Aca - Departamento de Física - Universidad Técnica Federico Santa ...

V Encuentro Sud Americano de Colisiones Inelásticas en la Materia
Fast atom diffraction from metallic surfaces
M.S. Gravielle 1 , G. Bocán 2 , and R. Díez Muiño 3
1 Instituto de Astronomía y Física del Espacio, CONICET, Bs. As, Argentina, and Depto de Física,
FCEN, UBA.
2 Centro Atómico Bariloche, CNEA and CONICET, S.C. de Bariloche, Río Negro, Argentina.
3 Donostia International Physics Center (DIPC) and C. Fís. Materiales CSIC-UPV/EHU, San Sebastián,
Spain.
E-mail: msilvia@iafe.uba.ar
1. INTRODUCTION
Diffraction from crystal surfaces produced by
grazing scattering of atoms in the keV energy
range is nowadays attracting considerable attention.
The first experimental evidences of this
effect were reported for insulator surfaces [1] for
which the presence of a band gap strongly suppresses
inelastic electronic processes, favoring
the conservation of quantum coherence. However,
this diffraction effect has recently been
observed at metallic materials [2,3] as well, although
electron excitations were supposed to
smudge interference signatures for these surfaces.
The aim of this work is to study the
atomic diffraction from metal surfaces by considering
keV nitrogen atoms impinging grazingly
on Ag(111) .
2. THEORETICAL MODEL
To describe the scattering process we employ
a distorted-wave model - the surface-eikonal
approximation [4] - that makes use of the eikonal
wave function to represent the elastic collision
with the surface, while the movement of the
fast projectile is described classically by considering
axially channeled trajectories for different
initial conditions. The surface-eikonal T-matrix
element reads:
r r
(eik)
Tif
= ∫ dRos
aif
( Ros
),
(1)
where R r
determines the initial position of the
os
projectile on the surface plane and
a
if
r
( R
os
r
-3
) = ( 2π)
∫ dt |vz
( RP
) |×
r r r
exp[ -iQ.R -i η(
R )] V
P
P
SP
r
( R
P
)
(2)
is the transition amplitude associated with the
r r
classical path RP
( Ros
, t)
, with Q r the projectile
r
momentum transfer and v z
( RP
) the component
of the projectile velocity perpendicular to the
surface plane. In Eq. (2), η ( R
r P
) is the eikonal-
Maslov phase that takes into account the action
of the projectile-surface potential V ( R
r
SP P)
along
the classical path.
An accurate potential energy surface for the
N/Ag(111) system was derived from density
functional theory (DFT) calculations as implemented
in the "Vienna Ab-initio Simulation
Program" (VASP) code [5], combined with an
elaborate interpolation method [6].
3. RESULTS
We apply the theoretical model to evaluate
angular projectile distributions for impact along
different low-index crystallographic directions.
The influence of the incidence energy and the
symmetry of the considered channel are both
analyzed.
32 Valparaíso, Chile