Postcollisional effects in multiple ionization of diatomic molecules by ion impact
Carmen A. Tachino, Mariel E. Galassi, and Roberto D. Rivarola
*
Instituto de Física Rosario (CONICET–UNR) and Facultad de Ciencias Exactas, Ingeniería y Agrimensura,
Universidad Nacional de Rosario, Avenida Pellegrini 250, 2000 Rosario, Argentina
Received 21 December 2007; published 17 March 2008
Multiple electron ionization of diatomic molecular targets is studied for impact of proton beams. It is shown
that for these molecular targets, as it was previously predicted for atomic ones, postcollision Auger type
emission following inner-shell vacancy production dominates the reaction at high enough impact energies.
Moreover, it is proven that this dominance increases as the ionization degree increases.
DOI: 10.1103/PhysRevA.77.032714 PACS numbers: 34.50.Gb
I. INTRODUCTION
The present work deals with multiple electron ionization
of diatomic molecular targets by impact of fast ion beams.
During the collision process, when the projectile interacts
with the target electrons, the emission to the continuum of
one or more of them can be produced, generating vacancies
in the different molecular orbitals. Thus in the time interval
corresponding to a first step, the residual molecule is pro-
moted to a highly excited state as a consequence of its direct
interaction with the ion beam. In a postcollisional step, once
the projectile is far away, the vacancies in the inner orbitals
are filled by residual target electrons, such that the excess of
energy is simultaneously employed to induce photon emis-
sion or to produce ionization of other bound electrons, which
are thus autoionized through Auger-type emission 1–3. In
fact, this Auger-type emission corresponds to intershell Au-
ger and/or intrashell Coster-Kronig electron ionization.
These ejected electrons generate new vacancies, so that pho-
ton emission and/or electron autoionization are again pro-
duced. These mechanisms can be subsequently repeated so
that additional Auger-type cascades can be observed in the
target 4.
Special attention was devoted to the Auger autoionization
reaction since its discovery in the 1920s, due to the important
role that it plays in different scientific areas see, for ex-
ample, Refs. 5,6. In more recent years, active research was
developed in order to understand the physical mechanisms
involved in multiple ionization of noble gases. In a pioneer-
ing work by DuBois and Manson 7, a detailed analysis for
the case of proton impact on Ar and Kr showed that Auger
transitions following the production of direct inner-shell va-
cancies dominate the multiple ionization cross sections at
high enough impact energies. It was also suggested that the
same behavior could be expected for Ne. Later, Heber et al.
8 measured single and multiple ionization cross sections
for impact of 1 MeV / amu-O
x+
and F
x+
ions on Ar atoms.
They found that to describe the experimental results it was
necessary to take into account a charge multiplication effect
of the recoil target via Auger decay of L-shell vacancies.
However, they claimed that this charge multiplication effect
should not appear for Ne targets.
An independent-particle model was employed to study
multiple electron loss of the target in the cases of Ne and Ar
atoms, where single-particle time-dependent Schrödinger
equations were solved using the basis generator model
BGM, describing the target bound state in terms of the
optimized potential model 9–11. For proton impact a good
representation of experimental data 7,12–15 was obtained.
However, for both targets and high ionization degree the the-
oretical description overestimates underestimates the exist-
ing measurements at intermediate high collision energies.
The discrepancy at high impact energies was corrected in-
cluding Auger type postcollisional emission in the theoretical
model 9–11,14. Instead of using a multinomial statistics as
it was done in previous works, Archubi et al. 16 solved the
transport equation for an ion traveling through an inhomoge-
neous electron density corresponding to the atom, which
leads to a Poisson distribution. Ionization probabilities were
obtained using the shell-to-shell local plasma approximation
with the Levine and Louie dielectric function 17 to take
into account the energy ionization threshold of each shell.
This model gives an adequate description of experimental
total cross sections, for different q-ionization degrees, for
proton impact on Kr and Xe targets. Also, a statistical
energy-deposition model SED was used by Kabachnik
et al. 18,19 to study multiple ionization of atoms and di-
atomic molecules. They include a free parameter in the cal-
culation of q-fold ionization probabilities to consider the
contribution of nondirect ionization processes. Recently, we
have studied the cases of the impact of protons on Ne and Ar
atoms, employing a binomial statistics in a similar way as it
has been done before in Refs. 9–11 but where single ion-
ization probabilities were calculated using the continuum
distorted wave eikonal initial state CDW-EIS and exponen-
tial models 20. The total cross section results for different
q-ionization degrees confirmed the behavior obtained by us-
ing the BGM model. At high enough energies, the multiple
ionization reaction is dominated by Auger type emission fol-
lowing previous process of direct ionization produced by the
projectile. Moreover, this dominance becomes more impor-
tant as q increases.
The main scope of the present work is to analyze the
influence of postcollisional Auger type emission on the pro-
cess of multiple electron ionization of diatomic molecules
after interaction with a fast ion beam. We select the case of
impact of protons on CO molecules for which experimental
data exist at impact energies high enough to neglect electron
*
rivarola@fceia.unr.edu.ar
PHYSICAL REVIEW A 77, 032714 2008
1050-2947/2008/773/0327146 ©2008 The American Physical Society 032714-1