Biomedical Engineering Reference
In-Depth Information
due to the multi-centered nature of the water target. In this context, we have
recently proposed a unified methodology to express the water molecular wave
functions in both phases by means of a single-centre partial-wave description (see
[
56
,
67
] for more details). In brief, the wave functions have been carried out by
using the Gaussian 03 program and computed at the Hartree-Fock level of theory
by using the augmented, correlation-consistent, polarized-valence quadruple-zeta
basis set (aug-cc-pvQZ) of Kendall
et al
.[
68
]. Geometry optimization has been
done by including electronic correlation energy at the second-order Møller-Plesset
perturbation theory (MP2, [
69
]). For the computations in the liquid phase we have
used the polarizable continuum model (PCM) developed by Tomasi et al. [
70
] based
on the representation of the liquid by a polarizable dielectric continuum having the
static dielectric constant of water
. Thus, a cavity was created in this
continuum and a water molecule was placed in it. The molecule was then described
quantum mechanically with a Hamiltonian including the electrostatic interactions
with the surrounding dielectric medium. Once polarized by the molecular charges,
the continuum creates a reaction potential inside the cavity, which in turn polarizes
the molecule. The wave function was also obtained by an iterative computation
using the so-called self-consistent reaction field approach.
The obtained wave functions were then here used as input data in our theoretical
treatment developed for describing the water ionization induced by electron and
positron impact in the energy range 10 eV-100 keV (see Fig.
13.3
).
.© D 78:39/
13.4.3
The excitation processes
Excitation includes all the processes that modify the internal state of the impacted
target molecule (without secondary electron creation), each of them giving a non
negligible contribution to the final energetic cartography. They include in particular:
i)
electronic transitions towards Rydberg states or degenerate states ( A
1
B
1
;
B
1
A
1
,
diffuse band),
ii)
dissociative attachment leading to the formation of negative ions,
iii)
dissociative excitation, leading to excited radicals (H
;
O
et OH
), and in a
minor part
iv)
vibrational and rotational excitations.
In order to account all the processes listed
i), ii)
and
iv)
,wehaveusedthe
semi-empirical approach of Olivero et al. [
71
], whereas the dissociative excitation
processes were treated via the approach proposed by Green and Dutta [
72
]. More-
over, following some experimental observations [
73
], we assume that excitation
induces no angular deflection.
Finally, note that we have here assumed that electron- and positron-induced
excitation could be treated in the same way considering recent experimental data
on Neon which only reported slight differences in terms of total excitation cross
sections between the two projectile types (see [
74
] for more details).
