Radiation and Scattering Processes

J Comparison Between Different Radiation and Scattering Processes... 

Table 4.1. Comparison between different radiation and scattering processes...

For the propagation of light inside the sample, only two flux channels are assumed upward and downward. The letters I and J represent radiation in the downward and the upward directions, respectively. The downward flux through a layer of thickness dr is decreased by absorption and scattering processes and increased by scattering process from the reflected light. As a result the flux equations can be written ... 

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. 

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. 

Attenuation—A process by which a beam from a source of radiation is reduced in intensity by absorption and scattering when passing through some material. 

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