Antonia Ruiz

Electronic structure and polarizabilities of icosahedral fullerenes: A Pariser–Parr–Pople approach

A Pariser–Parr–Pople approach, complemented with physical consistency criteria based on the expected molecular response to a weak electric field, has been used to predict the electronic level structure and polarizabilities of five icosahedral fullerenes in the range C60–C720. The behavior of the polarizability α as a function of the fullerene size is given by the expression α=0.75Re3, where Re is an effective molecular radius. It is argued that fullerenes would present the maximum polarizability values allowed for carbon shells, as if they were made of graphene.

Polarization effects in C60 fullerene complexes of alkali ions

We introduce a secular semiempirical model of the Pariser–Parr–Pople type to reproduce the electronic structure and polarizability of the C60 fullerene. The model is then used to simulate the response of this molecule to an electric charge and estimate its polarization energy. By expressing the charge potential at the C60-cage surface as a multipole expansion, an analytical form is obtained for the polarization energy. Application of these results to endo- and exohedral complexes of alkali ions gives data in rather good agreement with recent ab initio calculations [Hira and Ray, Phys. Rev. A 52, 141 (1995)].

A theoretical model of the static polarizability of carbon buckyonions

A theoretical approach to determine the static dipole polarizabilities of carbon buckyonions is presented. The microscopic electronic structure of the system is provided by an effective one-electron model and the screening effects are treated within the random phase approximation (RPA). The particular spherical geometry of these multishell fullerenes makes possible the use of electrostatic arguments to derive a simple expression for the RPA which gives the polarizability of the buckyonion and the dipole moment induced on each shell in terms of either the screened or unscreened polarizabilities of the isolated shells. A systematic analysis as a function of the buckyonion size is performed. The relevance of an adequate microscopic description of the electronic structure is demonstrated by contrasting the results provided by two different representations of the electron motion, namely a surface electron gas and a more realistic Huckel model. A comparison between our results and those derived from classical d...

Photoabsorption spectra of icosahedral fullerenes: A semiempirical approach

A semiempirical model has been used to predict the electronic photoabsorption spectra of five icosahedral fullerenes in the range C60–C720. The model parameters are first fixed in C60 by fitting its calculated spectrum to the available experimental data, and then conveniently adjusted to describe the larger fullerenes. The structures observed in the calculated spectra show a tendency to smooth π and σ plasmons as the fullerene size increases; however other finite-size features related to the particular geometry of these molecules are still visible at higher resolution. Some consequences of the strong electron screening effects on these spectra are discussed.

Low-temperature dynamics and spectroscopy in exohedral rare-gas C60 fullerene complexes

The adatom dynamics in exohedral C60 fullerene complexes of rare-gas atoms are studied with a three degrees of freedom model. The eigenvalue problem of the corresponding quantum Hamiltonian is solved and the electric-dipole spectra for ArC60, NeC60, and HeC60 in the low-temperature range from 5 to 40 K are simulated. The most important spectral features are related to the degree of angular anisotropy in the adatom–C60 interaction. The ArC60 and NeC60 complexes present very simple spectra which can be assigned in terms of three-mode oscillators; the corresponding motion takes place in the deep hexagon wells (also in the pentagon wells for NeC60) of the interaction potential. On the contrary, the HeC60 complex shows more complicated spectra with important tunneling effects due to the smaller angular anisotropy of the interaction. The onset of almost free internal rotation takes place in this complex at rather low energies, and this gives rise to a low-frequency rotational band in the spectra at temperatures...

Classical and quantum analysis of quasiresonance in grazing atom-surface collisions

The momentum transfer between the normal components to an index direction in the collision of an atom with a periodic surface is investigated. For fast atoms with grazing angle of incidence there is an interval of azimuthal angles around the index direction for which the energy transfer can be very efficient. This effect is reflected in quantum diffraction patterns with large non-specular peaks, associated with the parallel to the surface and normal to the index direction momentum component, and can be described in terms of quasiresonance. Although the classical dynamics does not reproduce the precise quantum diffraction probabilities, indicating the quantum nature of this effect, classical and quantum computations show that the span of the quasiresonance region coincides in both dynamics and can be classically estimated from the phase-space analysis.

A theoretical model of the photoabsorption spectra of carbon buckyonions

A theoretical model has been developed to predict the photoabsorption spectra of spherical carbon buckyonions in the region dominated by the pi-plasmon feature. This model makes use of the microscopic electronic structure of the system, which is provided by an effective Huckel one-electron model. The important screening effects are treated within the random phase approximation, whose form is an extension to the dynamic case of the one derived in a previous work for the static polarizabilities of these species. A systematic analysis as a function of the buckyonion size is performed. We compare the spectra obtained in this way with those derived from a different representation of the electron motion, namely a two-dimensional spherical electron gas, and from a classical dielectric model.

Effects of classical nonlinear resonances in grazing diatom-surface collisions

Energy transfer between vibrational, rotational, and translational degrees of freedom of a molecule during a collision process is enhanced when the classical frequencies associated with the initial state are in the proximity of nonlinear resonance conditions. We present an analysis of the classical resonant effects in the collisions of light diatoms with periodic surfaces, and discuss the initial conditions in which these effects can be observed. In particular, we find that for grazing incidence and resonant initial values of the classical frequencies, corresponding to specific vibro-rotational molecular states and translational energies, an efficient energy transfer between the intramolecular vibro-rotational degrees of freedom and the translational degree of freedom along a symmetry direction on the surface can be found. This efficient energy transfer manifests itself in the emergence of specific peaks in the molecular diffraction patterns. The predictions of the resonance analysis are contrasted with the results of classical trajectory calculations obtained in a diatom-rigid surface collision model.

Quasiresonances in atom-surface collisions

The only global autonomous system in which quasiresonant processes have been previously described is the seminal example of one atom colliding with a diatom molecule. In this work we describe classical quasiresonances in a different context, the grazing angle atom-surface collisions. While in the first system the actions related to the process correspond to the internal vibro-rotational degrees of freedom of the molecule, in this new example they turn to be the two components of the momentum of the atom parallel to the surface. We discuss the range of initial actions where quasiresonant processes arise, and suggest a new method to localize quasiresonance regions using the dwell time of the incoming atom in the vicinity of the surface.

Scattering cross sections for low-energy alkali cation +C60 collisions: The relevance of polarization

Total scattering cross sections for low-energy collisions of C60 fullerene with alkali ions are theoretically estimated using an accurate spherical potential approximation. These cross sections show the relevance of polarization effects. Our results indicate a way in which collisional experimental methods could be used to measure the high polarizabilities of C60 and other fullerenes.