Young´s double slit
experiment revisited
at the atomic level
An international team including groups from
the LCPMR (UPMC-CNRS), Paris; SOLEIL
Synchrotron; the University of Trieste (Italy)
and Uppsala University (Sweden), showed
that it is possible to obtain very accurate
information on the structure of molecules,
such as the distance between the atoms that
compose them. In particular, they have shown
that the emission of electrons from equivalent
atoms in a molecule gives rise to phenomena
similar to those observed in Young’s double-slit
interference experiment. This famous optical
experiment dating back 200 years consisted
of making two beams of light issued from
the same source interfere, by passing them
through two close slits. A periodic pattern was
observed, the distance between the fringes
depending on the distance between the slits
and the wavelength of the light.
It is shown that the distance between two
fringes (successive maxima or minima
of the curves describing the relative number
of electrons emitted from different equivalent
atoms of the molecule) is directly correlated
to the distance between the atoms from which
the electrons are emitted, i.e. provides a kind
of "ruler" to characterize distances on the
atomic scale. The experimental and theoretical
tools used in this study were relatively simple
and easily accessible. This pioneering study
was just a first step, as this method can be
extended to complex systems.
Interferences in coherent emission
of photoelectrons from two equivalent
atomic centers in a molecule are the
microscopic analogies of the celebrated
Young’s double-slit experiment. While
previous experiments had shown the
existence of such interference phenomena
in core-level ionization, this cannot
be a general method, because the most
effective way to detect the oscillations
is to take the ratio of the symmetric and
antisymmetric combinations (g/u) of core
orbitals of two equivalent atoms [1,2],
and this is experimentally possible for few
selected systems. A schematics of the
experiment is shown in Figure
➊
. In the
present work, the novelty is the extension
of the method to inner-valence orbitals,
whose symmetric and antisymmetric
combinations are well separated in binding
energy. We obtained photoelectron spectra
for the inner-valence region in the series
of simple hydrocarbons C
2
H
2
, C
2
H
4
,
and C
2
H
6
, in the photon energy range
of 70–700 eV, and derived the g/u cross-
section ratios. The experimental and
theoretical ratios are shown in Figure
➋
.
The experimental data have been collected
at the soft X-ray PLEIADES beamline at
Synchrotron SOLEIL. The experimental
results are compared with the theoretical
predictions by first-principles density
functional theory (DFT) calculations [3,4].
The excellent agreement between theory
and experiment is evident.
A strong dependence of the oscillation
period on the C–C distance is observed,
which can be used to determine bond
lengths between selected pairs
of equivalent atoms with an accuracy
of at least 0.01 Å. The oscillation shape
and period differ for each molecule.
This is expected because the C–C bond
length in C
2
H
2
, C
2
H
4
, and C
2
H
6
is different.
The sensitivity of the oscillations
to the bond lengths is providing a new
perspective for accurate estimation
of those.
The present work clearly demonstrates
the importance of interference phenomena
in photoionization. These structures, which
are easily experimentally accessible
and are most evident in terms of cross-
section ratios, are highly informative
of geometrical structure, conformational
equilibria, molecular electronic structure,
etc. The present results open the way
to extensive investigations of such
phenomena in a wide range of systems.
ATOMIC AND MOLECULAR PHYSICS, DILUTE MATTER, UNIVERSE SCIENCE
84
SOLEIL
HIGHLIGHTS
2013
