Creation of excited states as the result of x-ray photoionization Page: 2 of 30
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I. INTRODUCTION
In a normal photoionization process all the energy of the photon is ex-
pended in the ejection of a photoelectron according to the simple energy rela-
tionship
E = hv - EB (1)
where Ee is the kinetic energy of the photoelectron, hv the energy of the photon
and EB is the binding energy, which may be defined as
EB = Tf - T (2)
where Ti is the total energy of the initial neutral species and T is the total
energy of the resulting ion. If one is dealing with an atam and assumes the
validity of single electron wave functions, EB can be taken as the binding energy
of the atomic orbital in which the vacancy is formed. Thus, for a monochroatic
source of photons there is a series of photoelectron energies corresponding to
the binding energies of the atomic orbitals.
In a "normal" photoionization process each electron in the resultant
ion is described by the same set of quantum numbers that existed in the initial
atom. All the electrons except the photoejected electron, however, readjust or
relax to the new configuration containing a single vacancy. (Subsequent filling
of this vacancy may occur by x-ray emission or Auger process but these phenaiena
will not be discussed in this review.) In photoionization there is also the
possibility that multiple ionization or excitation will occur whereby one or
more electrons are promoted into an orbital with different quantum numbers or
even into the continuum. Mien this cc -urs, the energy of the final state is T ,
and frm equation (2) the photoelectron will be ejected with an energy that
*
differs from the "normal" photoelectron peak by Tf - Tf. In this paper we shall
review the types of phenomena that lead to such excited states, namely:
(1) electron shake up and shake off; (2) configuration interaction and (3) mul-
tiplet splitting.
In this review we shall concern ourselves with photoionization caused by
soft x rays. Carmonly used sources, for example, are the Al Ka and Mg Ka x rays
with energies of 1487 and 1254 eV respectively. These x rays induce photoioni-
zation of the core electron as well as the valence electrons. The photon energy
in general will be considerably above the threshold energy required for normal
photoionization or for multiple ionization and excitation, and will thus be in
the region where the sudden approximation is valid. Both atanic and molecular
systems will be discussed, and scme indication of how the study of excited
states produced by photoionization can be used in shedding light on the elec-
tronic structure of molecules will be pointed out.
II. ELECRON SHAKE UP AND SHAKE OFF
As the result of photoionization in the core of an ato, electrons in
the outer shell sense a sudden change in the central potential due to the re-
moval of an electron which had been shielding the nuclear charge. This sudden
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Carlson, T. A. Creation of excited states as the result of x-ray photoionization, article, January 1, 1973; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1026279/m1/2/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.