MAIN RESEARCH
TOPICS
1.- Energetics and Dynamics
of Small Molecular Clusters
To attain accurate description
of their properties and an understanding of the nanoscopic behavior of atomic
and
molecular clusters (and,
ultimately, of the condensed phase of materials) requires a correspondingly
accurate and
realistic knowledge of
their intermolecular forces. In particular, ensembles of variable size which
are made up of
fairly simple components
such as the neutral, identical rare gases have been for a long time the
preferred testing
ground for a broad variety
of theoretical and computational analysis of such intermolecular forces.
Small clusters of Helium
(in particular, dimers and trimers) present a series of unusual quantum
properties of
fundamental interest. These
properties can play a role not only in connection with the statistical behavior
of
collective modes of Helium
gas at low temperatures, but also with the Bose-Einstein condensation as
well as with
the appearance of the so-called
Efimov states in three body interaction. The recent years have brought to
the
forefront of both applied
and fundamental research the importance of knowing the mechanism for, in
general,
three body recombination
dynamics when only very weakly interacting species are involved and when
such events
are forced to happen at
very low temperatures of the gaseous thermal bath, preluding to the
occurrence of
mesoscopic effects such
as the Bose-Einstein condensation induced by photo-absorption, radiative cooling
or other
laser-induced processes.
2.- Quantum and Classical
Trajectories. Classical and Quantum Chaos
From its beginning Quantum Mechanics
has revealed as a very successful and powerful theory to describe nature.
However, the standard formalism
in terms of probabilities is often unable to provide a satisfactory intuitive
insight
into the underlying physical
processes, as it is the case for the corresponding classical description.
The situation is
even worse if one takes into
account that there must be some kind of correspondence between the predictions
of both
mechanics in the appropriate
limits. Some alternative formalisms of Quantum Mechanics have been proposed
in the
literature such as, for example,
Feynman path integrals, Madelung hydrodynamical formulation, Semiclassical-IVR
approach and Bohm formalism.
Bohm developed, in particular, a formalism in which the initial and final
states of a
physical process are connected
causally by the
so-called quantum trajectories. Bohmian Mechanics combines both the
prediction of Quantum Mechanics
and the capability of providing an intuitive interpretation of the physical
processes involved.
Particle diffraction experiements
have played a key role in the development of Quantum theory since its inception.
In
words of Feynman, the double-slit
experiment has in it the heart of Quantum Mechanics. Our work is running
along
this line.
We have tackled the diffraction of atoms from surfaces with different degrees
of corrugation and in presence
or not of single adsorbates as
well as the selective adsorption and threshold resonances in elastic
scattering. Our aim
now is to extent this type
of analysis to inelastic scattering and see the role played by the
temperature of the surface.
Due to the fact we are mainly
interested in interpretative aspects, we are obliged to analyze the
physical problems
aboved mentioned in terms of
classical, semiclassical and standard quantum methods. The physical processes
studied
are complex and non-linear showing
that the underlying dynamics display chaos and catastrophes.
3.-
Helium Atom Scattering. Diffraction, Diffusion and Adsorbate
Vibrations
Low-energy incident beams of
light neutral particles are able to probe, in a non-destructive way, the
outermost layer
of metallic, insulator and semiconductor
surfaces. In particular, experiments with Helium atoms have been extensively
carried out and, due to the small
value of the mass, the scattering is predominantly elastic and diffraction
dominates.
This has led to the obtaining
of a wealth of data and a lot of experimental information about the orientation
and size
of the surface unit cells, surface
corrugations, particle-surface physisorption interaction potentials, surface
diffusion,
condensation and growth phenomena.
Inelastic processes can also be studied and phonon dispersion curves are
obtained
by combining pulsed beams and
time-of-flight techniques. The role played by the surface temperature as well
as the
coverage of adsorbates on the
surface in most of the elementary processes ocurring at surfaces is our main
goal in this
topic. In particular, we are
currently investigating how the interaction between adsorbates is affecting
diffusion and
their low-energy modes.
4.- Quantum
Dissipative Systems. Stochastic Processes
Quantum dynamics of open systems is an interdiciplinary branch of Physics
where exchange of ideas and methods of
very broad fields of interest
meet together, processes which are ubiquitous in Biology, Chemistry, Economy,
etc. In
a very broad sense, all real physical
systems are open systems since the interaction with their environment can
never
be totally neglected. Interaction
with an environment generally causes systems to become mixed; the whole
is in a
definite state whereas the parts
are not (entanglement). Quantum interference
is an example and if the system and
the environment or bath are entangled
and then stop interacting, the entanglement remains. Usually we are interested
in the system since the bath is
an extended system and it is too difficult to deal with. The coupling between
the bath
and the system acts as a continuous
measuring apparatus leading to an incessant destruction of phase correlations
(decoherence).
Damping occurs because a system
interacts with the bath and the energy of the system is dissipated onto the
bath.
However, noise arise also since
the bath distributes some of its energy back to the system. In presence of
noise, we
have to speak of fluctuation which
is a stochastic variable with zero mean. The paradigm of a dissipative system
is
the Brownian motion which can
be regarded as stochastic because detailed information is lost for molecules
in the
medium. Similarly, if a large
number of degrees of freedom of a many-particle system is masked from observation,
the motion of the remaining degrees
of freedom is described as a stochastic process.
In our recent work on atom-surface
scattering, we have considered the surface as a heat bath and some of the
most
elementary processes such as diffraction
at surfaces with the presence or not of single adsorbates, selective
adsorption
resonances, diffusion and low-energy
vibrational relaxation are being currently analyzed.
Finally, some extension to Econophysics
is being developed. In particular, the Pricing Theory of Black-Scholes-Merton
to warrants and interest rates
are also subjects of our actual interest.