Fermion Molecular Dynamics Research Articles

Classical and semiclassical description of initial distributions of kaon capture by atomic hydrogen and deuterium

The moderation and capture of negative kaons by atomic hydrogen and deuterium were investigated by the classical trajectory Monte–Carlo (CTMC) and the semiclassical fermion molecular dynamics (FMD) methods. The dependence of ionization and capture cross sections on initial kaon energy was also studied. The initial populations of kaonic atom levels were calculated. The n distributions of kaonic atoms peaked close to the orbital giving optimum overlap with the displaced electronic orbital. The angular momentum distributions, l, are found to be approximately statistical but cut off at large l smaller than lmax = n − 1 in large n. The results are compared with the adiabatic ionization, diabatic states, and Born approximation methods. The FMD results were found to be in better agreement with quantum mechanical calculations than the ones from CTMC. The kaon kinetic energy spectrum prior to capture was calculated, which reveals that capture occurs at small collision energies up to the ionization energy. Also, the calculations show that kaonic hydrogen (deuterium) atoms have kinetic energies below 0.3 a.u. (0.15 a.u.) after formation.

Classical simulations of electron emissions from H2+ by circularly polarized laser pulses

With the classical fermion molecular dynamics model (FMD), we investigated electron emissions from H(2)(+) by circularly polarized laser pulses. The obtained electron momentum distribution clearly shows an angular shift relative to the expected direction for H(2)(+) aligned parallel to the polarization plane, which is in good agreement with the recent experimental result. By tracing the classical trajectory, we provide direct evidence for the electron delayed emission with respect to the instant when the electric field is parallel to the molecular axis, which was regarded as the origin of the angular shift in the electron momentum distribution. Furthermore, we find that the angular shift decreases with increasing the laser wavelength.

Quantum dynamics with fermion coupled coherent states: Theory and application to electron dynamics in laser fields

We present an alternate version of the coupled-coherent-state method, specifically adapted for solving the time-dependent Schr\odinger equation for multielectron dynamics in atoms and molecules. This theory takes explicit account of the exchange symmetry of fermion particles, and it uses fermion molecular dynamics to propagate trajectories. As a demonstration, calculations in the He atom are performed using the full Hamiltonian and accurate experimental parameters. Single- and double-ionization yields by 160-fs and 780-nm laser pulses are calculated as a function of field intensity in the range 10${}^{14}$--10${}^{16}$ W/cm${}^{2}$, and good agreement with experiments by Walker et al. is obtained. Since this method is trajectory based, mechanistic analysis of the dynamics is straightforward. We also calculate semiclassical momentum distributions for double ionization following 25-fs and 795-nm pulses at 1.5\ifmmode\times\else\texttimes\fi{}10${}^{15}$ W/cm${}^{2}$, in order to compare them with the detailed experiments by Rudenko et al. For this more challenging task, full convergence is not achieved. However, major effects such as the fingerlike structures in the momentum distribution are reproduced.

Electrical conductivity of a boron plasma through the OCP crystallization

We calculate the DC conductivity of a boron plasma along the 1 eV isotherm up to 25 times the normal density. We use both the quantum and Thomas Fermi molecular dynamics coupled with, respectively, the Kubo–Greenwood formulation and the semi-classical Ziman theory. We find that the DC conductivity obtained using a full quantum mechanical treatment exhibits a significant jump at the one component plasma phase transition – specifically the OCP crystallization of the ions – that is not reproduced using the semi-classical Ziman description. This difference – reaching up to a factor of four – remains well beyond the phase transition and up to the highest density explored. This shows that a full quantum mechanical treatment of the optical and electrical quantities is required in this regime even if semi-classical theories are reliable to obtain both the thermodynamical, and the ionic dynamical and structural properties.

Angular distributions of antiprotons elastically and inelastically scattered by helium

Angular differential cross sections are calculated for antiprotons elastically and inelastically scattered by the helium atom. The elastic cross sections are calculated quantum mechanically using both the adiabatic potential and a phenomenological optical potential. The absorptive effects, taken into account by the optical potential, are found to significantly suppress large-angle elastic scattering. Inelastic scattering (excitation and ionization) is treated by the quasiclassical fermion molecular dynamics method. The inelastic scattering is largely in the backward direction at laboratory collision energies within $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ of threshold, but moves to smaller angles as the energy increases.

Reactive collisions of atomic antihydrogen with the H2 and H2+ molecules

The fermion molecular dynamics (FMD) method is used to determine the protonium (Pn) formation and total destruction cross sections for collisions of antihydrogen with the H2 molecule and the H2+ molecular ion at collision energies above 0.1 au in the centre-of-mass system. The cross sections and initial quantum numbers are compared with the analogous cross sections for and previously calculated. Like the projectile, the protonium-formation cross sections for the projectile are much larger and extend to higher energies with the molecular targets than with the atomic target. The possibility is considered that a relatively long-lived state of the molecule may be formed in rearrangement scattering of at low energies.

Reactive collisions of atomic antihydrogen with H, He+ and He

The fermion molecular dynamics (FMD) method is used to determine the rearrangement and destruction cross sections for collisions of antihydrogen with H, He+ and He at collision energies above 0.1 au. The results for the H and He+ targets satisfactorily merge with previous calculations done for lower collision energies. Despite the absence of a critical distance, the destruction cross section for collisions of with He, previously uncalculated, is found to be comparable with the destruction cross sections for collisions with H and He+. All three cross sections are shown to be given quite reasonably by simple classical orbiting formulae at energies that are very low but still high enough for L > 0 partial waves to be dominant. The cross sections for formation of the antiprotonic atoms (Pn or ) and their initial quantum numbers with the projectile are found to be significantly different from the analogous cross sections for projectiles.

Dissociation and ionization in capture of antiprotons and negative muons by the hydrogen molecular ion

Cross sections and initial quantum number distributions are calculated for capture of the antiproton and negative muon (μ−) by the hydrogen molecular ion H+2 using the fermion molecular dynamics method. The capture of is found to be almost entirely adiabatic, occurring via target dissociation without ionization, but nonadiabatic effects are found to play a significant role in the capture of μ−, especially at the higher capture energies. Generally good agreement is obtained with the recent adiabatic classical-trajectory Monte Carlo calculation of Sakimoto (2004 J. Phys. B: At. Mol. Opt. Phys. 37 2255). The capture properties of H+2 are shown to be completely different from those of both the H atom and neutral H2 molecule.

Capture of antiprotons by some radioactive atoms and ions

Cross sections for antiproton capture are calculated using the fermion molecular dynamics method for ions of current interest in experiments determining nuclear structure of the radioactive nuclei ${}^{8}\mathrm{He},$ ${}^{11}\mathrm{Li},$ ${}^{11}\mathrm{Be},$ and ${}^{21}\mathrm{Mg}.$ The cross sections for the corresponding neutral atoms are also calculated. It is found that, except for helium, the cross sections for the ion and neutral atom at usual capture energies are similar, i.e., neither the enhanced trajectory curvature nor the absence of the most weakly bound electron have great effect. The behavior of the cross sections is also analyzed at very low collision energies, where the ion cross sections go as $1/E$ and the neutral cross sections as $1/\sqrt{E}.$

Introduction to the quantum trajectory method and to Fermi molecular dynamics

The quantum trajectory method (QTM) will be introduced, and an approximation to the QTM known as Fermi molecular dynamics (FMD) will be described. Results of simulations based on FMD will be mentioned for specific nonequilibrium systems dominated by Coulomb interactions.

Capture of negative muons and antiprotons by noble-gas atoms

Cross sections for capture of negative muons (μ - ) and antiprotons (p) by helium, neon. argon, krypton, and xenon atoms are calculated using the fermion molecular dynamics method. These cross sections are used to estimatethe capture ratios in mixtures.

Photoionization of atoms described by Fermi Molecular Dynamics: toward a firmer theoretical basis

The application of Fermi Molecular Dynamics (FMD) to the modeling of the photoionization of atoms by a short pulse of long wavelength laser radiation is examined in detail. Depression of the single ionization threshold to values of the electric field strength below the classical over-the-barrier threshold, a common occurrence in FMD, is shown to arise from the preexcitation of bound electrons into a continuum of unphysical low-lying excited states. A connection is made to analogous calculations performed with the quantum Hamilton-Jacobi equation, in which the time-dependent quantum potential ($Q$) plays a role similar to that played in FMD by the so-called Heisenberg potential (V H). Replacement of (V H) by Q in the FMD equations of motion results in a large reduction in the number of excitations to unphysical bound states, while producing no essential change in the photoionization probability.

Preliminary Results for Capture of Negative Muons and Antiprotons by Noble-Gas Atoms

Cross sections for capture of negative muons (μ−) and antiprotons ( $${\bar p}$$ ) by helium, neon, argon, krypton, and xenon atoms (incomplete for the two heaviest noble-gas atoms) are calculated using the fermion molecular dynamics (FMD) method. These cross sections are used to estimate the capture ratios in mixtures, but these ratios are not precise since the total energy-loss cross sections have not yet been determined.

Multiple ionization of argon atoms by long-wavelengthlaser radiation: a fermion molecular dynamicssimulation

We applied the method known as fermion molecular dynamics to thedescription of an isolated argon atom interacting with a shortpulse of intense, long-wavelength laser radiation, for both linearand circular polarization. We discuss the results of these calculations,insofar as they bear on the mechanisms leading to the production ofhigh-ionic-charge states during laser-atom interaction.

Negative pion capture in HD gas and inH2+D2gas mixtures: Resolution of the isotope puzzle

The cross sections, initial quantum numbers, and kinetic energies for pionic atoms formed by negative pion capture in mixtures of isotopic hydrogen molecules are calculated using the fermion-molecular-dynamics (FMD) method. With these cross sections, the reduced capture ratio for a ${\mathrm{H}}_{2}+{\mathrm{D}}_{2}$ mixture is found to be ${(P}_{p}^{({\mathrm{H}}_{2}{+\mathrm{D}}_{2})}{/P}_{d}^{({\mathrm{H}}_{2}{+\mathrm{D}}_{2})}{)/(c}_{p}{/c}_{d})=1.204$, and the capture ratio for HD is found to be ${P}_{p}^{(\mathrm{HD})}{/P}_{d}^{(\mathrm{HD})}=0.875$. In light of these results, the p-to-$d$ pion transfer probabilities Q are reevaluated using prior experimental data and determined to be larger than previously thought: $Q=0.28$ at deuterium fraction ${c}_{d}=0.5$ and $Q=0.42$ as ${c}_{d}\ensuremath{\rightarrow}1$. The puzzling relationship of the experimental data for HD to that for ${\mathrm{H}}_{2}+{\mathrm{D}}_{2}$ mixtures is explained.

Isotope effects on antiproton and muon capture by hydrogen and deuterium atoms and molecules

Cross sections for capture of the antiproton $(\overline{p})$ and negative muon $({\ensuremath{\mu}}^{\ensuremath{-}})$ by the ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ molecules are calculated using fermion molecular dynamics (FMD). All the cross sections are significantly larger than those for capture by the corresponding atom, also evaluated by the FMD method. The largest molecular cross sections are obtained when the negative projectile mass best matches the nuclear mass in the molecular target, thus for $\overline{p}+{\mathrm{H}}_{2}.$ The vibrational degree of freedom is shown to be most important in distinguishing the four reactions, but the effects of rotations, two-center electronic charge distribution, and nonadiabaticity are also significant. The predicted initial capture fractions (i.e., not taking subsequent transfer into account) in a ${\mathrm{H}}_{2}+{\mathrm{D}}_{2}$ mixture are ${P}_{\mathrm{capt}}^{(p)}{/P}_{\mathrm{capt}}^{(d)}{=qc}_{p}{/c}_{d},$ where $q=1.585$ for $\overline{p}$ and $q=1.186$ for ${\ensuremath{\mu}}^{\ensuremath{-}}$ independent of ${c}_{p}$ and ${c}_{d}.$ The energy-dependent quantum-number distributions of the exotic atoms formed, the angular distributions of antiprotonic atoms, and the initial kinetic energies of muonic atoms are also presented.

Quasiclassical modelling of helium double photoionization

We applied the method known as fermion molecular dynamics (FMD) to the description of a helium atom interacting with a short pulse of intense, long-wavelength laser radiation, for both linear and circular polarization. We describe the results of these calculations, insofar as they bear on the question of the mechanisms leading to double electron ejection. In the case of linear polarization, boomeranging trajectories leading to double ionization were observed at all laser intensities above threshold. The probability of occurrence was low, and almost independent of laser intensity. However, very near to threshold, boomeranging trajectories leading to double ionization were found to occur with a probability comparable to that for all other independent electron (sequential) processes. This produced a shoulder in the curve of double ionization probability versus laser intensity. The size of this shoulder was found to depend on laser wavelength and pulse length. No such trajectories were found for circular polarization.

Multiple ionization of helium clusters by long wavelength laser radiation

We applied the method known as Fermion Molecular Dynamics to the description of a small cluster of helium atoms interacting with a short pulse of intense, long wavelength laser radiation, for both linear and circular polarization. We discuss the results of these calculations, elucidating questions relating to the mechanisms of multiple ionization in small clusters.

Fermion molecular dynamics in atomic, molecular, and optical physics

Classical dynamics, often called 'molecular dynamics' when applied to atoms and molecules, is much easier than solving the many-body Schrodinger equation for a number of reasons. In particular, correlation and rearrangement are simple in classical dynamics. Fermion molecular dynamics (FMD) is a quasi-classical method for treating quantum-mechanical systems using classical equations of motion with momentum-dependent model potentials added to the usual Hamiltonian. These model potentials constrain the motion to satisfy the Heisenberg uncertainty and the Pauli exclusion principles. We discuss the foundations of the FMD model and its applications to atomic and molecular structure, ion-atom collisions, stopping powers, formation of antiprotonic atoms, and multiple ionization of atoms in strong laser fields.

Molecular effects on antiproton capture byH2and the states ofp¯pformed

Complete five-body dynamical calculations of antiproton $(\overline{p})$ capture by the hydrogen molecule $({\mathrm{H}}_{2})$ have been carried out using a generalization of the Kirschbaum-Wilets method (belonging to a class of quasiclassical methods sometimes called fermion molecular dynamics). The differences between capture by ${\mathrm{H}}_{2}$ and the H atom are found to be dramatic. The effects due to the two-center structure, rotational motions, and vibrational motions are distinguished. Of particular importance, the vibrational degree of freedom enables the molecule to capture antiprotons having lab energies above 100 eV, whereas atomic capture cuts off sharply above the ionization threshold of 27 eV (in the lab system). Antiproton capture by the atom is calculated by the same method as well as by the classical-trajectory Monte Carlo method, which is applicable only to the atom. The initial quantum numbers (assigned quasiclassically) of the $\overline{p}p$ formed are found to be shifted to significantly smaller values for the molecular target; the $n$ distribution is also narrower for the molecular target as compared with the atomic target.