Photons Inner Shells

In order to provide a more complete survey of the ionization cross sections of inner levels, some information on ionization by photons, that is photoionization, will now be given. Photons are often labelled as uncharged, indirectly ionizing particles. Photoionization, formerly called the photoelectric effect, contributes to the attenuation of (energetic) electromagnetic radiation (see p. 257). Cross sections and attenuation coefficients are evidently closely connected. Some important references will be given on various shell or subshell ionization by photons. 

A general view on the calculation of photoionization cross sections is given in [14]. 

Detailed numerical results of a theoretical relativistic treatment of the electrons moving in a Hartree-Slater central potential are given in [1], see references therein up to 1973. 

In the soft X-ray region, simple hydrogenic models in a single-electron process break down, as shown by comparing experimental observation of X-ray absorption spectra to theoretical predictions. Outer subshells make relatively important contributions to cross sections giving rise to Cooper minima [2 to 5]. 

Some papers give measured values of inner shell photoelectric cross sections Ox (X = K, L, . ..). Measurements imply X-ray spectroscopy and/or photoelectron spectroscopy. 

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Extended X-ray absorption fine structure (EXAFS) measurements based on the photoeffect caused by collision of an inner shell electron with an X-ray photon of sufficient energy may also be used. The spectrum, starting from the absorption edge, exhibits a sinusoidal fine structure caused by interferences between the outgoing and the backscattered waves of the photoelectron which is the product of the collision. Since the intensity of the backscattering decreases rapidly over the distances to the next neighbor atoms, information about the chemical surroundings of the excited atom can be deduced. 

Individual X-ray photons, 127 Induced birefringence, 89 Inner shell ionization, 123, 124 Inner-orbital ionization, 15 InP, 52... 

Spectroscopic techniques look at the way photons of light are absorbed quantum mechanically. X-ray photons excite inner-shell electrons, ultra-violet and visible-light photons excite outer-shell (valence) electrons. Infrared photons are less energetic, and induce bond vibrations. Microwaves are less energetic still, and induce molecular rotation. Spectroscopic selection rules are analysed from within the context of optical transitions, including charge-transfer interactions The absorbed photon may be subsequently emitted through one of several different pathways, such as fluorescence or phosphorescence. Other photon emission processes, such as incandescence, are also discussed. 

Exposure of elements to a broad spectrum of X-rays results in the ejection of electrons from their inner shells. Electrons from outer shells falling into these vacancies emit radiation of specific wavelengths (see Figure 14.13). Analysis of this radiation, referred to as X-ray fluorescence (XRF), allows for the identification of the element from which the photon is emitted. Instruments for carrying out this analysis can be either laboratory sized or can be handheld... 

Figure 5.1 The X-ray emission and Auger processes (Pollard and Heron 1996 37). An inner shell vacancy is created in the K shell by the photoelectric process (emitted photoelectron not shown), (a) shows the X-ray emission process, where an L shell electron drops down to fill the vacancy, and the excess energy (EK - EL) is carried away as an X-ray photon. In (b), an L shell electron drops down, but the excess energy is carried away by an Auger electron emitted from the M shell, with kinetic energy approximately equal to EK - EL — EM. Reproduced by permission of the Royal Society of Chemistry.

The probability that the inner shell vacancy will de-excite by one or other of these processes depends on the energy level of the initial vacancy and the atomic weight of the atom. The fluorescent yield, co, is defined as the number of X-ray photons emitted per unit vacancy, and is a measure of the... 

This technique is also known as electron spectroscopy for chemical analysis (ESCA). Although it is concerned with the detection of electrons, it is discussed here because the way in which the photoelectrons are produced is fundamental to the XRF process. As described above, an incident X-ray photon produces an excited ion by ejecting an inner shell electron. The excited... 

Thus it was not observed until lasers were invented. In principal, one-photon and two-photon excitation follow different selection rules. For example, the inner shell one-photon transitions in transition metal, rare earth, and actinide ions are formally forbidden by the parity selection rule. These ions have d- or/-shells and transitions within them are either even to even (d d) or odd to odd (f /). The electric dipole transition operator is equal to zero. 

The Fricke solution contains iron ion in its constituent, and one might expect the effect of inner-shell photoabsorption of iron around the energy region of the Fe K-shell absorption edge. No photon-energy dependence of the Fricke yield around the K-shell absorption edge of the ion was found, which was explained by the small abundance of iron in the sample. 

We should note that the photoelectric effect often leaves an inner shell vacancy in the atom that previously contained the ejected electron. This vacancy will be filled by an atomic transition, called fluorescence, and generally produces an X-ray photon. In an interesting twist of fate, the X-ray photon will have an energy that is just below the sharp rise in the attenuation coefficient due to conservation of momentum and can often escape from the absorber. Recall that the direction of the fluorescence photon will be uncorrelated with the direction of the incident photon and a fraction will be emitted backwards from the absorber. The absorber will thus emit its own characteristic X-rays when it is irradiated with high-energy photons. 

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