- Continuum-orientation
phenomena in ionization by positron impact J. Fiol, R. O. Barrachina Journal of Physics B: Atomic Molecular and Optical
Physics 44 5205 (ISSN:
0953-4075) (2011).
ABS
Ionization collisions
by positron impact at low and intermediate energies are
investigated by means of a classical trajectory Monte Carlo (CTMC)
simulation. Calculations of fully differential cross sections
unveil a remarkable dynamical orientation of the electron–positron
continuum dipole. Actually, by means of the CTMC simulation we show
that, for electron velocities close to the final positron velocity,
the dipole is narrowly oriented along the direction of motion of
its centre-of-mass, with the negative charge pointing towards the
residual target, i.e. emitted at lower energies than the positron.
The characteristic features of this phenomenon are studied. Also,
several future experiments and calculations are proposed. The
possible outcomes of this further research might help to unveil the
mechanisms responsible for the appearance of this orientation
effect.
- Classical
trajectory Monte Carlo calculation of the fully differential cross
section for ionization of H2 by positron
impact R. O.
Barrachina, J. Fiol
Journal of Physics Conference Series
199 12022 (ISSN: 1742-6588)
(2010).
ABS
The possibility of
performing kinematically complete measurements of ionization
collisions by positron impact brings about the promise of
encountering new unexpected phenomena, which otherwise would be
impossible to foresee. In this communication, we employ the
classical trajectory Monte-Carlo method to explore H2
ionization fully differential cross section (FDCS), depending on
the relative momentum of the electron- positron continuum dipole
and the deflection angle of its center of mass. We find a strong
structure in the FDCS at non-zero deflection angles. Finally, we
discuss some of these new effects, and the possibility of observing
them in actual experiments.
- Recoil-Ion Momentum
Distribution in Positron-Impact Ionization Collisions
R. O. Barrachina,
A. Delesque, R.
Tricard, F. O. Navarrete,
R. D. Picca, J. Fiol,
V. D. Rodríguez 63rd Gaseous Electronic Conference &7th International
Conference on Reactive Plasmas, Conference Proceedings
KWP111 (2010).
- Dynamics of
collisions with positrons J. Fiol, P. Macri, R. O.
Barrachina Nuclear Instruments
and Methods in Physics Research B 267 211-214 (ISSN: 0168-583X) (2009).
ABS
The measurement of
ionization by positron impact reveals that the maximum of the
electron capture to the continuum cusp is shifted from its
theoretical position. In this work the hypothesis that the observed
effect is the result of an anisotropic momentum distribution in the
projectile- electron reference system is considered. By elaborating
on the ansatz that the cusp asymmetry is qualitatively similar for
positron impact than the for for ion-atom collisions, we obtain
fully differential cross sections that show the same features than
those experimentally observed. The present estimation for the
position of the maximum agrees well with cross section measurements
performed in coincident electron-positron detection
experiments.
- FAST TRACK
COMMUNICATION: Dynamical orientation effects in the ionization of
atoms and molecules by positron impact J. Fiol, R. O. Barrachina Journal of Physics B: Atomic Molecular and Optical
Physics 42 23 231004 (ISSN: 0953-4075) (2009).
ABS
We present a novel
effect in positron impact ionization collisions at small and
intermediate energies, namely a strong dynamical orientation of the
electron-positron continuum dipole. By means of a classical
trajectory Monte Carlo simulation we show that the dipole is
narrowly oriented along the direction of motion of its center of
mass, with the negative charge pointing towards the residual
target. We show that the experimentally observed displacement of
the maximum of the electron- capture-to-the-continuum (ECC) cusp
from its theoretical position, an open issue for almost a decade,
is a by-product of this orientation phenomenon. We compare this
phenomenon, previously unforeseen by all quantum-mechanics
theories, with the apparently milder asymmetry of the ECC cusp in
ion impact collisions. Examples of future experimental and
theoretical opportunities are provided.
-
Quantum-trajectory calculations of proton-hydrogen model
collisions M.
A. Acuña, J. Fiol
Nuclear Instruments and Methods in Physics
Research B 267 2 288-291 (ISSN: 0168-583X) (2009).
ABS
We investigate a
proton-hydrogen model collision using a method based on the de
Broglie-Bohm formulation of quantum dynamics. By studying the
quantum-trajectories of the particles we obtain approximate
ionization and capture cross sections that are in good agreement
with the exact values. In particular, the implementation of this
high-order approximation method allows us to solve each trajectory
independently. The method has a relatively low computational cost
and can be straightforwardly parallelized for many bodies
systems.
- Threshold
effects in the ionization of atoms by positron impact
R. O. Barrachina,
J.
Fiol, P. Macri
Nuclear Instruments and Methods in Physics
Research B 266 402-406 (ISSN:
0168-583X) (2008).
ABS
We employ different
theoretical models, both classical and quantum-mechanical, to
explore the recoil-ion momentum distribution in positron atom
collisions. We pay special attention to the vicinity of the
kinematical threshold between ionization and positronium formation.
We demonstrate that it is intertwined by dynamical constraints to
the formation of highly excited and low-lying continuum electron
positron states. Finally we discuss how the study of recoil-ion
momentum distribution, which is characteristic of a reaction
microscopy technique, might represent an alternative approach to
the standard spectroscopy of electrons and positrons.
- Momentum-transfer
dependence of ionization cross sections for C6++He
collisions J. Fiol,
S. Otranto, R. E.
Olson Journal of Physics
Conference Series 58 161-164
(ISSN: 1742-6588) (2007).
ABS
In this work we study
the dependence of the fully differential cross sections (FDCS) for
single ionization of He by 100 MeV/amu C6+ on the
momentum transferred by the projectile to the target. Three
dimensional plots of the FDCS are used to illustrate how the
different configurations contribute when the theoretical results
are convoluted over the experimental uncertainties. This
convolution is shown to be essential in order to properly reproduce
the measured data.
- Fully differential
ionization cross sections in fast ion-atom collisions
J. Fiol, S. Otranto, R. E.
Olson Journal of Physics
Conference Series 88 12014
(ISSN: 1742-6588) (2007).
ABS
Fully differential
cross sections for helium ionization by fast ion collisions are
analyzed. The emitted-electron angular distributions for fixed
momentum transfer and electron energies are described theoretically
and compared to the experiment. The effect of uncertainties in the
determination of the momentum is considered within the theoretical
model. The cross sections are found to be extremely sensitive to
the inclusion of the uncertainties. Important quantitative and
qualitative modifications of the calculated cross sections are
obtained by including small uncertainties in the determination of
the momentum transferred by the projectile to the target.
- Critical
comparison between theory and experiment for C6+He fully
differential ionization cross sections J. Fiol, S. Otranto, R. E.
Olson Journal of Physics B:
Atomic Molecular and Optical Physics 39 L285-L290 (ISSN: 0953-4075) (2006).
ABS
Fully-differential
cross sections for single ionization of helium induced by impact of
100 MeVu-1 C6+ ions are examined within
a quantum-mechanical distorted wave model. The experimental
uncertainties are included in the theoretical calculations, leading
to a quantitative description of the experimental cross sections.
In particular, the ionization cross section for the electron
emitted in the plane perpendicular to the scattering plane is
reproduced for the first time by a full quantum-mechanical model.
The role of the internuclear interaction in this
perturbative-regime collision is discussed and found to be
unimportant for the present kinematical conditions.
- Angular
distributions and Dalitz plots for C6+ ionization of
He S.
Otranto, R. E. Olson,
J.
Fiol Journal of Physics B:
Atomic Molecular and Optical Physics 39 7 L175-L183 (ISSN:
0953-4075) (2006).
ABS
Single ionization
fully differential cross sections for 2 MeV/amu C6+ + He
collisions are obtained and analysed using the classical trajectory
Monte Carlo and continuum distorted wave models. The present
theoretical results are compared with the recent experimental data
of Fischer et al . The published experimental conditions are
considered in both theoretical models. The inclusion of the
momentum distribution of the target atom leads to an improved
description of the forward electron emission. Dalitz plots for
single ionization fully differential cross sections in ion-atom
collisions are presented and are used to help elucidate the
collision dynamics.
- Theory of ionization
processes in positron-atom collisions R. Della Picca, J. Fiol,
R. O. Barrachina, V. D. Rodríguez Nuclear Instruments and Methods in Physics Research
B 247 52-57 (ISSN: 0168-583X)
(2006).
ABS
We review past and
present theoretical developments in the description of ionization
processes in positron-atom collisions. Starting from an analysis
that incorporates all the interactions in the final state on an
equal footing and keeps an exact account of the few-body
kinematics, we perform a critical comparison of different
approximations, and how they affect the evaluation of the
ionization cross section. Finally, we describe the appearance of
fingerprints of capture to the continuum, saddle-point and other
kinematical mechanisms.
- Dynamics of
collisions revealed by classical methods R. E. Olson, J. Fiol,
J. Perez, P.
Beiersdorfer Nuclear
Instruments and Methods in Physics Research B 233 12-18 (ISSN: 0168-583X) (2005).
ABS
Single ionization of
helium by impact of 3.6 MeV/u Au53+ ions is investigated by means
of quantum and classical methods. Calculation of fully differential
cross sections are compared to recently published data for
ionization of low-energy electrons as a function of the momentum
transferred by the projectile to the target system. The results
show that inclusion of the resolution and uncertainties present in
the experiment has a major influence on both the shape and
magnitude of the calculated cross sections. The effect of using a
two-electron model for the He target along with including
electron-electron correlation are also investigated. However, after
incorporating all the experimental conditions within the
calculations, the one- and two-electron results present similar
behavior.
- Investigation
of saddle point electrons in positron-atom ionization
collisions R.
Della Picca, J. Fiol, R. O. Barrachina Nuclear Instruments and Methods in Physics Research
B 233 270-275 (ISSN: 0168-583X)
(2005).
ABS
We investigate the
appearance of fingerprints of saddle-point electrons in the
ionization of atoms and molecules by positron impact. We calculate
the quintuple differential cross section by means of a continuum
distorted wave approxi- mation that employs the correct kinematics.
We find a clear structure located at precisely the energy and
emission angle where a saddle-point electron is supposed to appear.
Unmistakable signatures of this structure can also been pinpointed
in the projectile dispersion and the recoiling of the target
ion.
- Extreme
Sensitivity of Differential Momentum Transfer Cross Sections to
Target Atom Initial Conditions R. E. Olson, J.
Fiol Physical Review
Letters 95 263203 (ISSN:
0031-9007) (2005).
ABS
Heavy-particle cross
sections differential in the momentum transferred to the target are
investigated using the classical trajectory Monte Carlo method.
With the 3.6 MeV/u Au53++He system as a test case, it is
shown that these cross sections are extremely sensitive to the
initial target temperature. In particular, when thermal motion is
varied for one of the target's initial momentum components between
0 and 25 K the absolute cross sections vary by orders of magnitude
and, in addition, their relative shapes undergo major changes. We
find that by setting one of the target's transverse momenta to a
temperature of 16 K, previously reported major discrepancies
between theory and experiment are removed.
- Three- and
four-body dynamics in fast heavy ion-atom ionization
J. Fiol, R. E. Olson Journal of
Physics B: Atomic Molecular and Optical Physics 37 3947-3960 (ISSN: 0953-4075) (2004).
ABS
Single ionization of
helium by impact of 3.6 MeV u-1 Au53+ ions is
investigated by means of quantum mechanical and classical methods.
Calculations of fully-differential cross sections are compared with
recently published data for ionization of low-energy electrons as a
function of the momentum transferred by the projectile to the
target system. A description of initial and final states of the He
atom in terms of a Hartree-Fock potential provides an improvement
over previous hydrogen-like models. The present results show that
inclusion of the resolution and uncertainties present in the
experiment has a major influence on both the shape and magnitude of
the cross sections. The effect of four-bodies and electron-electron
correlation are also investigated. However, after including the
experimental conditions into the calculations, the three- and
four-body results present similar behavior.
- Dynamics
underlying fully differential cross sections for fast
C6++He collisions R. E. Olson, J.
Fiol Journal of Physics B:
Atomic Molecular and Optical Physics 36 20 L365-L373 (ISSN:
0953-4075) (2003).
ABS
Single ionization of
helium by impact of
100 MeV u-1 C6+ ions is
investigated using the three-body classical trajectory Monte Carlo
method. Fully differential cross section calculations are compared
to recently published data for ionized electrons emitted both in
the scattering plane, and in the plane perpendicular to it that
contains the initial velocity. The present calculations show good
agreement with the experimental data, and improve upon
continuum-distorted-wave results. The collision mechanisms that
lead to the different structures observed in the fully differential
cross sections are investigated and include those of the binary and
recoil peaks for in-plane collisions as well as the maxima observed
in the out-of-plane scattering.
- Characteristic
low-energy electrons emitted in proton-atom collisions at
intermediate impact velocities J. Fiol, S. Suárez, D. Fregenal,
A. D. González, P.
D. Fainstein Physical Review
A 67 050702(R) (ISSN: 1050-2947)
(2003).
ABS
We report
experimental and theoretical electron spectra within the very
low-electron-energy range below 30 eV. Doubly differential cross
sections for ionization of different noble gases (He, Ne, Ar, Kr,
Xe) by impact of 100-keV and 225-keV protons are reported. A
pronounced structure in the low-energy region of emittedelectron
spectra is observed with different intensities for the different
targets. We compare our experimental findings with two theoretical
models: the first-order Born approximation and the
“continuum-distorted-wave– eikonal-initial-state” model using
different optimized central potentials to calculate the bound and
continuum states of the target. We attribute our experimental
observation to a signature of the predicted low-energy binary
peak.
- Correlation in
fast collisions of highly charged ions with atoms
J. Fiol, R. E. Olson Nuclear
Instruments and Methods in Physics Research B 205 474-478 (ISSN: 0168-583X) (2003).
ABS
Atomic hydrogen and
helium ionization by impact of 3.6 MeV/u Au53+ is
investigated within Classical Trajectory Monte Carlo and
quantum-mechanical CDW-B1 and CDW-EIS models. For helium
ionization, a description of initial and final electron states in
terms of Hartree-Fock potentials is compared with hydrogenic
models. Different ionization mechanisms are characterized. A strong
correlation of the electron, projectile and target nucleus is
observed. Good agreement is obtained between calculations performed
with the different models.
- Dynamical
electron-electron correlation in C2++He simultaneous
target-projectile collisional ionization J. Fiol, R. E. Olson, R.
Moshammer, J. Ullrich
Journal of Physics B: Atomic Molecular and
Optical Physics 36 L99-L105
(ISSN: 0953-4075) (2003).
ABS
Simultaneous target and
projectile single ionization in 3.6 MeV u-1
C2+ + He collisions is considered, within a six-body
classical trajectory Monte Carlo model. Analysis of the relative
azimuthal angle between the two emitted electrons allows one to
discriminate the ionization contribution produced by the two-centre
dynamical electron- electron interaction from that due to
nucleus-electron interactions. The present calculations agree well
with cross section measurements recently performed in kinematically
complete experiments.
- Nuclear-electron
versus nuclear-nuclear interactions in ionization of atoms by
highly charged ions impact J. Fiol, R. E. Olson 680
Application of Accelerators in Research and
Industry American Institute of
Physics 138-141 (ISSN: 0094-243X) (2003).
ABS
The correlated behavior of
the electron and recoil-nucleus ion is studied for atomic
ionization by collisions with fast, highly charged ions.
Calculations were performed using classical-trajectory Monte Carlo
and quantum-mechanical Continuum Distorted Wave methods that
incorporate all interactions including the internuclear potential.
Electron double and triple differential ionization cross sections
are presented as a function of projectile momentum transfer for
hydrogen ionization by 3.6 MeV/u Au53+ impact. It is
possible to identify collision mechanisms that give rise to
binary-like peak electrons, and a new class of electrons recently
predicted for highly charged ions impact, which were termed “swing
by electrons”. The relative importance of each mechanism is
determined by the range of the nuclear-nuclear versus that of the
nuclear- electron interactions. An experimentally accessible method
for the separation of these different mechanisms based on the study
of the azimuthal angle of the particles is proposed.
- Three-body
dynamics in the ionization of hydrogen by positron
impact J. Fiol,
R. E. Olson Journal of Physics B: Atomic Molecular and Optical
Physics 35 5 1173-1184 (ISSN: 0953-4075) (2002).
ABS
Three-body dynamics are
investigated for single ionization of hydrogen by 50
and 100 eV positron impact. The momentum distributions of
the electron, the projectile and the recoiling target nucleus are
investigated. The present results show that, despite the large mass
of the target nucleus, it actively participates in the dynamics of
the ionizing collisions. In particular, a strong correlation
between the positron and the recoil-ion momenta is found, which is
especially important at the lower collision energy of this
study.
- Dynamics of
secondary ion emission: Novel energy and angular
spectrometry T. Jalowy, R.
Neugebauer, M. Hattass,
J.
Fiol, F. Afaneh,
J. A. M. Pereira, V. Collado, E. F. da
Silveira, H. Schmidt-Böcking,
K. O. Groeneveld Nuclear Instruments and Methods in Physics Research
B 193 762-767 (ISSN: 0168-583X)
(2002).
ABS
A new spectrometer
has been developed based on the combination of standard
time-of-flight technique and position sensitive delay line
detectors. The basic features of the spectrometer, particularly of
the multi-hit capable detector, are described. To demonstrate the
performance of this new system, the dynamic emission
characteristics, i.e. the three- dimensional velocity distribution,
of desorbed H\char254 from Al target by Ar0 impact (570 keV) is
presented. It is found that the desorption yield is maximum for
radial and axial emission velocities at 1.2 and 12 km/s
respectively, corresponding to 1.5 eV ions emitted at
57\circ to normal (following the projectile radial
direction). The initial energy distribution spreads out over 16
eV.
- Cusp formation
in the single-particle momentum distributions of three-body
continuum states J. Fiol,
R. O. Barrachina, V. D. Rodríguez Journal of Physics B: Atomic Molecular and Optical
Physics 35 1 149-164 (ISSN: 0953-4075) (2002).
ABS
We investigate the
appearance of cusp structures in single- particle momentum
distributions for processes leading to three-body continuum states.
We discuss under which circumstances these structures are produced,
and identify some of their properties by means of a final- state
interaction theory. Finally, we show how they change with the
relative mass ratios of the three particles in the final
state.
- Three-body dynamics in
hydrogen ionization by fast highly charged particles
J. Fiol, R. E. Olson Journal of
Physics B: Atomic Molecular and Optical Physics 35 7 1759-1773 (ISSN:
0953-4075) (2002).
ABS
Electron double and
triple differential cross sections are calculated as a function of
projectile momentum transfer for ionization of ground and excited
state hydrogen by 3.6 MeV u -1 C.
6+ and Au 53+ ions. These three-body Coulomb
systems are investigated using the classical-trajectory Monte Carlo
and continuum distorted wave methods that incorporate all
interactions including the nuclear-nuclear potential. The
calculations allow one to distinguish cross section features
associated with three collision mechanisms. The first is due to
distance collisions that lead primarily to a binary interaction
between the projectile and electron. The second is a strong
three-body interaction by the projectile with both the electron and
the target ion when the projectile passes between the electron and
its parent nucleus. These two mechanisms result in binary peak
electrons located near the momentum transfer vector Q. with active
participation by the recoil ion along - Q. in order to determine
the overall momentum transfer magnitude. The third mechanism, which
is present only for fast highly charged ion impact, yields electron
spectra that here-to-fore have not been observed. Rather than the
electrons being scattered near the angle θ associated with the
momentum transfer vector Q. , they are found at an angle of
360\circ-\theta. Such electrons are due to a close
collision of the projectile with the target nucleus with the
electron being forced to swing by its parent. These electrons are
not so-called recoil electrons associated with the angle
180\circ+ \theta. Moreover, in this case neither the
recoil ion nor the electron spectra peak near Q. . Calculations are
also presented for collisions with excited hydrogen in order to
assess the collision dynamics as a function of the radial
dimensions of the target atom.
- Zero-energy resonances
efects in break-up collisions J. Fiol, C. Courbin, V. D.
Rodríguez, R. O.
Barrachina 13
Resonances in Few-Body Systems
Springer-Verlag Few-Body Systems 46-55 (2002).
ABS
We show that the analysis of
cusp and threshold structures in breakup collisions of nuclei,
atoms and molecules represents an alternative approach for the
study of elastic two-particles collisions at extremely
low-energies. We illustrate these general ideas by showing that the
“electron capture to the continuum” cusp in the ionization of atoms
by neutral projectiles can be understood in terms of a resonance
mechanism. This approach allows for the extraction of the s-wave
scattering length for the electron-projectile interaction.
- Mapping of the
Bethe surface in single-ionization ion-atom collisions
R. E. Olson,
J.
Fiol Journal of Physics B:
Atomic Molecular and Optical Physics 34 19 L625-L631 (ISSN:
0953-4075) (2001).
ABS
The three-body Coulomb
problem is explored using classical- trajectory Monte Carlo
and continuum distorted-wave methods that include all interactions,
including the nuclear-nuclear term. Calculations for low-energy
electron emission as a function of projectile deflection, the Bethe
surface, are compared with experiment for single ionization of He
by 100 MeV u -1 C6+ and
3.6 MeV u -1 Au 53+ . Good
agreement is realized for the C6+ system. For Au
53+ poor agreement with experiment is obtained,
indicating that the proposed lack of the nuclear-nuclear
interaction is not the source of the discrepancy. The calculations
further show that significant cross sections at large projectile
deflections are only realized for double ionization, not single
ionization.
- Electron
capture to the continuum by proton and positron impact
J. Fiol, V. D. Rodríguez, R. O.
Barrachina Journal of Physics
B: Atomic Molecular and Optical Physics 34 5 933-944 (ISSN:
0953-4075) (2001).
ABS
We describe recent
experimental triple-differential cross sections of ionization by
the impact of both protons and positrons by means of a single,
kinematically exact theory. This model incorporates all the
interactions in the final state on an equal footing and keeps an
exact account of the three-body kinematics. We show that these
provisions make it possible to evaluate any multiple-differential
cross section for any given mass configuration, and analyse how it
changes with the relative masses of the three particles in the
final state. We analyse the differences in the electron cusp
formation by both heavy and light projectile impact at the
double-differential cross section level.
- Simultaneous
Projectile and Target Ionization in He+ + Ne
Collisions J. Fiol,
R. E. Olson, A. C.
F. Santos, G. M. Sigaud,
E. C. Montenegro Journal of Physics B: Atomic Molecular and Optical
Physics 34 16 L503-L509 (ISSN: 0953-4075) (2001).
ABS
Cross sections for
single and multiple ionization of the target atom with simultaneous
ionization of the projectile have been measured for 1-4 MeV
collisions of He + with Ne. The observations are
interpreted using the n -body classical trajectory Monte Carlo
method. The 11-body calculations include the L-shell electrons of
the Ne and the K-shell electron on the He + . In this
model, each electron interacts with both nuclei and with all the
electrons belonging to the other centre. For single ionization from
both centres, it is found that the dynamical
electron-electron (e-e) interaction produces a clear signature
in the relative azimuthal angle distribution between the ionized
electrons. For low values of momentum transfer q , this interaction
is reflected in a pronounced enhancement near 180\circ
in the azimuthal angle between electrons. Integral values of the
azimuthal angle events, summed over all q values, indicate that it
is possible to determine the relative importance of the e-e versus
the nuclear-electron (N-e) interactions.
- Recent Experiments on
the Roles of Projectile Electrons in Ion-Atom Collisions
D. Fregenal,
J.
Fiol, S. Suárez,
G. Bernardi, P.
Focke, A. D. González
576 Application
of Accelerators in Research and Industry American Institute of Physics 164-167 (ISSN:
0094-243X) (2001).
ABS
By measuring
recoil-ion-electron, projectile-electron, and triple coincidences,
multiple ionization differential cross sections and probabilities
for impact of H, He, H+ He+, and He++ colliding with He,
Ne, Ar, and Kr have been obtained. Here we concentrate on
differential measurements of multiple ionization of heavy targets
produced by impact of neutral and charged H and He projectiles. In
the case of H+ impact, previous experiments have shown a
large enhancement of electron emission associated with the ratio of
Ar2+/Ar+, with a maximum at an electron
velocity close to the projectile velocity for electron emission
angles around 90 degrees. Both double ionization (DI) and transfer
ionization (TI) are responsible for the enhancement, which was also
observed for Ne and Kr targets. New results demonstrate that, for
100 keV H++Ar collisions, the contribution to the
enhancement due to DI is larger than TI. Neutral beams produce no
effect of this sort. A simultaneous projectile and target
ionization contribution is discussed.
- Double capture
with simultaneous ionization in He2+ on Ar
collisions D.
Fregenal, J. Fiol, G. Bernardi, S. Suárez,
P. Focke, A. D.
González, A. Muthig,
T. Jalowy, K. O.
Groeneveld, H. Luna
Physical Review A 62 12703 (ISSN: 1050-2947) (2000).
ABS
We have
investigated double electron capture with simultaneous target
ionization in collisions of 25-keV/amu 3He2+
projectiles on argon. Doubly differential cross sections for
electrons emitted in angles 0\mbox\circ,
20\mbox\circ, 45\mbox\circ,
90\mbox\circ, 128\mbox\circ,
175\mbox\circ, in coincidence with the neutralized
projectiles were obtained. To establish the relative importance of
the double capture channel in the electron emission at the present
intermediate impact velocity, differential cross sections for total
electron emission were also measured. A narrow symmetric cusp-
shaped structure for electrons with velocity close to the ion
velocity, associated with the neutral emerging projectile, is
observed. Binary encounter electrons, masked by other processes in
total emission, are clearly observed for the double capture plus
ionization channel.
- Cusp formation and
threshold effects in break-up collisions R. O. Barrachina,
J.
Fiol, V. D. Rodríguez,
P. Macri 500 1 THE PHYSICS OF ELECTRONIC AND ATOMIC COLLISIONS
AIP 510-519 (2000).
ABS
We show how the
single-particle double differential cross sections (DDCS) in
breakup collisions are intertwined by dynamical constraints. In
particular, we study the corresponding relations among threshold
and cusp structures and identify some of their properties by means
of a final- state interaction theory. We provide general
expressions for the cusp and threshold structures that any
theoretical description of the collision process has to fulfil.
Finally, we show how these structures change with the relative mass
ratios of the three particles in the final state. .
- A classical
interpretation of the off-shell Coulomb transition
matrix J. Fiol,
R. O. Barrachina Journal of Physics B: Atomic Molecular and Optical
Physics 33 15 2847-2857 (ISSN: 0953-4075) (2000).
ABS
We analyse the
transition matrix for the two-body Coulomb scattering problem out
of the energy shell. This quantity appears in high- order
perturbation treatments of multichannel processes and plays a
central role in the generalization of the scattering theory to
include long-range interactions. In particular, its branch-point
singularities on the initial and final half of the energy shell are
known to produce sizeable effects in multiple-scattering amplitudes
for rearrangement and ionization collisions. In this paper we
present a classical description of the off-shell collision process
which helps to clarify its essential concepts.
- Minimum in the
distribution of electrons emitted from solids in the backward
direction C.
D. Denton, T. Jalowy,
H. Luna, J. Fiol, D. Fregenal, S. Suárez,
G. Bernardi, A.
González, N. R. Arista,
K. O. Groeneveld Nuclear Instruments and Methods in Physics Research
B 164 927-932 (ISSN: 0168-583X)
(2000).
ABS
We have investigated
the electron emission from carbon and aluminum foils in the
backward direction induced by proton impact with energies in the
range 50-250 keV. Doubly differential angular emission spectra were
obtained for several electron energies covering the angular range
extended from 160 degrees to 200 degrees, A minimum in the electron
angular distribution centred at 180 degrees was observed for slow
electrons and it disappears with increasing electron energies. We
have interpreted these results as a combination of three effects:
the excitation of single conduction electrons in the target, the
transmission of the excited electrons across the surface barrier
and the effect of the image force acting on the emitted electrons
when they are in the vacuum.
- Classical
description of threshold effects in ion-atom ionization
collisions J. Fiol,
C. Courbin, V. D.
Rodríguez, R. O.
Barrachina Journal of Physics
B: Atomic Molecular and Optical Physics 33 23 5343-5355 (ISSN:
0953-4075) (2000).
ABS
We employ a three-body
classical trajectory Monte Carlo (CTMC) method to calculate
the recoil-ion momentum distribution at its kinematic threshold in
ion-atom ionization collisions. We analyse how this threshold is
intertwined by dynamical constraints to the electron capture to the
continuum cusp in the electron double differential cross section.
We compare these calculations with those from a full
quantum-mechanical description and explore how these structures
depend on the interactions among the three particles in the final
state.
- Emisión electrónica en el
continuo de un átomo neutro utilizando proyectiles de H0
and H+ on He and Ar M. Galassi, J. Revelli, J. Fiol, G. Bernardi Anales de
la Asociación Física Argentina 58 (1998).
- An alternative
classical approach to the quantum-mechanical definition of the
scattering cross section J. Fiol, R. G. Pregliasco, I.
Samengo, R. O. Barrachina
American Journal of Physics
65 433-436 (ISSN: 0002-9505)
(1997).
ABS
We discuss an
alternative classical description of the nonrelativistic scattering
process that is completely analogous to the time-independent
formalism employed in quantum-mechanics textbooks. This approach
provides an elementary introduction to the standard stationary
scattering theory, embodying many of its essential concepts.
- Una perspectiva clásica de
la función de onda coulombiana del continuo I. Samengo, R. G. Pregliasco, J. Fiol, R. O. Barrachina Anales de la Asociación Física Argentina 422
(1997).
ABS
In this work we
develop a method to calculate the classical density of particles
scattered by a central force. using this technique we find an
analytical expression for the classical density in the scattering
from a Coulomb potential. We compare it with the quantum-mechanical
result. We conclude that both distributions have the same global
behavior. We show that, not only in the asymptotic region but in
every point of the space, the classical distributions can be
separated in incoming and outgoing components. Furthermore, the
densities for the attractive and repulsive case can be mapped into
each other. These results lead to an interpretation of the
different features of the quantum-mechanical densities in terms of
classical concepts.
- Regularización del problema
de dispersión en presencia de potenciales de largo alcance
J. Fiol, R. O. Barrachina Anales de la Asociación Física Argentina 428
(1997).
ABS
Almost every process in
atomic collisions involves coulomb interactions either in the
asymptotic evolution of the system or in intermediate states in
charge exchange processes. However, this type of long range
interactions does not fit into the usual formal scattering theory,
which requires a free asymptotic evolution of the colliding
fragments. Many different methods have been proposed in order to
circumvent these difficulties and include long range interactions.
One of them consists in evaluating the on-energy-shell limit in a
regularized version of the off-energy-shell transition matrix.
Using a classical description, we show the close relation between
this method and another one suggested by Dettmann in 1971 which
consists in postponing the infinite time limit in a time-dependent
wave-packet formalism.
