53 research outputs found
Image processing for grazing incidence fast atom diffraction
Grazing incidence fast atom diffraction (GIFAD, or FAD) has developed as a
surface sensitive technique. GIFAD is less sensitive to thermal decoherence but
more demanding in terms of surface coherence, the mean distance between
defects. Such high quality surfaces can be obtained from freshly cleaved
crystals or in a molecular beam epitaxy (MBE) chamber where a GIFAD setup has
been installed allowing in situ operation. Based on recent publications by
Atkinson et al. and Debiossac et al, the paper describes in detail the basic
steps needed to measure the relative intensities of the diffraction spots. Care
is taken to outline the underlying physical assumptions.Comment: IISC-21 International Workshop on Inelastic Ion-Surface Collisions,
Dosnostia Sept. 2015. Elsevier, NIM-B (2016
Elastic and inelastic diffraction of fast atoms,\linebreak Debye-Waller factor and M\"{o}ssbauer-Lamb-Dicke regime
The diffraction of fast atoms at crystal surfaces is ideal for a detailed
investigation of the surface electronic density. However, instead of sharp
diffraction spots, most experiments show elongated streaks characteristic of
inelastic diffraction. This paper describes these inelastic profiles in terms
of individual inelastic collisions with surface atoms taking place along the
projectile trajectory and leading to vibrational excitation of the local Debye
oscillator. A quasi-elastic regime where only one inelastic event contributes
is identified as well as a mixed quantum-classical regime were several
inelastic collision are involved. These regimes describe a smooth evolution of
the scattering profiles from sharp spots to elongated streaks merging
progressively into the classical diffusion regime
Revisiting Atomic Collisions Physics with highly charged ions, A tribute to Michel Barat
Michel Barat passed away in November 2018 at the age of 80 after a rich
career in atomic and molecular collisions. He had participated actively in
formalizing to the electron promotion model, contributed to low energy reactive
collisions at the frontier of chemistry. He investigated electron capture
mechanisms by highly charged ions, switched to collision induced cluster
dissociation and finally to UV laser excitation induced fragmentation
mechanisms of biological molecules. During this highly active time he created a
lab, organized ICPEAC and participated actively in the administration of
research. This paper covers the ten years where he mentored my scientific
activity in the blossoming field of electron capture by highly charge ions
(HCI). In spite of an impressive number of open channels, Michel found a way to
capture the important parameters and to simplify the description of several
electron capture processes; orientation propensity, electron promotion, true
double electron capture, Transfer ionisation, Transfer excitation, formation of
Rydberg states, and electron capture by metastable states. Each time Michel
established fruitful collaborations with other groups.Comment: physics.atom-ph, submitted to Journal of Physics
Refraction of fast Ne atoms in the attractive well of LiF(001) surface
Ne atoms with energies up to 3 keV are diffracted under grazing angles of
incidence from a LiF(001) surface. For a small momentum component of the
incident beam perpendicular to the surface, we observe an increase of the
elastic rainbow angle together with a broadening of the inelastic scattering
profile. We interpret these two effects as the refraction of the atomic wave in
the attractive part of the surface potential. We use a fast, rigorous dynamical
diffraction calculation to find a projectile-surface potential model that
enables a quantitative reproduction of the experimental data for up to ten
diffraction orders. This allows us to extract an attractive potential well
depth of 10.4 meV. Our results set a benchmark for more refined surface
potential models which include the weak Van der Waals region, a long-standing
challenge in the study of atom-surface interactions
Energy loss and inelastic diffraction of fast atoms at grazing incidence
The diffraction of fast atoms at grazing incidence on crystal surfaces
(GIFAD) was first interpreted only in terms of elastic diffraction from a
perfectly periodic rigid surface with atoms fixed at equilibrium position.
Recently, a new approach have been proposed, referred here as the quantum
binary collision model (QBCM). The QBCM takes into account both the elastic and
inelastic momentum transfer via the Lamb-Dicke probability. It suggests that
the shape of the inelastic diffraction profiles are log-normal distributions
with a variance proportional to the nuclear energy loss deposited on the
surface. For keV Neon atoms impinging the LiF surface, the predictions of the
QBCM in its analytic version are compared with numerical trajectory
simulations. Some of the assumptions such as the planar continuous form, the
possibility to neglect the role of lithium atoms and the influence of
temperature are investigated. A specific energy loss dependence is identified in the quasi-elastic regime merging
progressively to the classical onset . The ratio of
these two predictions highlight the role of quantum effects in the energy loss.Comment: 9 pages 8 figures paper prepared for IISC-2
Elastic and inelastic diffraction of fast neon atoms on a LiF surface
Grazing incidence fast atom diffraction has mainly been investigated with
helium atoms, considered as the best possible choice for surface analysis. This
article presents experimental diffraction profiles recorded with neon
projectile, between 300 eV and 4 keV kinetic energy with incidence angles
between 0.3 and 1.5 along three different directions
of a LiF(001) crystal surface. These correspond to perpendicular energy ranging
from a few meV up to almost 1 eV. A careful analysis of the scattering profile
allows us to extract the diffracted intensities even when inelastic effects
become so large that most quantum signatures have disappeared. The relevance of
this approach is discussed in terms of surface topology.Comment: 9 page
Diffraction of fast atoms and molecules from surfaces
Prompted by recent experimental developments, a theory of surface scattering
of fast atoms at grazing incidence is developed. The theory gives rise to a
quantum mechanical limit for ordered surfaces that describes coherent
diffraction peaks whose thermal attenuation is governed by a Debye-Waller
factor, however, this Debye-Waller factor has values much larger than would be
calculated using simple models. A classical limit for incoherent scattering is
obtained for high energies and temperatures. Between these limiting classical
and quantum cases is another regime in which diffraction features appear that
are broadened by the motion in the fast direction of the scattered beam but
whose intensity is not governed by a Debye-Waller factor. All of these limits
appear to be accessible within the range of currently available experimental
conditions.Comment: 37 pages including 3 figure
Describing the scattering of keV protons through graphene
Implementing two-dimensional materials in technological solutions requires fast, economic, and non-destructive tools to ensure efficient characterization. In this context, scattering of keV protons through free-standing graphene was proposed as an analytical tool. Here, we critically evaluate the predicted effects using classical simulations including a description of the lattice's thermal motion and the membrane corrugation via statistical averaging. Our study shows that the zero-point motion of the lattice atoms alone leads to considerable broadening of the signal that is not properly described by thermal averaging of the interaction potential. In combination with the non-negligible probability for introducing defects, it limits the prospect of proton scattering at 5 keV as an analytic tool
Describing the scattering of keV protons through graphene
Implementing two-dimensional materials in technological solutions requires fast, economic, and non-destructive tools to ensure efficient characterization. In this context, scattering of keV protons through free-standing graphene was proposed as an analytical tool. Here, we critically evaluate the predicted effects using classical simulations including a description of the lattice’s thermal motion and the membrane corrugation via statistical averaging. Our study shows that the zero-point motion of the lattice atoms alone leads to considerable broadening of the signal that is not properly described by thermal averaging of the interaction potential. In combination with the non-negligible probability for introducing defects, it limits the prospect of proton scattering at 5 keV as an analytic tool
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