ELECTRON DRIVEN PROCESSES; SCIENTIFIC CHALLENGES
AND TECHNICAL OPPORTUNITIES
Nigel John Mason
Centre for Molecular and Optical Sciences, Department of Physics and Astronomy, The Open
University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
Abstract: Electron induced processes are prevalent in many disparate areas of science and technology. In this
paper we review some of the recent developments in our understanding of electron driven processes with particular
attention to electron interactions with biomolecules.
indeed recently it has been demonstrated
that discrete electron reactions may be
performed at the individual molecular
level using STM based technology thus
introducing the prospect of designer
synthesis on the nanoscale (Hla et al
(2000)).
INTRODUCTION
Electron induced reactions in both
gaseous and condensed phases initiate and
drive
the
basic
physical-chemical
processes in many different areas of
science and technology from industrial
plasmas to living tissues. For example
radiation damage in the DNA of living
systems has now been shown to arise
primarily from collisions of very low
energy
(sub-ionization)
secondary
electrons through dissociative attachment
to the components of DNA molecules or to
the water around them (Boudaïffa et al
(2000)). A collaborative project probing
electron induced fragmentation of ozone
has revealed the possibility of a direct
coupling mechanism between the lower
ionosphere and the Earth’s ozone layer
and thus predicted that alterations in
global ozone concentrations may directly
affect terrestrial radio transmissions,
providing a new methodology for probing
upper atmosphere ozone loss and
mesospheric chemistry (Senn et al (1999)).
In the technological field electron induced
reactions underpin most of the multibillion
dollar modern semiconductor industry
since it is those reactive fragments
produced by electron impact of etchant
gases that react directly with the silicon
substrate (Tanaka et al (2000)). Electron
induced
processes
are
also
of
extraordinary
importance
for
determination of structure and chemical
reactivity of species adsorbed on surfaces
The energy region below 3 eV is of
particular interest since here the cross
section for electron induced processes are
often dominated by the formation of
temporary negative ions (anions) These
quasi bound states are provided by the
short range polarization interaction
induced when an electron approaches a
target molecule the subsequent decay of
which may lead leave the target molecule
excited or even lead to dissociation. In the
case of dissociative electron attachment
(DEA) in many systems a 100 %
selectivity with respect to the cleavage of a
particular bond can be obtained ! This
opens interesting prospects for a selective
chemistry induced by electrons. It should
also be noted that electron induced
processes are often directly linked to
photon induced processes since photons of
sufficient energy excite or liberate
electrons from targets which may then
drive the local chemistry for example such
processes lead to the formation of
molecules in the interstellar medium,
molecules that may be the precursors of
life.
However at present only the rudiments
of such electron induced reactions are
understood such that the paucity of our
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
885
knowledge of such processes is limiting
further development in many of other
areas of science and technology. We
therefore
require
an
in
depth
understanding of the basic dynamics of
electron induced reactions in both the
gaseous and condensed phases of matter.
collisional ‘reactants’ has grown rapidly in
the last decade. Therefore there remains a
crucial need to provide extensive data on
electron-atom, electron-molecule and
electron-ion collision cross sections, yet
after eighty years of experimental study
we have ‘complete’ information on only a
few target systems (the rare gases) while
several targets (e.g. radioactive atoms and
free radical molecules) have yet to be
studied. Even if it were feasible to collect
all the data presently required by the
applied science community, it would
require the attention of the whole of the
world’s atomic and molecular research
teams for over fifty years! Hence in the
past twenty years great emphasis has been
placed upon the development of reliable
theoretical techniques that, once validated
experimentally, will be able to produce the
data required to an acceptable level of
accuracy. Great advances have been made
by the theoretical community e.g. the
development of the R-Matrix method by
Burke and co-workers and the adoption of
parallel
computational
techniques,
particularly in the understanding of
electron-atom scattering (see papers by
Whelan and co-workers in this volume)
but theory alone is not yet capable of
producing the volume and accuracy
required for all the many systems of
interest. Therefore experimentalists are
continuously developing techniques to
provide stringent tests of the developing
theoretical treatments of electron collision
processes, for example:
• The development of novel methods of
laser and collision spectroscopy to
prepare molecules in specific target
states provides the opportunity to study
state-to-state collision processes.
• The utilization of chemical synthesis
techniques to produce beams of
transient (radical) molecular targets has
opened the possibility of probing
electron/positron
energy
transfer
processes in highly reactive systems
pertinent to both industry and
biochemistry.
• The prospect of trapping molecules in
optical traps will provide an
opportunity to study well isolated
ELECTRON SCATTERING FROM
ATOMS AND MOLECULES
The interaction of electrons with atoms
and molecules has been studied
experimentally for almost a century, since
the pioneering work of Franck and Hertz
(1914) and Ramsauer (1921). Franck and
Hertz provided a validation of the Bohr
theory on the discrete nature of atomic
energy levels, while Ramsauer and Kollath
provided the first evidence of the
polarisation of the target charge cloud by
an incident charged particle, hence
demonstrating the need for a quantum
mechanical representation of the scattering
of discrete particles. The first quantum
mechanical calculations of Massey and
Mohr (1931) established the now
traditional interactive development of
theoretical and experimental techniques in
this research field. A good example of
such experimental/theoretical interactive
development was the experimental
discovery of short lived negative ion
‘resonances’ by Schulz (1963) that led to
the development of time dependent
calculations incorporating nuclear motion.
The intricacy of theoretical calculations
also led to the development of coincidence
experiments to test the theoretical
treatments of electron-atom excitation
processes.
The field of electron-atom/molecule
scattering is therefore well established and
as is shown in many of the papers in this
volume, we have a good understanding of
the major mechanisms and dynamics
involved in electron collision processes.
However, the demand for accurate
scattering cross sections and knowledge of
the final quantum mechanical states of the
886
shown in the upper portion of figure 1.
The majority of the copious secondary
electrons ( ~ 5 x 104 per MeV) created
within 10-15s along the radiation track have
energies below 20eV. These low-energy
electrons must undergo multiple inelastic
scattering events as they thermalise. The
primary energy-loss channels for electrons
with energies typical of the secondary
distribution
are
ionization,
direct
electronic excitation and most important
(but until recently little studied) resonance
scattering. The latter results in the
formation of Temporary Negative Ions
(TNIs), which decay via electron
autodetachment and dissociative electron
attachment (DEA) the latter process
leading to direct dissociation of the parent
molecule.
This process may be
summarised in the two step ‘reaction’;
cooled (no internal energy) molecules
(possibly biological) under controlled
experimental conditions.
• The ability to prepare and analyse the
effect of charged particle irradiation of
bio-molecules such as DNA.
• The development of new instrumental
techniques e.g. highly efficient
position-sensitive detectors, anglechanging electron spectrometers and
the use of. STM technology.
In the remainder of this article I will
give just one example of how electron
interactions
with
biomolecules
is
providing new information on radiation
damage in living tissues.
3. Electron
biomolecules
interactions
with
e + M-ABC ---> (M-ABC)(M-ABC)- ---> M- ABC
Many of the mutagenic or lethal effects
of ionizing radiation can be traced to
structural and chemical modification of
cellular DNA. The mechanisms by which
such degradation occurs have been the
subject of considerable research effort
with genotoxic effects of ionizing
radiation in living cells being commonly
attributed to direct impact of high-energy
quanta or by complex radical chemistry
(triggered by production of OH species by
primary ionizing radiation).
However
recently this explanation has recently been
questioned by the pioneering work of
Sanche and co-workers who suggest that
DNA lessions are induced by the lower
energy, secondary electrons generated by
the primary ionizing radiation. Sanche and
co-workers revealed that
• low energy electron irradiation
directly induces both single and
double strand breaks at energies
well below the ionization limit of
DNA (7.5eV) and
• that the probability of strand
breaks are one to two orders of
magnitude larger for electrons
than for corresponding energy
photons.
(1)
(2)
where (M-ABC)- is the temporary
negative ion which decays to a residual
anion (M-) and a (reactive) molecular
fragment ABC.
Figure 1. Top Frame: Energy distribution of
secondary electrons emitted during a primary
ionizing event. Lower Frame: Estimate of the
effective “damage” probability.
The typical energy distribution of
secondary electrons created in ionizing
radiation impact of mammalian tissue is
887
Hence the process of DEA provides a
direct low energy process for the
degradation of DNA (and other key
cellular material) by resonant electron
attachment to basic molecular components
(base,
deoxyribose,
phosphate,
or
hydration H2O) suggesting that single
strand damage is site specific and proceeds
through discrete molecular bond rupture.
This in turn suggest that double strand
breakage is simply induced through local
chemical reactivity. The DEA fragments
produced within the cellular DNA
subsequently reacting with adjacent bases
(at 3.4Å), or the phosphate-sugar
backbone ( at < 5Å) leading to clustered
damage within the DNA strand. Further
studies on electron interactions with the
molecular components of DNA and DNA
itself are on going as part of a European
Network programme involving over a
dozen research groups.
dynamics. The combination of traditional
gas phase electron spectroscopy with
condensed matter techniques is allowing
collective effects to be probed, while
electron scattering studies from clusters
provide a useful intermediate between free
atom/molecules in the gas phase and
collective effects in the condensed phases.
Experimental electron scattering is
therefore undergoing something of a
renaissance, and as we approach the
centenary of the first electron scattering
experiments we may at last, be able, to
understand and exploit the intricacies of
electron scattering interactions from atoms
and molecules..
REFERENCES
B. Boudaïffa, P Cloutier, D Hunting, M A
Huelks and L Sanche Science 287, 1658
(2000)
CONCLUSIONS
Franck and G Hertz 1914 D Phys Ges
Verhandlungen 16 512-6
Electron-atom/molecule scattering has
been the subject of intense research
activity,
both
experimentally
and
theoretically, for nearly a century, yet
particularly
for
electron-molecule
scattering our understanding of the
dynamics of such processes remains
limited. Due to their importance in a
myriad of industrial, astrophysical,
atmospheric and biological processes
absolute electron scattering cross sections
are required for most atomic and
molecular systems yet the current database
is limited to a few easily prepared systems.
Only recently have experimental studies
been extended to unstable reactive and
free
radical
species
while
new
experimental techniques incorporating
advances in atom trapping and laser
spectroscopy are being adapted to provide
new insight into electron collision
S. W. Hla, L. Bartels, G. Meyer, and K.-H.
Rieder, Phys. Rev. Lett. 85 (2000) 2777
H S W Massey and C B O Mohr 1931 Proc
Roy Soc A132 605-11
C Ramsauer 1921 Annln Phys 66 546-9
G J Schulz 1963 Phys Rev Lett 10 104
G Senn, JD Skalny, A Stamatovic, NJ Mason,P
Scheier,TD Mark (1999) Phys. Rev. Lett. 82
5028
H Tanaka and M Inokuti Fundamentals in
Plasma Chemistry ed M Inokuti Academic
Press San Diego (1999).
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