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Refereed Publications
- Quantum-Trajectory calculations of
proton-hydrogen model collisions Nucl. Instrum. Methods
B En prensa2008
Local CopyWe 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 \emphtrajectory independently. The method has a relatively low computational cost and can be straightforwardly parallelized for many bodies systems. - Dynamics of collisions with
positrons Nucl. Instrum. Methods
B En prensa2008
Local CopyThe 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. - Threshold effects in the ionization of
atoms by positron impact Nuclear
Instruments and Methods in Physics Research B 266 402-4062008
Local Copy External Link DOIWe 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 Journal of Physics: Conference Series 58 161-1642007
Local Copy External LinkIn 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 Journal of Physics: Conference Series 88 012014 (8pp)2007
Local Copy External LinkFully 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. Phys. B: At. Mol.
Phys. 39 L285-L2902006
Local Copy External LinkFully-differential cross sections for single ionization of helium induced by impact of 100 \mathrmMeV\mathrmu-1 \mathrmC6+ 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 J. Phys.
B: At. Mol. Phys. 39
7 L175-L1832006
Local Copy External LinkSingle 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
Nucl. Instrum. Methods B 247 52-572006
Local Copy External LinkWe 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. - Investigation of saddle point electrons in
positron-atom ionization collisions Nuclear Instruments and Methods in Physics Research
B 233 270-2752005
Local Copy External LinkWe 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
Phys. Rev. Lett. 95 26
2632032005
Local Copy External LinkHeavy-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. - Dynamics of collisions revealed by
classical methods Nuclear Instruments and Methods in Physics Research
B 233 12-182005
Local Copy External LinkSingle 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. - Three-body dynamics in fast heavy ion-atom
ionization collisions J. Phys. B: At. Mol.
Phys. 37 3947-39602004
Local Copy External LinkSingle 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. - Dynamical electron-electron correlation in
C2++He simultaneous target-projectile collisional
ionization J. Phys. B: At.
Mol. Phys. 36
L99-L1052003
Local Copy External LinkSimultaneous 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. - Dynamics underlying fully differential
cross sections for fast C6++He collisions
J. Phys. B: At. Mol. Phys. 36 20
L365-L3732003
Local Copy External LinkSingle 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. - Correlation in fast collisions of highly
charged ions with atoms Nucl. Instrum. Methods B
205 474-4782003
Local Copy External LinkAtomic 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. - Characteristic low-energy electrons emitted
in proton-atom collisions at intermediate impact velocities
Phys. Rev.
A 67 050702(R)2003
Local Copy External LinkWe 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. - Dynamics of secondary ion emission: Novel
energy and angular spectrometry Nucl. Instrum. Methods
B 193 762-7672002
Local CopyA 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 Hþ 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. - Three-body dynamics in hydrogen ionization
by fast highly charged particles J. Phys. B: At. Mol.
Phys. 35 7 1759-17732002
Local Copy External LinkElectron 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. - Cusp formation in the single-particle
momentum distributions of three-body continuum states
J. Phys. B: At. Mol.
Phys. 35 1 149-1642002
Local Copy External LinkWe 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. - Nuclear-electron versus nuclear-nuclear
interactions in ionization of atoms by highly charged ions
impact
Application of Accelerators in Research and
Industry Int. Conf. 680
138-1422002
Local Copy - Three-body dynamics in the ionization of
hydrogen by positron impact J. Phys. B: At. Mol.
Phys. 35 5 1173-11842002
Local Copy External LinkThree-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. - Mapping of the Bethe surface in
single-ionization ion-atom collisions J.
Phys. B: At. Mol. Phys. 34
19 L625-L6312001
Local Copy External LinkThe 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. - Simultaneous Projectile and Target
Ionization in He+ + Ne Collisions J. Phys. B: At. Mol.
Phys. 34 16 L503-L5092001
Local Copy External LinkCross 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. - Electron capture to the continuum by proton
and positron impact J. Phys.
B: At. Mol. Phys. 34
5 933-9442001
Local Copy External LinkWe 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. - Zero-energy resonances efects in break-up
collisions Few-Body
Systems 13 46-552001
Local CopyWe 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. - Recent Experiments on the Roles of
Projectile Electrons in Ion-Atom Collisions Application of
Accelerators in Research and Industry - Sixteenth Int. Conf.
576 164-1672001
Local Copy - Minimum in the distribution of electrons
emitted from solids in the backward direction Nucl. Instrum. Methods
B 164 927-9322000
Local CopyWe 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. Phys. B: At. Mol. Phys.
33 23
5343-53552000
Local Copy External LinkWe 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. - Cusp formation and threshold effects in
break-up collisions The
Physics of Electronic and Atomic Collisions
510-5192000
Local CopyWe show how the singe-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 fulfill. Finally, we show how these structures change with the relative mass ratios of the tree particles in the final state. - A classical interpretation of the off-shell
Coulomb transition matrix J. Phys. B: At. Mol.
Phys. 33 15 2847-28572000
Local Copy External LinkWe 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. - Double capture with simultaneous ionization
in ion-atom collisions Phys. Rev. A
62 127032000
Local Copy External LinkWe have investigated double electron capture with simultaneous target ionization in collisions of 25-keV/amu 3He(2+) projectiles on argon. Doubly differential cross sections for electrons emitted in angles 0 degrees, 20 degrees, 45 degrees, 90 degrees, 128 degrees, 175 degrees, 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. - Emisión electrónica en el continuo de un
átomo neutro utilizando proyectiles de H0 and
H+ on He and Ar Anales de la Asociación Física Argentina
581998
Local Copy - An Alternative Classical Approach to the
quantum-mechanical definition of the Scattering Cross
Section Am. J.
Phys. 65 433-4361997
Local CopyWe 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. - Regularización del problema de dispersión
en presencia de potenciales de largo alcance Anales de la Asociación Física Argentina
4281997
Local CopyAlmost 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. - Una perspectiva clásica de la función de
onda coulombiana del continuo
Anales de la Asociación Física
Argentina 4221997
Local CopyIn 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.
Non-refereed Publications
- Phd Thesis. Estudio teórico-experimental de
estructuras en las secciones eficaces de ionización por
colisión Instituto Balseiro, Universidad Nacional de Cuyo
1-1382000
Local CopyIn this work we investigate on the structures observed in the cross-sections of ionization by using theoretical and experimental tools. By keeping an exact account of the three-body kinematics we obtain general relations among the single-particle momentum-distributions. In particular we study the corresponding relations between cusp and threshold structures and how they change with the relative mass ratios of the three particles in the final state. Moreover, we analyse the effect of the internuclear interaction on the different cross-sections. By considering all the interactions with the same order of approximation in the final state and a full treatment of the kinematics we generalize previous quantum-mechanical models of ion-atom collisions to systems with arbitrary mass ratios. We investigate the cusp formation and threshold effects by using classical and quantum frameworks. We emphasize the similarities and differences between both approaches. Finally, we show experimental evidence of a structure that can be identified with a mechanism of binary-encounter emission. We discuss this and related mechanisms. In particular, we investigate the occurrence of multiple binary-collision processes. - Master Thesis. Teoría de colisiones en
presencia de potenciales de largo alcance Instituto Balseiro,
Universidad Nacional de Cuyo 1-1261997
Local Copy