Ionization electron decay process

Interatomic Coulombic decay (ICD) is an electronic decay process that is particularly important for those inner-shell or inner-subshell vacancies that are not energetic enough to give rise to Auger decay. Typical examples include inner-valence-ionized states of rare gas atoms. In isolated systems, such vacancy states are bound to decay radiatively on the nanosecond timescale. A rather different scenario is realized whenever such a low-energy inner-shell-ionized species is let to interact with an environment, for example, in a cluster. In such a case, the existence of the doubly ionized states with positive charges residing on two different cluster units leads to an interatomic (or intermolecular) decay process in which the recombination part of the two-electron transition takes part on one unit, whereas the ionization occurs on another one. ICD [73-75] is mediated by electronic correlation between two atoms (or molecules). In clusters of various sizes and compositions, ICD occurs on the timescale from hundreds of femtoseconds [18] down to several femtoseconds [76-79]. 

Fig. 7.15. Photophysics associated with x-ray photoelectron spectroscopy and x-ray fluorescence. As illustrated, in the XPS experiment one monitors the energy of the electron ejected from the M shell upon photoionization (process 1). In the XRF experiment, one monitors the fluorescence emitted from either the M shell after photoionization (process 2a), or from the L shell after photo ionization and radiationless decay (process 2b).

Note that this EC decay process does not involve capture of the orbital electron of the 7Be since it is fully ionized in a star but rather involves capture of a free continuum electron. As a consequence, the half-life of this decay is 120 d rather than the terrestrial half-life of 77 d. The resulting 7Li undergoes proton capture as ... 

Hitherto the discussion of Fig. 5.2 has neglected the possibility of non-radiative decay following 4d shell excitation/ionization. These processes are explained with the help of Fig. 5.2(h) which also reproduces the photoelectron emission discussed above, because both photo- and autoionization/Auger electrons will finally yield the observed pattern of electron emission. (In this context it should be noted that in general such direct photoionization and non-radiative decay processes will interfere (see below).) As can be inferred from Fig. 5.2(h), two distinct features arise from non-radiative decay of 4d excitation/ionization. First, 4d -> n/ resonance excitation, indicated on the photon energy scale on the left-hand side, populates certain outer-shell satellites, the so-called resonance Auger transitions (see below), via autoionization decay. An example of special interest in the present context is given by... 

Photoionization can result either from the direct interaction of a photon with the ionizing electron or by an indirect process. An example of the latter is autoionization where a photon is absorbed to produce an excited state of the molecule, M, by a resonance transition. The excited state then subsequently decays to the molecular ion and a photoelectron (equation 4) ... 

In AES, core-hole excitations are created when a beam of electrons, typically with energies between 1 to 10 KeV, is impinged onto the surface. In the decay process, one upper-level electron falls into the vacant core level and a second electron, the Auger electron, is ejected. Since the kinetic energy of the emitted Auger electron is characteristic of the (doubly-ionized) atom, AES is a sensitive technique that provides information on the elemental composition (except H and He) of the interfacial region. Analytical procedures that enable the use of AES as a quantitative technique have been suggested. " ... 

In these time-dependent kinetic studies, a variety of electron collision processes similar to those treated in the steady-state kinetics has been treated. In addition to these processes, nonconservative electron collision processes, such as ionization and attachment, and even the nonlinear electron-electron interaction have been taken into accoimt. Besides the various types of electron collisions, other electron generation and destruction processes, such as the chemo-ionization in collisions between excited heavy particles in decaying plasmas or the injection of beamlike electrons into plasma, have been included as particle sources or sinks... 

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