a lower level n emitting light of frequency differing from the laser frequency by —>< . This is the frequency for the Stokes process. The frequency for the anti-Stokes process would be + < . As the population of an upper level n is less than level k the intensity of the Stokes lines would be expected to be greater than the intensity of the anti-Stokes lines. This approach is inconsistent with the quantum mechanical treatment in which the third common level is introduced as a mathematical expedient and is not involved directly in the scattering process (9). 

When the fine electron beam of a STEM Instrument passes through a specimen, it generates secondary radiation through inelastic scattering processes. When inner shell electrons of the atoms are excited, the secondary radiation signals may be characteristic of the elements present and so provide a basis for the mlcroanalysls of the small specimen regions which are irradiated. 

RAT grinding operations. This surface layer was removed except for a remnant in a second grind. Spectra - both 14.4 keV and 6.4 keV - were obtained on the undisturbed surface, on the bmshed surface and after grinding. The sequence of spectra shows that nanophase Oxide (npOx) is eiu-iched in the surface layer, while olivine is depleted. This is also apparent from a comparison of 14.4 keV spectra and 6.4 keV spectra [332, 346, 347]. The thickness of this surface layer was determined by Monte-Carlo (MC)-Simulation to about 10 pm. Our Monte Carlo simulation program [346, 347] takes into account all kinds of absorption processes in the sample as well as secondary effects of radiation scattering. For the MC-simulation, a simple model of the mineralogical sample composition was used, based on normative calculations by McSween [355]. 

NES is an elastic and coherent scattering process, i.e., it takes place without energy transfer to electronic or vibronic states and is delocalized over many nuclei. Owing to the temporal and spatial coherence of the radiation field in the sample. 

Inelastic photon scattering processes are also possible. In 1928, the Indian scientist C. V. Raman (who won the Nobel Prize in 1930) demonstrated a type of inelastic scattering that had already been predicted by A. Smekal in 1923. This type of scattering gave rise to a new type of spectroscopy, Raman spectroscopy, in which the light is inelastically scattered by a substance. This effect is in some ways similar to the Compton effect, which occurs as a result of the inelastic scattering of electromagnetic radiation by free electrons. 

We have seen that the electrical field associated with electromagnetic radiation plays an important role in elastic scattering and in microparticle heating. It plays a no less important role in the inelastic scattering processes of fluorescence and Raman spectroscopy, which we examine next. 

BACKSCATTERING. The deflection of particles or of radiation by scattering processes through angles greater than 90° with respect to the original direction of motion. 

Finally we must deal with perturbations not to pressureless matter but to baryons and photons. At early times, the universe is radiation dominated, and pr = p/3 for radiation. Before the epoch of recombination, the same Thomson scattering processes that keep the baryons ionized also keep the radiation tightly-coupled to the ions. The nuclei have pressure pb = (5kT/dmp)p [Pg.182]

To understand the scattering process, we will distinguish it from other processes, starting with electron scattering, which was studied in 1906 by J J. Thomson. He found that the intensity scattered by an electron interacting with an x-ray radiation is given by following equation [20,26] ... 

These are analytical tools since the character of the interaction is related to the structure and composition of the materials under test. When IR radiation goes across a sample, some photons are absorbed or suffer an inelastic scattering process caused by the active vibrations of the atoms, molecules, and ions, which compose the test material. The frequencies of the absorbed, or scattered, radiation are exclusively related to a particular vibration mode. Consequently, the process reveals attributes of the test material. Subsequently, IR (absorption) and Raman (scattering) are vibration-based spectroscopic methods widely used for characterizing materials, because they allow qualitative structural information to be obtained. 

Both these effects require knowledge of the spectral and spacial radiation distributions of the radiation flux on a surface. The determination of both these distributions at a given location is a difficult instrumentation problem. In this paper, the effect of scattering processes in the atmosphere on the available energy of solar radiation on a surface is examined. Both the spectral and spacial effects of Rayleigh scattering are demonstrated. 

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