Search Results (48 total)

Electron-impact ionization cross sections for Li₂ and Li₂⁺ are calculated using a configuration-average distorted-wave method. Bound orbitals for the molecule and its ions are calculated using a single-configuration self-consistent-field method based on a linear combination of Slater-type orbitals. The bound orbitals are transformed onto a two-dimensional lattice (r,θ), which is variable in the radial coordinate and constant in the angular coordinate, from which Hartree with local exchange potentials are constructed. The single-particle Schrödinger equation is then solved for continuum distorted waves with S-matrix boundary conditions. Total ionization cross sections for Li₂ at an equilibrium internuclear separation of R=5.0  a.u. and for Li₂⁺ at an equilibrium internuclear separation of R=5.9  a.u. are presented.

A comprehensive study is made of the energy relaxation rates between ions and electrons in a dense hydrogen plasma. Results of classical molecular dynamics (MD) simulations are compared with quantal calculations using the Fermi golden rule and using dimensional continuation. The rates from the molecular dynamics simulations employing a screened potential are found to be in reasonable agreement with the Landau-Spitzer relaxation rates, and are around 30% higher than the Fermi golden rule rates. By inverting the classical MD relaxation rate vs the quantal result, a semiclassical value for the screening length is suggested. We present energy relaxation rates relevant for radiation-hydrodynamic simulations of inertial confinement fusion devices.

Electron-impact ionization cross sections for atoms in high-temperature dense plasmas are calculated in a configuration-average distorted-wave method. For those conditions in which the Coulomb coupling parameter is small, we use the Debye-Hückel potential to screen the projectile electron from the nucleus and target electrons. Ionization cross sections are calculated for both Ne⁴⁺ and Au⁴⁷⁺ at high temperatures and various electron densities up to 1.0×10²⁵ cm⁻³. In general we find that as the density increases, the ionization cross section decreases at all incident energies. We also find that as the charge on the atomic ion becomes larger, the relative density effect becomes smaller.

Electron-impact excitation and ionization cross sections are presented for all atomic ions in the Si, Cl, and Ar isonuclear sequences. These data contribute to the continuing effort to provide accurate collisional data for magnetic fusion and astrophysical modeling. For excitation processes, level-resolved cross section calculations are presented which were made using first-order many-body perturbation theory. For ionization processes, we present calculations made with the configuration-average approximation using a distorted-wave method. A selection of excitation and ionization cross sections are compared with experiment, where available. For ionization of Si²⁺ and Si³⁺, we also compare with calculations made using time-dependent close-coupling theory, to assess the accuracy of the distorted-wave calculations. Our cross sections will be tabulated in several atomic collision databases for use in future kinetics modeling efforts.

A physical interpretation is given for the variation with internuclear separation of the fully differential cross section for double photoionization of H₂. This effect is analyzed in a geometry where the fourbody interaction is completely probed. Excellent agreement is found between experiment and time-dependent close-coupling theory after convoluting the latter over the relevant solid angles. We show the observed variations are purely due to the ε_Σ component of the polarization vector ε along the molecular axis, a conclusion which is supported through calculations of the photoionization of H₂⁺.

We present total cross sections for single and double ionization of helium by antiproton impact over a wide range of impact energies from 10  keV/amu to 1  MeV/amu. A nonperturbative time-dependent close-coupling method is applied to fully treat the correlated dynamics of the ionized electrons. Excellent agreement is obtained between our calculations and experimental measurements of total single and double ionization cross sections at high impact energies, whereas for lower impact energies, some discrepancies with experiment are found. At an impact energy of 1 MeV we also find that the double-to-single ionization ratio is twice as large for antiproton impact as for proton impact, confirming a long-standing unexpected experimental measurement.

Electron-impact double-ionization cross sections for the helium atom are calculated by direct solution of a nine-dimensional Schrödinger equation using a time-dependent close-coupling method. Previous calculations are extended to higher incident electron energies using a simple factor-of-2 reduction in the mesh spacing for the three-dimensional radial lattice. The recent calculations, computationally more than an order of magnitude more difficult, are found to be in good agreement with experiment from threshold to beyond the peak of the cross section at 275  eV.

Electron-impact ionization cross sections for diatomic molecules are calculated in a configuration-average distorted-wave method. Core bound orbitals for the molecular ion are calculated using a single-configuration self-consistent-field method based on a linear combination of Slater-type orbitals. The core bound orbitals are then transformed onto a two-dimensional (r,θ) numerical lattice from which a Hartree potential with local exchange is constructed. The single-particle Schrödinger equation is then solved for the valence bound orbital and continuum distorted-wave orbitals with S-matrix boundary conditions. Total cross section results for H₂ and N₂ are compared with those from semiempirical calculations and experimental measurements.

A study is made of the differential cross sections arising from the photoionization of H₂⁺. Previous studies indicated surprising differences in the shapes of the angular distributions calculated from exterior complex scaling and 2C methods. To further explore these differences, we have calculated the angular distributions from the photoionization of H₂⁺ using an independent two-body Coulomb function (2C) method and a distorted wave approach. As a final test, we also present calculations using a time-dependent technique. Our results confirm the discrepancies found previously and we present possible reasons for these differences.

Triple differential cross sections arising from the break up of the H₂ molecule by a single photon are presented. The time-dependent close-coupling technique is used to calculate differential cross sections for various geometries. Excellent agreement is found between current work and recent exterior complex-scaling calculations, confirming, for the first time, the absolute magnitude of the triple differential cross sections. Our calculations also compare favorably with recent synchrotron light source measurements.

A finite-temperature random-phase approximation (FTRPA) is applied to calculate oscillator strengths for excitations in hot and dense plasmas. Application of the FTRPA provides a convenient, self-consistent method with which to explore coupled-channel effects of excited electrons in a dense plasma. We present FTRPA calculations that include coupled-channel effects. The inclusion of these effects is shown to cause significant differences in the oscillator strength for a prototypical case of ¹P excitation in neon when compared with single-channel and with average-atom calculations. Trends as a function of temperature and density are also discussed.

Double- and triple-differential cross sections are presented for the electron-impact ionization of ground-state hydrogen at incident electron energies of 15.6 and 17.6  eV. The time-dependent close-coupling method is used to calculate the differential ionization cross sections, and comparisons are made with previous theoretical calculations and with experimental measurements. Excellent agreement is obtained between our calculations and previous work.

The double autoionization rate for the He⁻(2s²2p  ²P) hollow atom state is calculated using a nonperturbative time-dependent close-coupling method. The nine-dimensional wave function for the three electron atom is expanded in coupled spherical harmonics. The time-dependent Schrödinger equation is then reduced to a set of close-coupled partial differential equations for the three-dimensional radial expansion functions. Relaxation in imaginary time subject to orthogonality constraints yields a 2s²2p  ²P hollow atom state with real energy, while propagation in real time yields total and double Auger rates. The total Auger rate is in reasonable agreement with previous saddle-point complex-rotation calculations. The double Auger rate is found to be 10% of the total Auger rate, in keeping with the relatively large double to total ratio found in recent experiments for He⁻(2s2p^{2  4}P).

A study of inner-shell electron-impact ionization of heavy neutral atoms is presented. A relativistic distorted-wave method is used to calculate K-shell ionization of neutral Mn, Fe, Ni, and Cu, and also the L-shell ionization of neutral W. These calculations are compared with measurements made by electron-impact ionization from a thin target of the atomic species in question. Good agreement is found between the calculations and measurements.

Electron-impact ionization cross sections for H₂ are calculated using a nonperturbative time-dependent close-coupling method. In a standard frozen-core approximation, the six-dimensional wave function for the valence target electron and the incident projectile electron is expanded in products of rotational functions. The time-dependent Schrödinger equation for the two-electron system is then reduced to a set of close-coupled partial differential equations for the four-dimensional expansion functions in (r₁,θ₁,r₂,θ₂) center-of-mass spherical polar coordinates. The nonperturbative close-coupling results are found to be over a factor of 2 lower than perturbative distorted-wave results, but in excellent agreement with experimental measurements, at incident electron energies near the peak of the total integrated cross section.

A study is made of low-energy electron-impact single ionization of ground-state helium. The time-dependent close-coupling method is used to calculate total integral, single differential, double differential, and triple differential ionization cross sections for impact electron energies ranging from 32 to 45  eV. For all quantities, the calculated cross sections are found to be in very good agreement with experiment, and for the triple differential cross sections, good agreement is also found with calculations made using the convergent close-coupling technique.

We present calculations for the double photoionization (with excitation) and the triple photoionization of Li and Be. We extend and more fully discuss the previous calculations made for Li by Colgan [Phys. Rev. Lett. 93, 053201 (2004)] and present calculations for Be. The Be triple photoionization cross sections are compared with previous double shake-off model calculations of Kheifets and Bray [J. Phys. B 36, L211 (2003)], and our calculations are found to be significantly lower.

A time-dependent close-coupling method for three-electron atomic systems is formulated to calculate the double autoionization of hollow-atom states. Initial excited states are obtained by relaxation of the Schrödinger equation in imaginary time, while autoionization rates are obtained by propagation in real time. A 12-coupled-channels nonperturbative calculation on a three-dimensional radial lattice yields a double-autoionization rate for the Li(2s²2p)→Li²⁺(1s)+2e⁻ transition that that is somewhat smaller than earlier many-body perturbation theory calculations and in reasonable agreement with rates extracted from resonance profiles found in more recent γ+Li experiments.

A configuration-average distorted-wave method is developed to calculate electron-impact excitation and ionization cross sections for diatomic molecules and their ions. The method is based on the construction of bound and continuum orbitals on a two-dimensional numerical lattice in (r,θ) center-of-mass polar coordinates. Our first applications are the calculation of 1sσ→2pσ and 1sσ→2pπ excitation cross sections and 1sσ→ϵlλ ionization cross sections for H₂⁺. Comparisons are made with plane-wave Born, distorted-wave, and R-matrix calculations, as well as experimental measurements.

Symmetrized complex amplitudes for the double photoionization of helium are computed by the time-dependent close-coupling and exterior complex scaling methods, and it is demonstrated that both methods are capable of direct calculation of these amplitudes. The results are found to be in excellent agreement with each other and in very good agreement with results of other ab initio methods and experiments.

Electron-impact ionization cross sections for helium are calculated using time-dependent close-coupling theory. The total wave function for the three electron system is expanded in nine dimensions, where three dimensions are represented on a radial lattice and a coupled channels expansion is used to represent the other six dimensions. Collision cross sections are obtained by t→∞ projection onto fully antisymmetric spatial and spin functions, with care as to orthogonality of different representations. Cross sections are also obtained using time-independent first- and second-order perturbative distorted-wave theory. Total cross sections are calculated at incident energies above the double ionization threshold for electron-impact single ionization leaving He⁺ in the 1s, 2s, and 2p states and for electron-impact double ionization. Both the single ionization cross section, leaving He⁺ in the 1s ground state, and the double ionization cross section are in excellent agreement with previous absolute experimental measurements.

Calculations are presented for the double photoionization (with excitation) and triple photoionization of the Li atom. The motion of all three electrons is treated equally by solving the time-dependent Schrödinger equation in nine dimensions. A radial lattice is used to represent three of the nine dimensions, while a coupled channel expansion is used to represent the other six dimensions. Probabilities for photoionization are obtained by t→∞ projection onto fully antisymmetric spatial and spin functions. Double photoionization cross sections for lithium leaving the ion in the 1s, 2s, and 2p states are presented. Good agreement is found with the measurements of Huang et al. [Phys. Rev. A 59, 3397 (1999)] for the total double photoionization cross section and with the measurements of Wehlitz et al. [Phys. Rev. Lett. 81, 1813 (1998)] for the triple photoionization cross section.

The sensitivity of lithium plasma models to the underlying atomic data is investigated. Collisional-radiative modeling is carried out with both the Los Alamos and ADAS suite of codes. The effects of plane-wave Born, distorted-wave, and nonperturbative R-matrix with pseudostates and time-dependent close-coupling electron impact atomic data on derived plasma quantities such as the ionization balance and radiated power are studied. Density and temperature regimes are identified where nonperturbative excitation and ionization rate coefficients must be used. The electron temperature and density ranges investigated were 0.2  eV≤T_e≤90  eV and 10¹⁰  cm⁻³≤N_e≤10¹⁴  cm⁻³.

A time-dependent method is used to study the photoionization of the simplest one-electron molecule, H₂⁺. We use the variational principle to solve the time-dependent Schrödinger equation for H₂⁺ in spherical coordinates (r,θ) centered on the center of mass of the H₂⁺ system in a time-varying electromagnetic field, in the fixed-nuclei approximation. Bound and continuum states of H₂⁺ are obtained by diagonalizing the two-dimensional Hamiltonian for H₂⁺ on a uniform lattice. Two different algorithms for the time propagation of the Schrödinger equation are described, the first an explicit time propagator involving matrix multiplication and the second an implicit time propagator involving matrix inversion. Single-photoionization cross sections for H₂⁺ are presented for the cases where the laser field is oriented both parallel and perpendicular to the internuclear axis. Excellent agreement is found between the present calculations and previous work. Two- and three-photon ionization cross sections are also presented for the cases where the laser field is oriented both parallel and perpendicular to the internuclear axis. Comparison with previous work is available only for the parallel orientation case where good agreement is found with a previous time-independent calculation.

Inelastic electron scattering from light atomic species is of fundamental importance and has significant applications in fusion-plasma modeling. Therefore, it is of interest to apply advanced nonperturbative, close-coupling methods to the determination of electron-impact excitation for these atoms. Here we present the results of R matrix with pseudostate (RMPS) calculations of electron-impact excitation cross sections through the n=4 terms in Be, Be⁺, Be²⁺, and Be³⁺. In order to determine the effects of coupling of the bound states to the target continuum in these species, we compare the RMPS results with those from standard R-matrix calculations. In addition, we have performed time-dependent close-coupling calculations for excitation from the ground and the metastable terms of Be⁺ and the metastable term of Be³⁺. In general, these results are found to agree with those from our RMPS calculations. The full set of data resulting from this work is now available on the Oak Ridge National Laboratory Controlled Fusion Atomic Data Center web site, and will be employed for collisional-radiative modeling of Be in magnetically confined plasmas.