Radiationless transition. Auger

Burhop, E. H. S. (1952) The Auger Effect and other Radiationless Transitions, Cambridge University Press, Cambridge. 

If certain quanta suitable for the excitation of a line are absorbed without photon emission, a radiationless transition is likely. This transition is known as the Auger effect,39 and it may be thought to involve an absorption by the atom of the photon produced when the hole in the K shell is filled by an electron from one of the external shells such as the L shell. The absorption of this photon results in the ejection of a second electron from one of the shells to leave a doubly charged residue of what had been a normal atom. The atom in this condition is described by naming the two states in which the electron holes are to be found e.g., the atom is in the LL or LM or LN state. An atom in such a state is, of course, vastly different from the usual divalent cation. 

The competitive relaxation process known as the Auger effect involves radiationless transitions and the ejection of valency electrons. X-ray fluorescence spectrometry has developed as a... 

In this case, an M electron is emitted as an Auger electron. The Auger process is termed a radiationless transition. The probability that an inner shell vacancy... 

This is termed a radiationless transition, and the emitted electron is called an Auger electron. Figure 5.1 shows a schematic diagram of the X-ray emission and Auger processes. 

Auger electron spectroscopy (29) is a type of electron spectroscopy that is used for determining solid surface elemental and electronic composition. An experiment is conducted by bombarding a solid surface with an electron beam of energy ranging from 1 keV to 10 keV. Alternatively, an x-ray source can be used. The Auger electrons, emitted from an atom by means of a radiationless transition, are... 

Figure 5.7 Spectrum of electrons ejected from magnesium atoms after interaction with 80 eV photons (measured at the quasi-magic angle in order to allow the extraction of relative intensities). The 3s and 2p photolines are shown together with their satellites and the radiationless transitions following the 2p main and satellite processes, i.e., L2 3-M,M1 normal Auger transitions and Auger satellites, respectively. From [HKK88].

Capture may take place to bound states of the negative ion that undergo radiationless transitions to repulsive states of the negative ion resulting in dissociative electron capture. These radiationless intramolecular transitions (Auger transitions) are a result of overlap of the discrete states with a continuum of states of AB-. These types of processes are illustrated for diatomic molecules in Figure 2.2b. Electrons are first captured into the bound state represented by curve 1, and before autodetachment can take place an intramolecular radiationless transition occurs to curve 2 resulting in dissociation into A and B-. 

Figure 9.7. Fluorescence yield co and Auger yield 1 - co for the K and L shells as a function of the atomic number. (According to E. H. S. Burshop The Auger Effect and other Radiationless Transitions. Cambridge University Press, London 1952.)...

Auger electrons are caused by the ejection of an electron in an outer shell to a state in the continuum. If a primary vacancy exists in the K shell, the vacancy is filled from the Lg shell and the energy released in this transition results in the expulsion of an electron from the L3 shell the electron will be denoted as an auger electron of the KL2L3 type. Thus, there are nine forms of radiationless transition in the KLL group, each with a specific energy unique to the element. 

Eq. (13) suggests an upper limit of 5 about 0.001 eV, 2000 times less than observed. This is a safe proof that radiationless transitions of the Auger type are far more effective in decreasing t 2 and increase 5 much above what can be accounted for by X-ray emission ( fluorescence ) due to 4f - 4d transitions. Furthermore, Eq. (13) suggests an increase of 6 by a factor of ten from Z = 71 to 90 in complete disagreement with the observed 6 known to vary less than 15 percent (45). Hence, the Coster-Kronig transi-... 

Both autoionisation and the Auger effect are often referred to as radiationless transitions, because the initial reorganisation of the atom, takes place on very short timescales (typically 10 13s) without the emission of radiation. This does not, however, mean that no radiation at all is emitted by the atom during or after either of these processes. It is merely that... 

In Auger electron spectroscopy an atom is ionized by X-ray, electron or ion bombardment. This ionic state has a certain probability to decay in a radiationless transition whereby an additional electron is emitted (the so-called Auger electron). Thus the atom is finally left in a double ionized state. The energy distribution of the Auger electrons is recorded. 

Auger spectroscopy prepares a system in a core-hole state by ionizing radiation and measures the kinetic energy of secondary electrons produced when the highly excited core-hole state makes a radiationless transition to a continuum state with two valence-holes and a free electron. The initial photoelectron and the secondary (Auger) electron make this a two-electron detachment process leading to the two-particle two-hole propagator... 

The probability amplitude for the radiationless transition of the Auger process then is... 

The probability of radiationless transitions can be computed wave-mechanically. In what follows we will consider the special case where an atom ionised in the /C-shell emits an L-electron. The number of emitted Z/-electrons in relation to the emitted K a-light quanta is obtained in agreement with the experiments of P. Auger and H. Robinson. 

The dependence on the atomic number, expressed in Eqs. (10), (11) and (12), is wholly obtained without dimensional considerations for general quantum jumps. It has the same precision as Moseley s law, E Z ze is independent of Z and zl is proportional to Z, so zeIzl is proportional to Z . If one considers that the number of outer electrons that can be emitted on a radiationless transition increases with increasing atomic number, then one would expect that all in all zeIzl varies with a power a little bit smaller that Z. The following table confirms this using the observations by Auger. 

For X = K (no subshell) the number of X-ray photons emitted is given by Nk=N(Dk where N is the total number of K holes involved. Here N is equal to the sum of radiative and radiationless transitions. To a first approximation, K radiationless transition probability is nearly independent of Z, while radiative electric-dipole probability is proportional to Z. It justifies useful semiempirical laws based on q)kOcZ /(Z +c), c=constant. They are discussed in [4] which also gives many references on Auger and related processes up to 1971. Due to Coster-Kronig transitions, experimental and theoretical problems are more complicated for X = L, M,. .. Experimental data depend on the primary vacancy distribution which must remain unaltered before the vacancies are filled. Literature provides either total X-shell data(X = L, M,. ..) or partial Xj-subshelldata (Xj = L., Lg, L3,. ..). Definitions... 

Burhop, E.H.S. (The Auger Effect and Other Radiationless Transitions, Cambridge Univ. Press, Cambridge 1952). 

Once an inner shell vacancy is created in an atom the atom may then remrn toward its ground state via emission of a characteristic X ray or through a radiationless Auger transition. The probability of X-ray emission is called the fluorescence yield. 

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