arXiv subjects

F. A. Gianturco

Publications and source records attributed to F. A. Gianturco.

29 records · Page 2Linked to original sources

arXiv2009

Fast LiH destruction in reaction with H: quantum calculations and astrophysical consequences

We present a quantum-mechanical study of the exothermic 7LiH reaction with H. Accurate reactive probabilities and rate coefficients are obtained by solving the Schrodinger equation for the motion of the three nuclei on a single Born-Oppenheimer potential energy surface (PES) and using a coupled-channel hyperspherical coordinate method. Our new rates indeed confirm earlier, qualitative predictions and some previous theoretical calculations, as discussed in the main text. In the astrophysical domain we find that the depletion process largely dominates for redshift (z) between 400 and 100, a range significant for early Universe models. This new result from first-principle calculations leads us to definitively surmise that LiH should be already destroyed when the survival processes become important. Because of this very rapid depletion reaction, the fractional abundance of LiH is found to be drastically reduced, so that it should be very difficult to manage to observe it as an imprinted species in the cosmic background radiation (CBR). The present findings appear to settle the question of LiH observability in the early Universe. We further report several state-to-state computed reaction rates in the same range of temperatures of interest for the present problem.

astro-ph.CO↗

arXiv2007

Correlation-Polarization Effects in Electron/Positron Scattering from Acetylene: A Comparison of Computational Models

Different computational methods are employed to evaluate elastic (rotationally summed) integral and differential cross sections for low energy (below about 10 eV) positron scattering off gas-phase C$_2$H$_2$ molecules. The computations are carried out at the static and static-plus-polarization levels for describing the interaction forces and the correlation-polarization contributions are found to be an essential component for the correct description of low-energy cross section behavior. The local model potentials derived from density functional theory (DFT) and from the distributed positron model (DPM) are found to produce very high-quality agreement with existing measurements. On the other hand, the less satisfactory agreement between the R-matrix (RM) results and measured data shows the effects of the slow convergence rate of configuration-interaction (CI) expansion methods with respect to the size of the CI-expansion. To contrast the positron scattering findings, results for electron-C$_2$H$_2$ integral and differential cross sections, calculated with both a DFT model potential and the R-matrix method, are compared and analysed around the shape resonance energy region and found to produce better internal agreement.

physics.chem-ph↗

arXiv2007

Collisional Quenching at Ultralow Energies: Controlling Efficiency with Internal State Selection

Calculations have been carried out for the vibrational quenching of excited H$_2$ molecules which collide with Li$^+$ ions at ultralow energies. The dynamics has been treated exactly using the well known quantum coupled-channel expansions over different initial vibrational levels. The overall interaction potential has been obtained from the calculations carried out earlier in our group using highly correlated ab initio methods. The results indicate that specific features of the scattering observables, e.g. the appearance of Ramsauer-Townsend minima in elastic channel cross sections and the marked increase of the cooling rates from specific initial states, can be linked to potential properties at vanishing energies (sign and size of scattering lengths) and to the presence of either virtual states or bound states. The suggestion is made that by selecting the initial state preparation of the molecular partners, the ionic interactions would be amenable to controlling quenching efficiency at ultralow energies.

physics.chem-ph↗

arXiv2007

Density Functional Theory for the Photoionization Dynamics of Uracil

Photoionization dynamics of the RNA base Uracil is studied in the framework of Density Functional Theory (DFT). The photoionization calculations take advantage of a newly developed parallel version of a multicentric approach to the calculation of the electronic continuum spectrum which uses a set of B-spline radial basis functions and a Kohn-Sham density functional hamiltonian. Both valence and core ionizations are considered. Scattering resonances in selected single-particle ionization channels are classified by the symmetry of the resonant state and the peak energy position in the photoelectron kinetic energy scale; the present results highlight once more the site specificity of core ionization processes. We further suggest that the resonant structures previously characterized in low-energy electron collision experiments are partly shifted below threshold by the photoionization processes. A critical evaluation of the theoretical results providing a guide for future experimental work on similar biosystems.

physics.chem-ph↗

arXiv2007

Rotational cooling efficiency upon molecular ionization: the case of Li$_2(a^3Σ_u^+)$ and Li$_2^+(X^2Σ_g^+)$ interacting with $^4$He

The low-temperature (up to about 100K) collisional (de)excitation cross sections are computed using the full coupled-channel (CC) quantum dynamics for both Li$_2$ and Li$_2^+$ molecular targets in collision with $^4$He. The interaction forces are obtained from fairly accurate {\it ab initio} calculations and the corresponding pseudo-rates are also computed. The results show surprising similarities between sizes of inelastic flux distributions within final states in both systems and the findings are connected with the structural change in the molecular rotor features when the neutral species is replaced by its ionic counterpart.

physics.chem-ph↗

arXiv2007

Microsolvation of cationic dimers in $^4$He droplets: geometries of A$_2^+$(He)$_N$ (A=Li,Na,K) from optimized energies

Ab initio computed interaction forces are employed in order to describe the microsolvation of the A$_2^+(^2Σ)$ (A=Li,Na,K) molecular ion in $^4$He clusters of small variable size. The minimum energy structures are obtained by performing energy minimization based on a genetic algorithm approach. The symmetry features of the collocation of solvent adatoms around the dimeric cation are analyzed in detail, showing that the selective growth of small clusters around the two sides of the ion during the solvation process is a feature common to all three dopants.

physics.chem-ph↗

arXiv2007

Bosonic Helium droplets with cationic impurities: onset of electrostriction and snowball effects from quantum calculations

Variational MonteCarlo and Diffusion MonteCarlo calculations have been carried out for cations like Li$^+$, Na$^+$ and K$^+$ as dopants of small helium clusters over a range of cluster sizes up to about 12 solvent atoms. The interaction has been modelled through a sum-of-potential picture that disregards higher order effects beyond atom-atom and atom-ion contributions. The latter were obtained from highly correlated ab-initio calculations over a broad range of interatomic distances. This study focuses on two of the most striking features of the microsolvation in a quantum solvent of a cationic dopant: electrostriction and snowball effects. They are here discussed in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the clusters features.

physics.chem-ph↗

arXiv2007

Electron Scattering from Gaseous Ocs($^1Σ$): Comparing Computed Angular Distributions and Elastic Cross Sections with Experiments

Differential cross sections are computed for the title polar molecule using static interaction, exchange forces and correlation-polarisation effects as described in detail in the main text. The dipole effect is also reported via the dipole Born correction procedure and the final angular distributions are compared with existing experimental data. The shape and location of the prominent low-energy resonance are computed and compared with experiments. The comparison shows that the present treatment of the interaction forces and of the quantum dynamics can indeed afford good agreement between measured and computed quantities for a multielectron target as OCS.

physics.chem-ph↗

arXiv2005

Microsolvation of Li$^+$ in small He Clusters. Li$^+$He$_n$ species from Classical and Quantum Calculations

A structural study of the smaller Li$^+$He$_n$ clusters with $n\le30$ has been carried out using different theoretical methods. The structures and the energetics of the clusters have been obtained using both classical energy minimization methods and quantum Diffusion Monte Carlo. The total interaction acting within the clusters has been obtained as a sum of pairwise potentials: Li$^+$-He and He-He. This approximation had been shown in our earlier study \cite{8} to give substantially correct results for energies and geometries once compared to full ab-initio calculations. The general features of the spatial structures, and their energetics, are discussed in details for the clusters up to $n=30$ and the first solvation shell is shown to be essentially completed by the first ten helium atoms.

physics.chem-ph↗

arXiv2005

Neutral and ionic dopants in helium clusters: interaction forces for the $Li_2(a^3Σ_u^+)-He$ and $Li_2^+(X^2Σ_g^+)-He$

The potential energy surface (PES) describing the interactions between $\mathrm{Li_{2}(^{1}Σ_{u}^{+})}$ and $\mathrm{^{4}He}$ and an extensive study of the energies and structures of a set of small clusters, $\mathrm{Li_{2}(He)_{n}}$, have been presented by us in a previous series of publications [1-3]. In the present work we want to extend the same analysis to the case of the excited $\mathrm{Li_{2}}(a^{3}Σ_{u}^{+})$ and of the ionized Li$_{2}^{+}(X^{2}Σ_{g}^{+})$ moiety. We thus show here calculated interaction potentials for the two title systems and the corresponding fitting of the computed points. For both surfaces the MP4 method with cc-pV5Z basis sets has been used to generate an extensive range of radial/angular coordinates of the two dimensional PES's which describe rigid rotor molecular dopants interacting with one He partner.

physics.chem-ph↗

arXiv2002

Transition from molecular complex to quantum solvation in OCS(He)_N

We present quantum calculations of the rotational energy levels and spectroscopic rotational constants of the linear OCS molecule in variable size clusters of 4He, using spectral evolution quantum Monte Carlo methods that allow excited states to be accessed without nodal constraints. The rotational constants of OCS are found to decrease monotonically from the gas phase value as the number of helium atoms increases to N=6, after which the average constant increases to saturation at a value in excellent agreement with experimental measurements made on significantly larger clusters (N>1000). The minimum is shown to indicate a transition from a molecular complex to a quantum solvated molecule, with the former characterized by floppy but near rigid behavior, while the latter is characterized by non-zero permutation exchanges and a smaller extent of rigid coupling.

cond-mat↗