4
A. V. Glushkov
Generally speaking, an autoionization falls within the general class of phenomena
known as the Auger effect [1]. In the Auger effect an atomic system “seemingly”
spontaneously decays into a partition of its constituent parts [6, 7]. Traditionally the
phenomenon of autoionization, or more particularly the autoionization state decay
itself is treated as decay of a bound state. Nevertheless, many scientists have indicated
that the process is rigorously a part of the scattering continuum. Due mostly to the
known work of Feshbach [5], a rigorous formulation can be established whereby
the main element of the theory can be made into a bound state problem with the
scattering elements built around it. The major constituent of both these features is
accomplished with projection operators. In our opinion [2, 3], an autoionization state
should be considered as a resonance in a continuum and we will treat it for the most
part in this Chapter. As it is well known the autoionization states can be formed by
scattering processes and photoabsorption. These are the inverses of the autoionization
and photon emission processes by which they can decay. In the scattering process,
formation of the autoionization state corresponds to a resonance in the scattering
cross section. Autoionization is the process which corresponds to the decay of the
resonance. The decay of the resonance (autoionization) is then seen to be the last
half of the resonant scattering process. Numerous studies of autoionization states in
spectra of different atoms and ions have shown that the resonant or autoionization
state, although it may be long-lived, is not completely stationary. According to Ref.
[4, 5] that is the reason that the word “seemingly” was used to describe the Auger
process. Since the early work of Beutler the interaction of a discrete (bound) atomic
state with a continuum channel has received a great deal of attention. A detailed
treatment of such autoionization processes by configuration interaction has been
given by Fano [4].
In recent years a rapid progress has been made in studying and understanding
autoionization processes in two-quasiparticle atomic systems. Experimentally, multistep laser excitation of the heavier earth-alkaline atoms resulted in a wealth of data
on doubly-excited configurations, particularly for the barium atom (e.g. [21–33]).
Multi-channell quantum defect theory (MQDT) in combination with eigenchannel
R-matrix calculations is quite successful in describing autoionization spectra of these
atoms even in cases involving large numbers of continuum channels [21–31]. In a
semiempirical approach model potentials are used that are adjusted to the ionic
spectrum for each value of the orbital angular momentum l. Much progress has been
achieved in the theoretical and computational treatment of electron-impact ionization
and autoionization processes due to using such methods as exterior complex scaling
(ECS)], time-dependent close-coupling (TDCC), convergent close coupling (CCC)
or R-matrix with pseudo-states (RMPS) [2–60]. Nicolaides et al. (e.g. Refs. in [29,
30]) have applied the time-dependent Schrödinger equation approach to treating the
doubly excited autoionizing states in simple atomic systems. Nikitin, Ostrovsky et al.
developed an analytical framework to compute autoionization rates in two-electron
systems using single configuration wavefunctions (e.g. [73]). Its applicability is limited to doubly excited states where the outer electron has a large orbital angular
momentum l > 3. Poirier [35] included core polarization effects into the NikitinOstrovsky model [31] and performed computing radial matrix elements. This model
A. V. Glushkov
Generally speaking, an autoionization falls within the general class of phenomena
known as the Auger effect [1]. In the Auger effect an atomic system “seemingly”
spontaneously decays into a partition of its constituent parts [6, 7]. Traditionally the
phenomenon of autoionization, or more particularly the autoionization state decay
itself is treated as decay of a bound state. Nevertheless, many scientists have indicated
that the process is rigorously a part of the scattering continuum. Due mostly to the
known work of Feshbach [5], a rigorous formulation can be established whereby
the main element of the theory can be made into a bound state problem with the
scattering elements built around it. The major constituent of both these features is
accomplished with projection operators. In our opinion [2, 3], an autoionization state
should be considered as a resonance in a continuum and we will treat it for the most
part in this Chapter. As it is well known the autoionization states can be formed by
scattering processes and photoabsorption. These are the inverses of the autoionization
and photon emission processes by which they can decay. In the scattering process,
formation of the autoionization state corresponds to a resonance in the scattering
cross section. Autoionization is the process which corresponds to the decay of the
resonance. The decay of the resonance (autoionization) is then seen to be the last
half of the resonant scattering process. Numerous studies of autoionization states in
spectra of different atoms and ions have shown that the resonant or autoionization
state, although it may be long-lived, is not completely stationary. According to Ref.
[4, 5] that is the reason that the word “seemingly” was used to describe the Auger
process. Since the early work of Beutler the interaction of a discrete (bound) atomic
state with a continuum channel has received a great deal of attention. A detailed
treatment of such autoionization processes by configuration interaction has been
given by Fano [4].
In recent years a rapid progress has been made in studying and understanding
autoionization processes in two-quasiparticle atomic systems. Experimentally, multistep laser excitation of the heavier earth-alkaline atoms resulted in a wealth of data
on doubly-excited configurations, particularly for the barium atom (e.g. [21–33]).
Multi-channell quantum defect theory (MQDT) in combination with eigenchannel
R-matrix calculations is quite successful in describing autoionization spectra of these
atoms even in cases involving large numbers of continuum channels [21–31]. In a
semiempirical approach model potentials are used that are adjusted to the ionic
spectrum for each value of the orbital angular momentum l. Much progress has been
achieved in the theoretical and computational treatment of electron-impact ionization
and autoionization processes due to using such methods as exterior complex scaling
(ECS)], time-dependent close-coupling (TDCC), convergent close coupling (CCC)
or R-matrix with pseudo-states (RMPS) [2–60]. Nicolaides et al. (e.g. Refs. in [29,
30]) have applied the time-dependent Schrödinger equation approach to treating the
doubly excited autoionizing states in simple atomic systems. Nikitin, Ostrovsky et al.
developed an analytical framework to compute autoionization rates in two-electron
systems using single configuration wavefunctions (e.g. [73]). Its applicability is limited to doubly excited states where the outer electron has a large orbital angular
momentum l > 3. Poirier [35] included core polarization effects into the NikitinOstrovsky model [31] and performed computing radial matrix elements. This model