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The following article appeared as: Chiari, L., Anderson, E.,
Tattersall, W., Machacek, J.R., Palihawadana, P.,
Makochekanwa, C., Sullivan, J.P., Garcia, G., Blanco, F.,
McEachran, R.P., Brunger, M.J. and Buckman, S.J., 2013.
Total, elastic, and inelastic cross sections for positron and
electron collisions with tetrahydrofuran. The Journal of
Chemical Physics, 138, 074301.
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http://dx.doi.org/10.1063/1.4789584
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Total, elastic, and inelastic cross sections for positron and electron
collisions with tetrahydrofuran
Luca Chiari, Emma Anderson, Wade Tattersall, J. R. Machacek, Prasanga Palihawadana et al.
Citation: J. Chem. Phys. 138, 074301 (2013); doi: 10.1063/1.4789584
View online: http://dx.doi.org/10.1063/1.4789584
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THE JOURNAL OF CHEMICAL PHYSICS 138, 074301 (2013)
Total, elastic, and inelastic cross sections for positron and electron
collisions with tetrahydrofuran
Luca Chiari,
1,2
Emma Anderson,
2
Wade Tattersall,
2,3
J. R. Machacek,
2
Prasanga
Palihawadana,
2
Casten Makochekanwa,
2
James P. Sullivan,
2,a)
Gustavo García,
4,5
Francisco Blanco,
6
R. P. McEachran,
2
M. J. Brunger,
1,7
and Stephen J. Buckman
2,7
1
ARC Centre for Antimatter-Matter Studies, School of Chemical and Physical Sciences, Flinders University,
GPO Box 2100, Adelaide, SA 5001, Australia
2
ARC Centre for Antimatter-Matter Studies, Research School of Physics and Engineering, The Australian
National University, Canberra, ACT 0200, Australia
3
ARC Centre for Antimatter-Matter Studies, School of Engineering and Physical Sciences, James Cook
University, Townsville, QLD 4810, Australia
4
Instituto de Física Fundamental, Consejo Superior de Investigationes Científicas (CSIC), Serrano 113-bis,
28006 Madrid, Spain
5
Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia
6
Departamento de Física Atómica, Molecular y Nuclear, Universidad Complutense de Madrid,
28040 Madrid, Spain
7
Institute of Mathematical Sciences, University of Malaya, 50603 Kuala Lumpur, Malaysia
(Received 14 November 2012; accepted 15 January 2013; published online 15 February 2013)
We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahy-
drofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium
formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations
and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies,
have been measured for positron collisions with THF. The positron beam used to carry out these ex-
periments had an energy resolution in the range 40–100 meV (full-width at half-maximum). We also
present TCS results for positron and electron scattering from THF computed within the indepen-
dent atom model using the screening corrected additivity rule approach. In addition, we calculated
positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations).
While our integral and differential positron cross sections are the first of their kind, we compare
our TCS with previous literature values for this species. We also provide a comparison between
positron and electron-impact cross sections, in order to uncover any differences or similarities in
the scattering dynamics with these two different projectiles. © 2013 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4789584]
I. INTRODUCTION
Research on the effects of ionizing radiation in biomolec-
ular systems has attracted intense interest, in the last decade or
so, within the atomic and molecular physics and medical sci-
ence communities.
1
In particular, the importance of electron-
induced chemical processes at low energies has recently been
highlighted by the discovery that electrons at sub-ionization
2
and even sub-excitation energies
3
can attach to and cause
the fragmentation of the nucleic acids, the proteins, and their
components, such as the nucleobases, the sugars, and water.
4
These processes can eventually result in important cell and
tissue damage.
5,6
Positrons can also potentially trigger damage in
biomolecular systems, e.g., by the liberation of significant
numbers of secondary low-energy electrons, as the positrons
thermalize within the biological medium, through processes
such as direct ionization.
1
However, the mechanisms leading
to biological damage are qualitatively and quantitatively dif-
ferent for positrons and electrons.
7
In particular, the presence
a)
Author to whom correspondence should be addressed. Electronic mail:
james.sullivan@anu.edu.au.
of the positronium (Ps) formation channel in positron impact
phenomena leads to gamma rays originating from the Ps an-
nihilation which potentially add to the ionizing effect inside
the organic tissue.
7
In fact, the detection of the annihilation radiation emitted
by the positrons, as they enter the human body and annihilate
with the molecular electrons, is at the heart of positron emis-
sion tomography scans, an imaging technique of metabolic
activity, and an early detection tool for tumours. The ap-
plication of positrons in medical science, however, has ex-
panded to beyond purely diagnostic purposes. Indeed, they
have also been employed in therapeutic or clinical proce-
dures, e.g., as probes for protein syneresis,
8
in bioactive
molecule encapsulation,
9
and even in the treatment of tu-
mours (positherapy).
10
Nevertheless, there are many unan-
swered questions about the interactions of positrons with
biomolecules,
11
because the science behind the processes that
take place at the atomic and molecular level remains mostly
unknown. As a consequence, our group has embarked on a
project to explore the interaction of positrons with molecules
of biological interest. This project has started with experi-
ments on the primary constituent of all living organisms, i.e.,
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074301-2 Chiari
et al.
J. Chem. Phys. 138, 074301 (2013)
FIG. 1. Schematic diagram of an unrolled segment of DNA with the THF
molecule highlighted in the phosphate-deoxyribose backbone structure.
water, and the simplest organic acid, namely, formic acid.
12
In the present study, we shift our focus to tetrahydrofuran
(THF: C
4
H
8
O). THF represents a suitable model or analogue
for the sugar rings (ribose or deoxyribose) that are contained
in all nucleoside bases and which play an essential role in the
structure of both DNA
13–15
and RNA.
16
In fact, as specifically
shown by the schematic diagram of an unrolled segment of
DNA in Figure 1, the backbone structure of the nucleic acids
can be viewed as a series of THF-like molecules held together
by phosphate bonds, to which the nucleobases are covalently
linked to form the nucleotides. Therefore, THF can be thought
of as a prototypical analogue of the building blocks for living
matter.
17,18
THF also has some physico-chemical properties that
make it an interesting species to investigate from a more fun-
damental perspective. For example, it is a heterocyclic organic
compound containing an ether group with an exposed oxy-
gen atom available for hydrogen bonding. This peculiarity
in its structure makes it one of the most polar of the sim-
ple ethers. In fact, THF possesses both a large permanent
dipole moment μ = 1.63 D,
19
and static dipole polarizability
α = 47.08 a.u.
18
However, THF is also known to be conforma-
tionally impure. In fact, the THF molecule comes in at least
three symmetric and two asymmetric conformers
20
and this
needs to be borne in mind when carrying out experiments on
this target, as it may potentially complicate the interpretation
of the results. Nevertheless, at room temperature the popu-
lation of conformers in our THF sample is expected to be
essentially dominated by nearly the same proportion of the
two most energetically stable conformers, i.e., those in the C
2
(55.5%) and C
s
(44.5%) geometry.
21
It is interesting to note
that these two lowest energy conformers share the same val-
ues for the dipole moment and the dipole polarizability (see
above).
18
Therefore, the presence of more than one conformer
in our THF sample might not significantly complicate the in-
terpretation of our experimental results.
Given its interesting properties, and the biological role
of THF that we have outlined above, it is not surprising to
find a considerable number of papers in the literature on low-
energy electron collisions with this target. In particular, we
note the total cross section (TCS) measurements of Zecca
et al.,
22
Mo˙zejko et al.,
23
and Baek et al.
24
Elastic differential
cross section (DCS) measurements at selected impact ener-
gies have been carried out by Milosavljevi
´
c et al.,
25
Allan,
26
Colyer et al.,
27
Dampc et al.,
28
Gauf et al.,
29
and Baek
et al.
24
Except for the work of Milosavljevi
´
c et al.,
25
these
groups also reported elastic integral cross sections (ICS). To
the best of our knowledge, the only electron-impact, dis-
crete electronic-state excitation cross sections are those of
Do et al.,
30
while a review of all the available earlier work
and a recommended database for electron-THF scattering can
be found in Fuss et al.
31
Among the theoretical studies on
electron-THF scattering, we note the TCS calculation using
the R-matrix method of Bouchiha et al.
32
Computations of
the elastic DCS have been reported by Mo˙zejko and Sanche
33
with the independent atom model (IAM), Trevisan et al.
34
us-
ing the complex Kohn variational method, and Winstead and
McKoy
35
and Gauf et al.
29
both employing the Schwinger
multichannel method (SMC). These authors, with the excep-
tion of Trevisan et al.,
34
calculated the elastic ICS as well.
For completeness, we also cite the elastic ICS computation
by Tonzani and Greene
36
using the R-matrix approach.
With positron projectiles, however, there is only one ear-
lier experimental investigation on THF from the group at the
University of Trento
22
and there are no calculated cross sec-
tions. From a theoretical point of view, this reflects the diffi-
culty in making an accurate quantum-mechanical description
of a large target molecule such as THF, as well as the difficul-
ties posed by incorporating Ps formation into the formalism.
In this paper, we report on measurements of the TCS and
Ps formation cross section for positron scattering from THF in
the energy range from 1 to 190 eV. In addition, the summed
inelastic ICS for electronic excitations and direct ionization
have been measured up to 20 eV. Furthermore, measurements
of elastic DCS at selected incident energies between 1 and 25
eV are also described. The experiments were carried out at
the Australian Positron Beamline Facility,
37
using a positron
beam with an energy resolution in the range 40–100 meV
(full-width at half-maximum). In order to partially fill the gap
in the theoretical results for positron scattering from THF, we
have also computed the TCS, the elastic ICS and the sum over
all inelastic ICS (except for rotations and vibrations) within
the IAM approach with screening (SCAR) corrections ap-
plied. The energy range of these computations is between 1
and 5000 eV. In addition, in order to facilitate a comparison
of the cross sections for positron and electron collisions with
THF, we report on new theoretical results for electron scatter-
ing from this target species, calculated with the same method
and in the same energy range.
In Sec. II of this paper, we discuss the experimental
procedures of our measurements, and Sec. III presents the
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074301-3 Chiari
et al.
J. Chem. Phys. 138, 074301 (2013)
positron and electron scattering models and the computational
details that we employed in our calculations. We then report
and discuss our results in Sec. IV, before drawing some con-
clusions from the present investigation.
II. EXPERIMENTAL DETAILS
The measurements presented here were carried out us-
ing the low-energy positron beamline at the Australian Na-
tional University. This apparatus is based on a “Surko” trap
and beam system
38
and has already been described in detail
in Ref. 37, so that only an overview of its operation will be
presented here. Positrons are obtained from a
22
Na radioac-
tive isotope, which had an activity of approximately 40 mCi
for the present measurements. A solid neon moderator is used
to moderate the high-energy positrons emitted by this source.
The low-energy positron beam is then radially confined us-
ing a solenoidal magnetic field (100 G), and is transported
into a two-stage buffer-gas trap where a uniform and strong
magnetic field (530 G) is present. The trap electrodes form
a stepped electrostatic potential well structure, so that the
positrons that lose energy through inelastic collisions with a
mixture of N
2
and CF
4
buffer gases are trapped inside the
well. In this process, the positrons thermalize to room tem-
perature and form a cloud of particles that becomes the reser-
voir for the formation of a pulsed positron beam. The trap
is typically operated at a repetition rate of 60–100 Hz with
up to ∼1000 positrons emitted in each individual pulse. The
positrons released by the trap are directed into a scattering cell
where they interact with the molecules of the target of interest,
THF in this case. The scattering cell is made of gold-plated
copper and is 20 cm long, with entrance and exit apertures
that are 5 mm in diameter. The strong magnetic fields (530
G) present in the scattering cell region, and downstream from
it, ensure that all the positrons, except for those positrons that
form Ps and annihilate within the cell, are transmitted. Those
positrons subsequently pass through a retarding potential an-
alyzer (RPA), which is sensitive only to the parallel energy
component (E
) of the beam. The positrons transmitted by
the RPA are finally detected by a double-stack, micro-channel
plate assembly.
In our experimental configuration, the incident energy, at
which the positrons scatter from the target molecules, is set
by the potential of the scattering cell. The zero for the energy
scale is established with a retarding potential analysis of the
beam, i.e., with the energy scale defined relative to the “cut-
off” position of the beam. With this procedure the uncertainty
on the energy scale is estimated to be ±25 meV. The same
retarding potential analysis enables us to estimate the energy
distribution of the beam. Careful control over the beam for-
mation in the last stage of the trap cycle allows the energy
width of the beam to be comparable to the temperature of the
trapped positron cloud. For these measurements, the energy
resolution of the beam varied between 40 and 100 meV due
to variations in the beam formation characteristics.
Several precautions need to be taken in order to accu-
rately carry out the measurements. For instance, the target
pressure inside the cell is set to a value such that the number
of scattering events is no more than 10% of the unscattered
beam intensity, in order to minimize multiple scattering ef-
fects. In addition, the pressure measurements inside the scat-
tering cell need to be corrected for the thermal transpiration
effect, because the scattering cell temperature (∼24 ± 2
◦
C)
was different from the pressure gauge temperature (45
◦
C).
The thermal transpiration correction was calculated accord-
ing to the model of Takaishi and Sensui,
39
and was ∼3% in
the magnitude of the measured cross sections. The value for
the molecular diameter of THF that we used in this correction
was 4.63 Å.
28
Throughout the present measurements we used
a high-purity (>99.86%) THF sample. Although THF is a liq-
uid at room temperature, it is volatile enough (vapour pressure
∼176 hPa at 25
◦
C)
40
to easily provide the gas number density
in the cell to achieve the required beam attenuation. Note that
THF is rather hygroscopic,
22
but it comes as a mostly anhy-
drous sample from the supplier. We also performed numerous
freeze-pump-thaw cycles, in order to degas the target sample
and remove any impurities present in that sample that might
affect the results of our measurements.
The basic principle behind the TCS measurements, in
all linear transmission scattering experiments, is the Beer-
Lambert attenuation law. This method allows one to derive the
TCS from attenuation measurements of the beam intensity,
the target pressure in the cell, and the length of the interac-
tion region. The techniques used in the present experiment to
measure the TCS, the Ps formation cross sections, the elastic
DCS, and the details of the data analysis, have been presented
previously.
41–43
In short, the various cross sections are deter-
mined by measuring specific fractions of the positron beam
transmitted through the RPA with the target vapor present in
the scattering cell. In a collision with a target molecule, the
positron can be elastically scattered through some angle θ and
lose some of its E
in the process. It can also lose some of its
total energy if inelastic processes, such as electronic excita-
tions or direct ionization, are energetically allowed. As the
RPA discriminates against E
only, a retarding potential anal-
ysis provides a simple measurement of the total scattering. We
note here that, owing to our finite beam energy resolution, we
cannot resolve rotations and many of the vibrational modes
from the elastic scattering channel and, therefore, we actually
make measurements of the quasi-elastic DCS. Ps formation is
also possible above the Ps formation threshold (E
Ps
). As the
first adiabatic ionization potential (E
i
) of the most stable con-
former of THF is 9.57 eV,
44
and since E
Ps
= E
i
− 6.8 eV, we
obtain that E
Ps
= 2.77 eV for THF. Note that Ps formation is
a loss process, so it manifests as a loss of positron intensity in
the RPA transmission curve.
As with any experiment based on a linear transmission
technique, our method suffers from some angular discrimina-
tion limitations. They arise from the inability to distinguish
between positrons that are elastically scattered at small for-
ward angles from those in the primary, unscattered beam. The
fact that the number of “unscattered” events is overestimated
results in a measured TCS that is underestimated with re-
spect to its “true” value. At any given energy, the extent of
this effect depends on the angular discrimination of the mea-
surement and on the nature of the elastic DCS for the tar-
get species of interest in this forward angular region.
45
In
the present measurements, the angular discrimination is not
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