Yong Jun Cheng

Positronium formation in positron-hydrogen collisions with Debye potentials

Positronium (Ps) formation cross sections (n = 1, 2) in positron-hydrogen collisions in Debye plasma environment are calculated using the screening approximation model for various Debye screening lengths from the Ps formation thresholds to 50 eV. The effect of the screened Coulomb potential on Ps formation process is investigated by using the Debye-Huckel potential. The present results are compared with available theoretical calculations.

Effective oscillator strength distributions of spherically symmetric atoms for calculating polarizabilities and long-range atom–atom interactions

Effective oscillator strength distributions are systematically generated and tabulated for the alkali atoms, the alkaline-earth atoms, the alkaline-earth ions, the rare gases and some miscellaneous atoms. These effective distributions are used to compute the dipole, quadrupole and octupole static polarizabilities, and are then applied to the calculation of the dynamic polarizabilities at imaginary frequencies. These polarizabilities can be used to determine the long-range C6, C8 and C10 atom–atom interactions for the dimers formed from any of these atoms and ions, and we present tables covering all of these combinations.

Tune-out wavelengths for the alkaline-earth-metal atoms

The lowest three tune-out wavelengths of the four alkaline-earth-metal atoms Be, Mg, Ca, and Sr are determined from tabulations of matrix elements produced from large first-principles calculations. The tune-out wavelengths are located near the wavelengths for the 3P1o and 1P1o excitations. The measurement of the tune-out wavelengths could be used to establish a quantitative relationship between the oscillator strength of the transition leading to existence of the tune-out wavelength and the dynamic polarizability of the atom at the tune-out frequency. The longest tune-out wavelengths for Be, Mg, Ca, Sr, Ba, and Yb are 454.9813, 457.2372, 657.446, 689.200, 788.875, and 553.00 nm, respectively.

Transport properties of electron swarms in gaseous neon at low values of E/N

A detailed analysis of electron swarm transport through neon gas at applied reduced electric fields of E/N < 2 Td is presented. The root mean square difference of transport parameters calculated from a recent all-order many-body perturbation theory treatment (Cheng et al 2014 Phys. Rev. A 89 012701) with drift velocity measurements by the Australian National University group (Robertson 1972 J. Phys. B 5 648) is less than 1%. Differences of about 3% exist with characteristic energies, DT/μ, (Koizumi et al 1984 J. Phys. B 17 4387) indicating an incompatibility at the 3% level between drift velocity and transverse diffusion coefficient measurements. Multi-term solutions of the Boltzmann equation indicate that the two-term approximation gives transport parameters accurate to better than 0.01%. The diffusion constant at thermal energies is found to be sensitive to the numerical representation of the cross section. A recommended elastic momentum transfer cross section has been constructed that has a maximum difference of 0.5% with all ANU drift velocity data for E/N < 1.6 Td and a root mean square difference that is about a factor of 2 smaller.

Long-range interactions between alkali and alkaline-earth atoms

Dispersion coefficients between the alkali metal atoms (Li–Rb) and alkaline-earth metal atoms (Be–Sr) are evaluated using matrix elements computed from frozen core configuration interaction calculations. Besides dispersion coefficients with both atoms in their respective ground states, dispersion coefficients are also given for the case where one atom is in its ground state and the other atom is in a low-lying excited state.

Coupled channels optical method of electron impact on excited helium 21S

Ionization cross sections, excitation (differential and integral) cross sections and total cross sections for electron scattering on the metastable level 21S of helium are calculated at low and intermediate energies and compared with other theoretical and experimental results. The method used is the coupled channels optical method with an ab initio complex polarization potential.

Momentum-space coupled-channel calculation for positron-helium scattering

The momentum-space coupled-channel optical method has been applied to positron-helium scattering. The ionization continuum and the positronium formation channels are included via a complex equivalent-local optical potential. The positronium formation cross sections at the energy range from the threshold to 400 eV and the total scattering cross sections at the energies from 17 to 500 eV are reported and compared with available experimental measurements and theoretical calculations. The agreements between the present results and the corresponding measurements are satisfactory.

Blackbody radiation shift of the Ga+ clock transition

The blackbody radiation shift of the Ga+ clock transition is computed to be ?0.0140 ? 0.0062 Hz at 300?K. The small shift is consistent with the blackbody radiation shifts of the clock transitions of other group III ions which are of a similar size. The polarizabilities of the Ga+ , , and states were computed using the configuration interaction method with an underlying semi-empirical core potential. Quadrupole and non-adiabatic dipole polarizabilities were also computed. A byproduct of the analysis involved calculations of the low-lying spectrum and oscillator strengths, including polarizabilities, of the Ga2 + ion.

Blackbody radiation shift of the B+ clock transition

A calculation of the blackbody radiation shift of the B+ clock transition is performed. The polarizabilities of the B+2s2 1Se, 2s2p 1Po, and 2s2p 3Po states are computed using the configuration interaction method with an underlying semiempirical core potential. The recommended dipole polarizabilities are 9.64(3)a03, 7.78(3)a03 and 16.55(5)a03, respectively. The derived frequency shift for the 2s2 1Se → 2s2p 3P0o transition at 300 K is 0.0160(5) Hz. The dipole polarizabilities agree with an earlier relativistic calculation [Safronova et al., Phys. Rev. Lett. 107, 143006 (2011)] to better than 0.2%. Quadrupole and octupole polarizabilities and nonadiabatic multipole polarizabilities are also reported.

Resonance phenomena in e{sup +}-Li collisions

Energy-dependent phenomena for positron-lithium scattering in the energy region 1.45-4.5 eV have been investigated using the momentum-space coupled-channels optical method. Positions and widths of resonance states with angular momenta of J=0-2 and several threshold behaviors have been obtained. The present results are compared with other theoretical calculations.