arXiv subjects

Julio A. Alonso

Publications and source records attributed to Julio A. Alonso.

11 recordsLinked to original sources

arXiv2021

Structure, stability and optical absorption spectra of small Ti$_n$C$_x$ clusters: a first-principles approach

Titanium-carbide molecular clusters are thought to form in the circumstellar envelopes (CSEs) of carbon-rich Asymptotic Giant Branch stars (AGBs) but, to date, their detection has remained elusive. To facilitate the astrophysical identification of those clusters in AGBs and post-AGBs environments, the molecular structures and optical absorption spectra of small Ti$_n$C$_x$ clusters, with n = 1-4 and x = 1-4, and some selected larger clusters, Ti$_3$C$_8$, Ti$_4$C$_8$, Ti$_6$C$_{13}$, Ti$_7$C$_{13}$, Ti$_8$C$_{12}$, Ti$_9$C$_{15}$, and Ti$_{13}$C$_{22}$, have been calculated. The density functional formalism, within the B3LYP approximation for electronic exchange and correlation, was used to find the lowest energy structures. Except the clusters having a single Ti atom, the rest exhibit three-dimensional structures. Those are formed by a Ti fragment surrounded in general by carbon dimers. The optical spectra of Ti$_n$C$_x$, computed by time-dependent density functional theory, using the corrected CAM-B3LYP functional, show absorption features in the visible and near infrared regions which may help in the identification of these clusters in space. In addition, most of the clusters have sizable electric dipole moments, allowing their detection by radioastronomical observations.

astro-ph.GA↗

arXiv2019

An Improved Descriptor of Cluster Stability. Application to Small Carbon Clusters

The mass spectra of gas-phase clusters in cluster beams have a rich structure where the relative heights of the peaks compared to peaks corresponding to clusters of neighbor sizes reveal the stability of the clusters as a function of the size $N$. In an analysis of the published mass spectrum of carbon clusters cations $C_N^{+}$ with $N\leq$16 we have employed the most common descriptor of cluster stability, which is based on comparing the total energy of the cluster of size $N$ with the averaged energies of clusters with sizes $N$+1 and $N$-1. Those energies have been obtained from density functional calculations. The comparison between the stability function and the mass spectrum leaves some experimental features unexplained; in particular, the correlation with the detailed variation of the height of the mass peaks as a function of size $N$ is not satisfactory. We then propose a novel stability descriptor which improves matters substantially, in particular the correlation with the detailed variation of the height of the mass peaks. The new stability index is based on comparing the atom-evaporation energy of the cluster of size $N$ with the averaged atom-evaporation energies of clusters with sizes $N$+1 and $N$-1. The substantial improvement achieved is attributed to the fact that evaporation energies are quantities directly connected with the processes controllig the cluster abundances in the beam.

physics.chem-ph↗

arXiv2012

First-principles Structural and Electronic Characterization of Ordered SiO2 Nanowires

Density functional theory and molecular dynamics simulations have been used to optimize the structure of nanowires of SiO2. The starting structures were based on b-cristobalite, orthotridymite, b-tridymite, and rutile crystals. The analysis of the electronic structure has been validated by many-body perturbation calculations using the G0W0 and GW + Bethe-Salpeter equation approximations, in order to account for quasi-particle and excitonic effects. The calculations indicate that many of these nanowires have semiconducting character, while the corresponding bulk solids are insulators. In the case of thick rutile-like nanowires we found the gap congested with surface states. Electronic charge is transferred from silicon atoms to oxygen atoms, and the bonding in the nanowires is partly ionic and partly covalent. The magnitude of the band gap can be engineered by changing the structure and the thickness of the nanowires.

cond-mat.mtrl-sci↗

arXiv2011

Scattering of a proton with the Li4 cluster: non-adiabatic molecular dynamics description based on time-dependent density-functional theory

We have employed non-adiabatic molecular dynamics based on time-dependent density-functional theory to characterize the scattering behaviour of a proton with the Li$_4$ cluster. This technique assumes a classical approximation for the nuclei, effectively coupled to the quantum electronic system. This time-dependent theoretical framework accounts, by construction, for possible charge transfer and ionization processes, as well as electronic excitations, which may play a role in the non-adiabatic regime. We have varied the incidence angles in order to analyze the possible reaction patterns. The initial proton kinetic energy of 10 eV is sufficiently high to induce non-adiabatic effects. For all the incidence angles considered the proton is scattered away, except in one interesting case in which one of the Lithium atoms captures it, forming a LiH molecule. This theoretical formalism proves to be a powerful, effective and predictive tool for the analysis of non-adiabatic processes at the nanoscale.

physics.atm-clus↗

arXiv2002

Excited states dynamics in time-dependent density functional theory: high-field molecular dissociation and harmonic generation

We present a theoretical description of femtosecond laser induced dynamics of the hydrogen molecule and of singly ionised sodium dimers, based on a real-space, real-time, implementation of time-dependent density functional theory (TDDFT). High harmonic generation, Coulomb explosion and laser induced photo-dissociation are observed. The scheme also describes non-adiabatic effects, such as the appearance of even harmonics for homopolar but isotopically asymmetric dimers, even if the ions were treated classically. This TDDFT-based method is reliable, scalable, and extensible to other phenomena such as photoisomerization, molecular transport and chemical reactivity.

cond-mat↗

arXiv2001

Can optical spectroscopy directly elucidate the ground state of C20?

The optical response of the lowest energy members of the C20 family is calculated using time-dependent density functional theory within a real-space, real-time scheme. Significant differences are found among the spectra of the different isomers, and thus we propose optical spectroscopy as a tool for experimental investigation of the structure of these important clusters.

physics.atm-clus↗

arXiv2000

Ab Initio Calculation of the Lattice Distortions induced by Substitutional Ag- and Cu- Impurities in Alkali Halide Crystals

An ab initio study of the doping of alkali halide crystals (AX: A = Li, Na, K, Rb; X = F, Cl, Br, I) by ns2 anions (Ag- and Cu-) is presented. Large active clusters with 179 ions embedded in the surrounding crystalline lattice are considered in order to describe properly the lattice relaxation induced by the introduction of substitutional impurities. In all the cases considered, the lattice distortions imply the concerted movement of several shells of neighbors. The shell displacements are smaller for the smaller anion Cu-, as expected. The study of the family of rock-salt alkali halides (excepting CsF) allows us to extract trends that might be useful at a predictive level in the study of other impurity systems. Those trends are presented and discussed in terms of simple geometric arguments.

cond-mat.mtrl-sci↗

arXiv2000

Melting in large sodium clusters: An orbital-free molecular dynamics study

The melting-like transition in sodium clusters Na_N, with N=55, 92, and 142 is studied by using constant-energy molecular dynamics simulations. An orbital-free version of the Car-Parrinello technique is used which scales linearly with system size allowing investigation of the thermal behaviour of large clusters. The ground state isomer of Na_142 (an uncomplete three-shell icosahedron) melts in two steps: the first one (at approx. 240 K) is characterized by the high mobility of the atoms located on the cluster surface; the second, homogeneous melting (at approx. 270 K), involves diffusive motion of all the atoms across the cluster. For the case of Na_92, the icosahedral structure has a larger number of surface vacancies, and melts in two well separated steps, surface melting at approx. 130 K and homogeneous melting at approx. 240 K. Na_55, a complete two-shell icosahedron, melts in a single stage at approx. 190 K. Our results on homogeneous melting for Na_142 and Na_92 are in excellent agreement with recent experimental determinations of melting temperatures and latent heats. However, the experimentally observed enhancement of the melting temperature around N=55 is not reproduced by the calculations.

physics.atm-clus↗

arXiv1999

Orbital-Free Molecular Dynamics Simulations of Melting in Na8 and Na20: Melting in Steps

The melting-like transitions of Na8 and Na20 are investigated by ab initio constant energy molecular dynamics simulations, using a variant of the Car-Parrinello method which employs an explicit electronic kinetic energy functional of the density, thus avoiding the use of one-particle orbitals. Several melting indicators are evaluated in order to determine the nature of the various transitions, and compared with other simulations. Both Na8 and Na20 melt over a wide temperature range. For Na8, a transition is observed to begin at approx. 110 K, between a rigid phase and a phase involving isomerizations between the different permutational isomers of the ground state structure. The ``liquid'' phase is completely established at approx. 220 K. For Na20, two transitions are observed: the first, at approx. 110 K, is associated with isomerization transitions between those permutational isomers of the ground state structure which are obtained by interchanging the positions of the surface-like atoms; the second, at approx. 160 K, involves a structural transition from the ground state isomer to a new set of isomers with the surface molten. The cluster is completely ``liquid'' at approx. 220 K.

physics.atm-clus↗

arXiv1999

Calculation of The Band Gap Energy and Study of Cross Luminescence in Alkaline-Earth Dihalide Crystals

The band gap energy as well as the possibility of cross luminescence processes in alkaline-earth dihalide crystals have been calculated using the ab initio Perturbed-Ion (PI) model. The gap is calculated in several ways: as a difference between one-electron energy eigenvalues and as a difference between total energies of appropriate electronic states of the crystal, both at the HF level and with inclusion of Coulomb correlation effects. In order to study the possibility of ocurrence of cross luminescence in these materials, the energy difference between the valence band and the upmost core band for some representative crystals has been calculated. Both calculated band gap energies and cross luminescence predictions compare very well with the available experimental results.

cond-mat.mtrl-sci↗

arXiv1998

Lattice Distortions Around a Tl+ Impurity in NaI:Tl+ and CsI:Tl+ Scintillators. An Ab Initio Study Involving Large Active Clusters

Ab initio Perturbed Ion cluster-in-the-lattice calculations of the impurity centers NaI:Tl+ and CsI:Tl+ are pressented. We study several active clusters of increasing complexity and show that the lattice relaxation around the Tl+ impurity implies the concerted movement of several shells of neighbors. The results also reveal the importance of considering a set of ions that can respond to the geometrical displacements of the inner shells by adapting selfconsistently their wave functions. Comparison with other calculations involving comparatively small active clusters serves to assert the significance of our conclusions. Contact with experiment is made by calculating absorption energies. These are in excellent agreement with the experimental data for the most realistic active clusters considered.

cond-mat.mtrl-sci↗