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

V Encuentro Sud Americano de Colisiones Inelásticas en la Materia
Supression of binary and recoil peaks in ionization of H 2 by electron impact
Fojón O A, Stia C R and Rivarola R D
Instituto de Física Rosario (CONICET-UNR), Pellegrini 250 (2000) Rosario, Argentina
email address corresponding author: fojon@ifir-conicet.gov.ar
We study theoretically the single
ionization of H 2 molecules by fast electron
impact. Our aim is to show that interferences
coming from the coherent emission from the
molecular centers may produce unexpected
consequences in the physical features of the
observables of the reaction.
Interference phenomena have been of
crucial importance in the foundation of quantum
mechanics. Analogies with the Young two-slit
experiment played a fundamental role in the
description and comprehension of the dual
nature of quantum objects such as electrons. A
fascinating alternative way of observing
interference patterns is provided by the electron
spectra resulting from the ionization of
molecular diatomic targets. In the sixties, it was
suggested that the coherent emission from these
molecules may give rise to specific oscillations
in the differential cross sections of the ejected
electrons, the two molecular centers acting as the
analogues of the two slits in the Young
experiment [1]. However, this kind of
oscillations was measured for the very first time
with fast krypton ions impacting on H 2 [2]. In
previous works, we have shown that these
interference patterns may be observed also for
electron impact [3-7].
We focus here on electron emission at
high impact energies from fixed-in-space H 2
molecules impacted by fast electrons. We study
transitions at fixed equilibrium internuclear
distance from the ground state of H 2 to the
ground (gerade) and first excited (ungerade)
state of the H +
2 residual target. We consider
coplanar geometries in which the incident,
scattered and ejected momenta lie all in the same
plane. In addition, we analyze asymmetric
kinematics situations in which one slow and one
fast electron are detected in the final channel.
We employ a first order model obtained
in the framework of a two-effective center
approximation (TEC). The idea exploited in the
TEC model is that although electrons in the
ground state of H 2 are shared by both nuclei, the
electronic density is peaked at the nuclei
positions. Then, it is argued that ejection occurs
in the neighbourhoods of one nucleus while the
nuclear charge of the other one is screened
completely by the non ionized electron.
Consequently, a unique final effective
continuum function satisfying the correct
asymptotic long range conditions is used to
represent the ejected electron in the final channel
of the reaction. This model gives reasonably
good agreement with experiments [8]
constituting thus a good approximation to the
final state of the reaction in which three charged
bodies interact through Coulomb potentials. In
order to take into account the complexities of
this interaction in an approximate way, one can
take a more elaborated final function such as the
one used in Ref. [9]. This function describes the
final interactions through a product of three
Coulomb functions associated to the three twobody
pairs present in the final channel. The
approximation obtained using this function for
molecular targets gives an excellent agreement
with experiments [10].
We show here for the first time that under
definite conditions, destructive interferences
coming from the coherent emission from both
molecular centers provoke the supression of the
binary peak in the multiple differential cross
sections corresponding to transitions leading to
final ground state of H 2 + . This is a surprising
result as is well known that ejection is classically
more likely to be produced in the binary region.
Moreover, this finding is shocking as so far and
up to our knowledge the presence of the binary
peak was assumed for every ionization reaction
with either atomic or molecular targets [11].
57 Valparaíso, Chile