164 research outputs found
Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals
Based on an accurate first principles description of the energetics in
H-bonded KDP, we conduct a first study of nuclear quantum effects and of the
changes brought about by deuteration. Cluster tunneling involving also heavy
ions is allowed, the main effect of deuteration being a depletion of the proton
probability density at the O-H-O bridge center, which in turn weakens its
proton-mediated covalent bonding. The ensuing lattice expansion couples
selfconsistently with the proton off-centering, thus explaining both the giant
isotope effect, and its close connection with geometrical effects.Comment: 4 two-column pages, 4 figure
Molecular effects in the ionization of N, O and F by intense laser fields
In this paper we study the response in time of N, O and F to
laser pulses having a wavelength of 390nm. We find single ionization
suppression in O and its absence in F, in accordance with experimental
results at nm. Within our framework of time-dependent density
functional theory we are able to explain deviations from the predictions of
Intense-Field Many-Body -Matrix Theory (IMST). We confirm the connection of
ionization suppression with destructive interference of outgoing electron waves
from the ionized electron orbital. However, the prediction of ionization
suppression, justified within the IMST approach through the symmetry of the
highest occupied molecular orbital (HOMO), is not reliable since it turns out
that, e.g. in the case of F, the electronic response to the laser pulse is
rather complicated and does not lead to dominant depletion of the HOMO.
Therefore, the symmetry of the HOMO is not sufficient to predict ionization
suppression. However, at least for F, the symmetry of the dominantly
ionized orbital is consistent with the non-suppression of ionization.Comment: 19 pages, 5 figure
Inelastic electron injection in a water chain
Irradiation of biological matter triggers a cascade of secondary particles
that interact with their surroundings, resulting in damage. Low-energy
electrons are one of the main secondary species and electron-phonon interaction
plays a fundamental role in their dynamics. We have developed a method to
capture the electron-phonon inelastic energy exchange in real time and have
used it to inject electrons into a simple system that models a biological
environment, a water chain. We simulated both an incoming electron pulse and a
steady stream of electrons and found that electrons with energies just outside
bands of excited molecular states can enter the chain through phonon emission
or absorption. Furthermore, this phonon-assisted dynamical behaviour shows
great sensitivity to the vibrational temperature, highlighting a crucial
controlling factor for the injection and propagation of electrons in water
Ab initio determination of ion-traps in silver-doped chalcogenide glass
We present a microscopic picture of silver dynamics in GeSe:Ag glass
obtained from {\it ab initio} simulation. The dynamics of Ag is explored at two
temperatures, 300K and 700K. In the relaxed network, Ag occupies bond centers
between suitably separated host sites. At 700K, Ag motion proceeds via a
trapping-release dynamics, between "super traps" or cages consisting of
multiple bond-center sites in a small volume. Our work offers a first
principles identification of trapping centers invoked in current theories, with
a description of their properties. We compute the charge state of the Ag in the
network, and show that it is neutral if weakly bonded and Ag if in a
trapping center.Comment: submitted to pr
Dipole-quadrupole interactions and the nature of phase III of compressed hydrogen
A new class of strongly infrared active structures is identified for phase
III of compressed molecular H2 by constant-pressure ab initio molecular
dynamics and density-functional perturbation calculations. These are planar
quadrupolar structures obtained as a distortion of low-pressure quadrupolar
phases, after they become unstable at about 150 GPa due to a zone-boundary soft
phonon. The nature of the II-III transition and the origin of the IR activity
are rationalized by means of simple electrostatics, as the onset of a
stabilizing dipole-quadrupole interaction.Comment: 4 pages, 3 figures. To appear in Phys. Rev. Let
Water radiolysis by low-energy carbon projectiles from first-principles molecular dynamics
Water radiolysis by low-energy carbon projectiles is studied by first-principles molecular dynamics. Carbon projectiles of kinetic energies between 175 eV and 2.8 keV are shot across liquid water. Apart from translational, rotational and vibrational excitation, they produce water dissociation. The most abundant products are H and OH fragments. We find that the maximum spatial production of radiolysis products, not only occurs at low velocities, but also well below the maximum of energy deposition, reaching one H every 5 Ã… at the lowest speed studied (1 Bohr/fs), dissociative collisions being more significant at low velocity while the amount of energy required to dissociate water is constant and much smaller than the projectile’s energy. A substantial fraction of the energy transferred to fragments, especially for high velocity projectiles, is in the form of kinetic energy, such fragments becoming secondary projectiles themselves. High velocity projectiles give rise to well-defined binary collisions, which should be amenable to binary approximations. This is not the case for lower velocities, where multiple collision events are observed. H secondary projectiles tend to move as radicals at high velocity, as cations when slower. We observe the generation of new species such as hydrogen peroxide and formic acid. The former occurs when an O radical created in the collision process attacks a water molecule at the O site. The latter when the C projectile is completely stopped and reacts with two water molecules.EA ackowledges financial support from the following grants: Electron-Stopping, from the European Commission under the Marie-Curie CIG, program; FIS2012-37549-C05 from the Spanish Ministry of Science; and Exp. 97/14 (Wet Nanoscopy) from the Programa Red Guipuzcoana de Ciencia, TecnologÃa e Innovación, Diputación Foral de Gipuzkoa. Simulations were performed in the High Performance Computer at the University of Cambridge, and at the HECToR facility under the UKCP consortium EP/F037325/1
Nonadiabatic forces in ion-solid interactions: the initial stages of radiation damage
The Born-Oppenheimer approximation is the keystone for molecular dynamics simulations of radiation damage processes; however, actual materials response involves nonadiabatic energy exchange between nuclei and electrons. In this work, time dependent density functional theory is used to calculate the electronic excitations produced by energetic protons in Al. We study the influence of these electronic excitations on the interatomic forces and find that they differ substantially from the adiabatic case, revealing a nontrivial connection between electronic and nuclear stopping that is absent in the adiabatic case. These results unveil new effects in the early stages of radiation damage cascades
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