Photoionization outer shell

Summarizing the individual decay branches of the 4d5/2 -> 6p resonance, one finds that all final ionic states can also be reached by outer-shell photoionization, in (a) and (b) by main processes, and in (c)-(i) by discrete and continuous satellite processes. The effect of the resonance decay will then be a modification of these otherwise undisturbed direct outer-shell photoionization processes which turns out to be an enhancement in the present case. Therefore, these outer-shell satellites are called resonantly enhanced satellites. In this context it is important to note that outer-shell photoionization also populates other satellites, attached, for example, to electron configurations 5s25p4ns and 5s25p4nd. However, the parity of these satellites is even, while the decay branches (c)-(/) lead to odd parity. Therefore, both groups of final ionic states can be treated independently of each other (if configuration interaction in the continuum is neglected). 

The existence of satellite lines has already been mentioned several times, for example, in the explanation of the photoelectron spectra of rare gases (see Fig. 2.4), and in the discussion of 2p photoionization in magnesium. In this section the satellite spectrum related to outer-shell photoionization in argon will be treated... 

The complete experiment for 2p photoionization in magnesium described previously depends on the validity of the non-relativistic LSJ-coupling scheme and on the existence of a simple subsequent Auger transition. However, such conditions are rarely met, since in heavier elements spin-orbit effects cannot be neglected, and for outer-shell photoionization no subsequent decay is possible. In order to perform a complete experiment for such cases,f measurement of the spin-polarization of the photoelectrons is necessary. As an example, 5p photoionization in xenon will be discussed. 

Calculations show that cross-sections obtained in the Hartree-Fock approximation utilizing length and velocity forms of the appropriate operator, may essentially differ from each other for transitions between neighbouring outer shells, particularly with the same n. However, they are usually close to each other in the case of photoionization or excitation from an inner shell whose wave function is almost orthogonal with the relevant function of the outer open shell. In dipole approximation an electron from a shell lN may be excited to V = l + 1, but the channel /— / + prevails. For configurations ni/f1 n2l 2 an important role is... 

Figure 1.3 Illustration of the two classes of two-electron processes caused by photoionization using magnesium as an example, using, on the left the model-picture of Fig. 1.1 and on the right an energy-level diagram (not to scale) (a) direct double photoionization in the outer 3s shell (b) 2p inner-shell photoionization with subsequent Auger decay where one 3s electron jumps down to fill the 2p hole and the other 3s electron is ejected into the continuum (Auger electron). The wavy line represents the incident photon (which is often omitted in such representations) electrons and holes are shown as filled and open circles, respectively arrows indicate the movements of electrons continuum electrons are classified according to their kinetic energy e.

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... 

Double photoionization in the outer shell of rare gases by a single photon is an important manifestation of electron correlations. One specific aspect which has received much attention over the years is double photoionization in the vicinity of the double-ionization threshold. On the theoretical side, this attention is due to the possibility of deriving certain threshold laws without a full solution of the complicated three-body problem of two electrons escaping the field of the remaining ion. On the experimental side, the study of threshold phenomena always provides the challenge for mastering extremely difficult experiments. 

As a first approximation, direct double photoionization will be neglected. This is often justified because the cross section for double photoionization in outer shells, and hence also the corresponding amplitude, is much smaller than the cross section for single photoionization in an inner shell. Therefore, the Auger decay has... 

The deep inner shell orbitals such as Is, 2s and 2p are not very sensitive both to the scaling parameter a and to the oxidation state. On the other hand, the shallow inner shells like 3s and 3p and outer-shell orbitals 3d and 4s strongly depend upon a and the effective charge. Accordingly, the theoretical photoionization cross section computed by equation (12) is affected by the change of spatial extent of the atomic orbital. The theoretical photoionization cross sections for Fe orbitals shown in Table 2 are calculated for the photon energy of hv = 1487eV (A1 Ka) and indicated in Table 3. In the case of a=l. 0, the atomic orbitals are somewhat contracted compared with... 

Inner-shell photoionization leaves a core hole that is stabilized via relaxation processes (cf. Sec. 2.3). The core hole is filled by an outer-shell electron leading either to the emission of an Auger electron or an X-ray photon (cf. Fig. 4). The branching ratio between both relaxation processes depends on the nuclear charge... 

The filling of the inner shell vacancy left after photoionization by an outer shell electron produces a fluorescence X-ray photon. Again, the energies of fluorescent photons are characteristic of the elements, as they correspond to the differences of binding energies of electrons from different levels, which are different for different elements. 

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