RESEARCH
Line 1: Doped helium clusters
The experimental manifestation of the superfluidity at a microscopic level has been attained through Infra-red (IR) spectra of different molecules, in particular OCS, embedded in 4He nanodroplets. They show well-defined P and R peaks close to those corresponding to the isolated molecule: It is like the molecule is almost freely rotatinginside the helium environment. On the contrary, an unstructured and broad band does appear when fermion atoms, i. e. 3He, constitute the cluster. This fact has been partially explained by using classical arguments, though several issues remain open. The many-body problem involved can be studied from the quantum point of view using different approaches. However, the usual procedures employed exhibit some weaknesses. For instance, the Quantum Diffusion Montecarlo method is hard to be applied to the fermion scenario, while semi-empirical treatments based on the Functional Density Theory do not provide with the necessary wave-functions to perform spectroscopic simulations thus allowing a direct comparison with the experiment. Since the field of ab initio electronic structure calculations is the frame in which, perhaps, the many-body problem has been studied at the deepest level, we propose the use of a similar methodology to the case under study: a simple diatomic molecule surrounded by helium (fermion and/or boson) atoms. The later play the role of electrons, and the two atoms building up the molecule act as nuclei. In this way, we started with Hartree (Hartree-Fock) calculations on pure boson (fermion) helium clusters doped with a bromine molecule in its ground electronic state, while for different mixtures of the two species we resort also to a Self Consistent Field-like treatment. The potential energy function is modeled as the simple addition of triatomic He-Br2 potentials plus He-He interactions. In this approach, the distorted (by the helium environment) diatomic molecule reveals trough IR Raman spectra simulations that the main responsible for the experimental behavior above mentioned is the spin multiplicity. By means of Diffusion Montecarlo calculations , we are now analyzing the incidence of the potential energy surface used to describe each He-interaction on the size-dependent results (energies and spatial distributions) for pure boson clusters. We are also comparing such results with those obtained through Hartree-like models which incorporate the He-He correlation more accurately and where the helium atoms are allowed to occupy different orbitals. Besides the application of this kind of treatments to different dopant molecules (HF, CO, OCS), the goal is to accommodate powerful ab initio packages (Gaussian, Molpro) by replacing the standard Coulomb interactions appearing there by the molecular ones of relevance in our case. Extension of this approach to several fields in Physics in which the many-body problem is the main issue would be, in principle, feasible and profitable.Line 2: Reactive and inelastic collisions of relevance in astrophysics and atmospheric chemical physics
Several systems of astrophysical (H+ + D2, H+2 + H2, Li+ + H2 etc.) and atmospherical (O2 + O2, O + HX, etc.) interest are studied. Other systems, that react through harpooning mechanisms, such as Li + HF , Ca + Hcl, Ba + FCH3, and for which a large number of experimental data are available, are also studied. The aim in these systems is the computation of rate constants -total and/or state-to-state- as well as cross-sections. These magnitudes allow us, in some cases, to explain the available experimental observations, or to predict new observable effect in some others. In most cases under study there are several electronic states involved, since reactants and products are in many cases open shell systems. Those electronic states are coupled by means of non-adiabatic or spin-orbit terms. For this reason, we are working in several methods that can make use of a diabatic representation with several coupled potential energy surfaces. In this line, we also work in the development of different quantal treatments applied to colision dynamics. The methods cover timeindependent formalisms,by solving a set of coupled differential equations, as well as time-dependent ones, in which we solve the temporal evolution of wave packets. Along this line, the main goal is to be able to get an increasing performance of the dynamical methods to smoothly shift towards more complex systems involving a large number of degrees of freedom. Finally, and related to Line 8 (photodissociation), we study photoinitiated reactive processes, from precursors formed by van der Waals clusters or aninons. These studies give detailed information on the transition state, the place where reaction is taken place, on real time. The optimization of the laser pulse is also possible way for the control of such reactions. In this research line, although essentially theoretical, we are in close contact with serveral experimental groups devoted to the aforementioned processes.Line 3: Photodissociation of van der Waals and hydrogen-bonded clusters
Characterization of potential-energy surfaces of X2 Rgn (X = Cl, Br; Rg = He, Ne, Ar; n = 1, 2), XY - Rgn (X = I, Y = Cl; Rg = He; n = 1, 2) clusters
The study of weak van der Waals (vdW) interactions is fundamental in order to understand a wide variety of physical and chemical processes in small clusters and larger condensed matter systems. In this sense, a first step is the characterization of the potential-energy surfaces associated to those weak interactions, both in the ground and in excited electronic states of the cluster. At present this is still a challenge for theory. In this line of work the potential-energy surfaces of the excited electronic state B3Πu were characterized for clusters like I2-Ne, Cl2-He, and Br2-He. The surfaces obtained reproduce the available spectroscopic and dynamical experimental data typically within experimental error.Vibrational predissociation of X2 Rgn (n ≤ 2)
The investigation of photoinduced fragmentation processes in vdW clusters provides valuable information on the weak interactions and the solvation effects exerted by the rare gas atoms on the chemical molecule X2. One of such fragmentation processes is the vibrational predissociation, which was studied for the triatomic clusters above mentioned, and for more complex, tetraatomic clusters like I2-Ne, Cl2-Ne2, and Cl2-He2. The results obtained made possible to rationalize the experimental measurements concerning the energy transfer and fragmentation mechanisms involved.Photodissociation of hydrogen-bonded clusters
Another cluster of great interest are those containing a weak hydrogen bond, of the type Rg-HX (X = halogen, Rg = rare gas). Photodissociation of these complexes by means of laser excitation of the HX chromophore produces a very energetic hydrogen atom. Such an atom may collide several times with the Rg and X atoms before escaping. Such "intracluster" collisions produce signals in time-of-flight spectrum of the H fragment, which provide information on the electronic state to which the system was excited. Another relevant aspect is the formation of Rg-X radical complexes after departure of the H fragment. Experimental evidence of formation of such species has been detected. In this line of work the conditions of formation of Rg-X radical complexes were investigated for a wide variety of Rg-HX (X = F,Cl,Br,I; Rg = Ar,Kr) clusters. As a result, a global explanation for the formation of these radicals has been proposed, which can be extended to other similar clusters. In addition, mechanisms which allow some control on the probability of radical formation as well as on the final state distributions of the radical products, have been proposed.Line 4: Secondary electron interactions in materials of biological and environmental interest
When high energy particles interact with matter produce secondary electrons which are the main responsible, through successive collisions with the atoms and molecules constituting the medium, of the energy transfer and the radiation damage. Important biological, medical and environmental applications require a detailed description, at molecular level, of the location and amount of energy involved in any collision process as well as the identification of the type of interaction that is taking place. This model then provides a radiation damage evaluation in terms of molecular dissociation that could be the origin of further functional modifications. The main objective of this research line is to obtain energy deposition models in materials of biological and environmental interest. For this purpose, we propose a method that combines experimental techniques with theoretical calculations to determine the interaction probabilities (cross sections) with energetic and angular resolution, both for high energy primary particles (photons, ions and electron) and secondary electrons, following the energy degradation procedure down to thermal energies. Finally, cross sectional data obtained by this procedure are used as input parameters in a Monte Carlo simulation of each single particle track, providing a map of the energy transfer points (clusters of damage) and giving information of the specific interaction that took place and the energy transferred at each point. The experiments are being carried out at the following laboratories:Line 5: Interdisciplinary Physics
This interdisciplinary line of research can be divided in the following sublines:Line 6: Reduced Density Matrices and Correlation Matrices Methodology
This line of research went through an intense initial development during the years 50-70. In 1983, C. Valdemoro re-initiated the quest for a direct evaluation of the second-order Reduced Density Matrix (2-RDM)by looking at the problem from a new point of view. This quest lead to the proposal in the 90's by C.Valdemoro and her collaborators of a method for the iterative solution of the second-order Contracted Schrödinger Equation (2-CSE),(see Physics Today Feb.2001 pg.59-60). In its second phase of development, this line of research has been following four main directions: