Atoms photon cross sections

Figure 20. Energy dependence of atomic photoionization cross sections of Cu 3d and Cl 3p orbitals dashed lines indicate He I, He II, and Mg K a sources. Insert fj ive high photon energy region at higher sensitivity, lmb = 10 cm Reproduced from Ref. 28. Copyright 1985, American Chemical Society.

Monoenergetic photons excite a core hole. The modulation of the absorption cross section with energy at 100 - 500 eV above the excitation threshold yields information on the radial distances to the neighbouring atoms. The cross section can be measured by fluorescence as the core holes decay or by attenuation of the transmitted photon beam. EXAFS is one of the many fine -structure techniques. 

The spectra discussed above indicate the presence of the ene-dithiolate electronic buffering effect, but not its cause. Insight into this cause can be gained by comparing spectra of a compound that have been collected with different photon energies to evaluate the photoionization cross sections. From previous experimental studies and calculations of atomic photoionization cross sections,it is expected that ionizations from orbitals with significant Mo 4d contributions will increase in intensity compared to ionizations of primarily S 3p character when data collected with a Hell photon source are compared to data collected with a Hel photon source. Mixing... 

The relative importance of the different interaction modes is expressed by means of the cross section which describes the probability that an interaction will occur between the particle (here photon) and the target (here atom). The cross section is usually expressed in Barn/atom. 

Ionization cross-section. A measure of the probability that a given ionization process will occur when an atom or molecule interacts with an electron or a photon. 

The saturation of the absorption. Roughly speaking, saturation of the absorption occurs when each of the sodium atoms in a column of section equal to the cross section of the D2 transition have absorbed a photon per time interval equal to the hfetime of the upper energy level. There is not so much sodium in the mesosphere ( 1 metric ton all around the Earth). Thus, in spite of the relatively high cross section of the 3S i/2 3P3/2 transition, saturation occurs at a quite low level, ... 

Local surface structure and coordination numbers of neighbouring atoms can be extracted from the analysis of extended X-ray absorption fine structures (EXAFS). The essential feature of the method22 is the excitation of a core-hole by monoenergetic photons modulation of the absorption cross-section with energy above the excitation threshold provides information on the distances between neighbouring atoms. A more surface-sensitive version (SEXAFS) monitors the photoemitted or Auger electrons, where the electron escape depth is small ( 1 nm) and discriminates in favour of surface atoms over those within the bulk solid. Model compounds, where bond distances and atomic environments are known, are required as standards. 

The essence of the XSW technique now lies in the effect these modulations have on the photoelectric cross-section of a target atom a distance c above the mure surface. The incident X-rays can eject a core electron from the atom so generating a vacancy and resulting in the emission of a fluorescent X-ray photon The probability of an incident photon ejecting the core electron, the photoelectric cross-section, is directly proportional to the electric field experienced by the atom Hcncc. the fluorescence yield, T(0.for an atom or ion distribution A (z) a distance above the mirror surface can be written... 

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