Biomedical Engineering Reference
In-Depth Information
Fig. 10.4
The Hartree-Fock molecular orbitals of pyrimidine for the four lowest molecular
orbitals [
7
]
The intensities of the RAE spectra depend on the overlap of the wave functions of
the intermediate and final states, i.e. the RAE decay favours the population of final
ionic states with similar localisation of the charge distribution as in the intermediate
state. This can be deduced on the basis of the electronic assignment and charge
distribution of the intermediate and final states reported in Table
10.2
and Fig.
10.4
,
respectively. So for example the decay to the n
N
state is very weak in all cases of
s
1
/
Fig.
10.3
b, while the decay to the
2
state is strongly favoured from C
4=6
.1
.
10.3
The photofragmentation of pyrimidine
The pioneering work of Sanche and co-workers has shown that low-energy electrons
of even less than 3 eV can cause single- and double-strand breaks of plasmid
DNA [
16
] via dissociative electron attachment (DEA) processes. Other efficient
mechanisms for molecular fragmentation involve the direct collision with heavy
ions [
17
, and references therein] and the absorption of high energy radiation [
18
,
and references therein]. The latter process, with formation of inner shell vacancies,
whose subsequent decay inevitably lead to the molecular fragmentation with one or
more charged fragments in the final state, is the subject of the present discussion.
The photon energy dependency of the molecular fragmentation of pyrimidine
has been investigated by measuring the time-of-flight (TOF) mass spectrum from the
appearance potential regions [
19
] up to the N(1 s) ionization threshold, see Fig.
10.5
.
The mass spectrum of pyrimidine can be roughly divided into the six regions
labelled A-F in Fig.
10.5
. As a rule of thumb, these regions can be interpreted
as charged fragments with one to six ring atoms, going from A to F respectively.
Atomic fragments H
C
=
N
C
form group A while the parent ion C
4
N
2
H
4
C
belongs to group F. Within each group several fragmentation channels, each one
with multiple hydrogen losses are present and make the detailed analysis far more
complicated than the rough assignment just presented. As a general feature, we
can observe that
C
C
=
) fragments with five-members of the original pyrimidine ring
(group E) are not observed at any of the investigated photon energies. This could
be explained by the unstable species corresponding to these masses, not living long
i
