D. R. Schultz

Elastic scattering and charge transfer in slow collisions: isotopes of H and H+ colliding with isotopes of H and with He

Differential and integral cross sections are calculated in the centre of mass energy range 0.1-100 eV for elastic scattering of protons (H+, D+, T+) by helium and of symmetric combinations of protons (H+, D+, T+) and hydrogen atoms (H, D, T) by hydrogen atoms (H, D, T). In addition, we derive from the elastic differential cross sections the momentum transfer and viscosity cross sections of use in the study of plasma transport, and compute the charge transfer cross section. Fully quantal and semiclassical approaches are utilized in these calculations, as are very accurate electronic potential energy curves. The results are compared with available data.

Charge Transfer in Collisions of C+ with H and H+ with C

Charge transfer rate coefficients for collisions of C+ with H and H+ with C are presented for temperatures from 30,000 to 107 K and from 10 to 107 K, respectively. The rate coefficients were calculated from recommended cross sections deduced in a recent theoretical and experimental investigation that took into account previous measurements. Nonadiabatic radial coupling is the dominant mechanism for both reactions above ~50,000 K, but for lower temperatures the reaction of H+ with C proceeds primarily by radiative charge transfer. Implications, due to the magnitude of the rate coefficients, for various astrophysical environments are discussed.

Elastic and vibrationally inelastic slow collisions: H + H2, H+ + H2

We report on a comprehensive study of the scattering of hydrogen atoms on the ground electronic surface of hydrogen molecules in the range of centre of mass energies 0.1-100 eV. Differential and integral elastic cross sections, the related transport cross sections, and vibrationally inelastic cross sections starting from both ground and excited vibrational states, are calculated using a fully quantal, coupled-channel approach in a truncated vibrational basis set, while the rotational dynamics of H2 is treated with the infinite order sudden approximation prescription. For comparison and to highlight the major physical mechanisms revealed in these collisions, a parallel study is carried out for scattering of protons on hydrogen molecules.

The time-dependent close-coupling method for atomic and molecular collision processes

We review the development of the time-dependent close-coupling method to study atomic and molecular few body dynamics. Applications include electron and photon collisions with atoms, molecules, and their ions.

H+ + H Scattering and Ambipolar Diffusion Heating

We report new and highly accurate quantum mechanical calculations of the astrophysically interesting scattering of H+ by H atoms. The effects of the quantum indistinguishability of the two protons are treated consistently throughout, including the calculation of the momentum transfer cross section where elastic scattering and charge transfer cannot be separated at low energies. We are able to resolve the numerous oscillations in the energy variation of the angle-integrated cross sections. With decreasing energy below 1 eV, the oscillations grow in amplitude until a smooth, approximately 1/v2 dependence on velocity is reached below 10-5 eV. The 1/v behavior, traditionally associated with a constant Langevin rate coefficient, is never realized down to the lowest energy calculated (10-10 eV). We use the momentum transfer cross section to calculate accurately the transport rate coefficient that characterizes the drag force between ionic and neutral fluids and the strength of ambipolar diffusion heating for temperatures and ion-neutral drift speeds relevant for astrophysical applications. An early fit of this rate coefficient by Draine is sustained except at low energies. The application of these results to the role of ambipolar diffusion in heating jets from young stellar objects is discussed.

Jovian X-Ray Aurora and Energetic Oxygen Ion Precipitation

The X-ray line spectra of highly charged oxygen ions excited by charge transfer interaction with the molecular hydrogen in the auroral atmosphere of Jupiter are calculated. The calculations utilize our calculated cross sections of state-selective charge transfer and the available cross-section data of ionization and stripping. Comparison of these spectra with high-resolution spectral observations may provide a sensitive probe of the characteristics of the heavy ions precipitating into the Jovian auroral atmosphere. On the basis of the much higher X-ray efficiency of heavy ions than of electrons, it is concluded that the Jovian aurora may be accounted for by a combination of energetic heavy-ion precipitation and energetic electron precipitation, which produces the auroral X-ray and ultraviolet emissions, respectively.

State-to-state rotational transitions in H2+H2 collisions at low temperatures

We present quantum mechanical close-coupling calculations of collisions between two hydrogen molecules over a wide range of energies, extending from the ultracold limit to the superthermal region. The two most recently published potential energy surfaces for the H(2)-H(2) complex, the so-called Diep-Johnson (DJ) [J. Chem. Phys. 112, 4465 (2000); 113, 3480 (2000)] and Boothroyd-Martin-Keogh-Peterson (BMKP) [J. Chem. Phys. 116, 666 (2002)] surfaces, are quantitatively evaluated and compared through the investigation of rotational transitions in H(2)+H(2) collisions within rigid rotor approximation. The BMKP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the potential energy surface compared to previous calculations of H(2)-H(2) interaction. We found significant differences in rotational excitation/deexcitation cross sections computed on the two surfaces in collisions between two para-H(2) molecules. The discrepancy persists over a large range of energies from the ultracold regime to thermal energies and occurs for several low-lying initial rotational levels. Good agreement is found with experiment B. Mate et al., [J. Chem. Phys. 122, 064313 (2005)] for the lowest rotational excitation process, but only with the use of the DJ potential. Rate coefficients computed with the BMKP potential are an order of magnitude smaller.

Studies of laser wakefield structures and electron acceleration in underdense plasmas

Experiments on electron acceleration and optical diagnostics of laser wakes were performed on the HERCULES facility in a wide range of laser and plasma parameters. Using frequency domain holography we demonstrated single shot visualization of individual plasma waves, produced by 40TW, 30fs laser pulses focused to the intensity of 1019W∕cm2 onto a supersonic He gas jet with plasma densities ne<1019cm−3. These holographic “snapshots” capture the variation in shape of the plasma wave with distance behind the driver, and resolve wave front curvature seen previously only in simulations. High-energy quasimonoenergetic electron beams were generated using plasma density in the range 1.5×1019≤ne≤3.5×1019cm−3. These experiments demonstrated that the energy, charge, divergence, and pointing stability of the beam can be controlled by changing ne, and that higher electron energies and more stable beams are produced for lower densities. An optimized quasimonoenergetic beam of over 300MeV and 10mrad angular divergence i...

Momentum Transfer and Viscosity from Proton-Hydrogen Collisions Relevant to Shocks and Other Astrophysical Environments

The momentum transfer and viscosity cross sections for proton-hydrogen collisions are computed in the velocity range of ~200-20,000 km s−1 relevant to a wide range of astrophysical environments such as supernova remnant shocks, solar wind, winds within young stellar objects or accretion disks, and interstellar protons interacting with the heliosphere. A variety of theoretical approaches are used to arrive at a best estimate of these cross sections in this velocity range that smoothly connect with very accurate results previously computed for lower velocities. Contributions to the momentum transfer and viscosity cross sections from both elastic scattering and charge transfer are included.

The Ion-Induced Charge-Exchange X-Ray Emission of the Jovian Auroras: Magnetospheric or Solar Wind Origin?

A new and more comprehensive model of charge-exchange induced X-ray emission, due to ions precipitating into the Jovian atmosphere near the poles, has been used to analyze spectral observations made by the Chandra X-ray Observatory. The model includes for the first time carbon ions, in addition to the oxygen and sulfur ions previously considered, in order to account for possible ion origins from both the solar wind and the Jovian magnetosphere. By comparing the model spectra with newly reprocessed Chandra observations, we conclude that carbon ion emission provides a negligible contribution, suggesting that solar wind ions are not responsible for the observed polar X-rays. In addition, results of the model fits to observations support the previously estimated seeding kinetic energies of the precipitating ions (~0.7-2 MeV u–1), but infer a different relative sulfur-to-oxygen abundance ratio for these Chandra observations.