Isao Shimamura

Time-delay matrix analysis of resonances: application to the positronium negative ion

A procedure to analyse resonances in scattering and photoionization calculations is discussed. It is theoretically connected to a general relation between the trace of the time-delay matrix and the energy derivative of the eigenphase sum. This procedure is especially useful in analysing resonances with strong background contributions, such as shape resonances lying just above the threshold of a new channel with an overcritical dipole field. In this case the background eigenphase sum diverges towards the threshold, and it can easily change the time delay due to an S-matrix pole into a negative time delay, or time advance. As an application, we analyse results of hyperspherical close-coupling calculations of S-, P-, D-, F-, G- and H-wave resonances in the positronium negative ion Ps− near the Ps(n = 2, 3) thresholds. We find examples of a strong background leading to an overall time advance and the dipole oscillation in the cross section above and near a threshold.

Absorption and emission spectra of alkaline-earth atoms in liquid helium: a theoretical study

Abstract The absorption and emission spectra of Mg, Ca, Sr and Ba atoms in superfluid He are calculated systematically by using an adiabatic line-broadening theory modified by inclusion of the bubble-oscillation effect. The present study, which incorporates a more accurate description of molecular electronic states fora diatomic system than that used in an earlier model calculation, results in satisfactory agreement with experimental spectra.

Time-delay matrix analysis of several overlapping resonances: applications to the helium atom and the positronium negative ion

A useful method of analysing sets of several overlapping multichannel resonances, which are extremely difficult to analyse by other means, is presented. It exploits the Lorentzian profiles of eigenvalues {qi(E)} of the energy-dependent time-delay matrix, avoided at their crossings. The contribution to {qi(E)} from eachS-matrix pole stands out clearly and uniquely from the contributions from the other poles. Thus the resonances that can be easily missed by the conventional technique of studying the eigenphase sum δ(E)or its derivative dδ/dE are singled out asdiabatic resonances in a visually unambiguous way. The method is illustrated with the 1 P o continuum states of the helium atom and the positronium negative ion Ps − below the threshold for He + (n = 5) or Ps(n = 5). Many new resonances are found for Ps − . (Some figures in this article are in colour only in the electronic version)

Nonadiabatically coupled hyperspherical equations applied to the collisional spin flip of muonic hydrogen

Nonadiabatically coupled equations in terms of hyperspherical coordinates are solved for the elastic and hyperfine transitions F→F′ of muonic hydrogen isotopes pμ, dμ, and tμ in collisions pμ(F)+p→p+μp(F′), dμ(F)+d→d+μd(F′), and tμ(F)+t→t+μt(F′), as an extension of our previous work on pμ [Igarashi et al., Phys. Rev. A58 (1998), 1166]. Converged cross sections are obtained for collision energies 10−3 to 102 eV and for the total orbital angular momentum L=0 and 1 including the spin–spin interactions VS explicitly in the Hamiltonian, and for L≥2 neglecting VS because of the large interparticle separations. The inclusion of VS for L=0 and 1 reproduces the correct hyperfine-splitting energy Δ∈ in the separated-atom limit, and is found to be of vital importance for the calculation of the spin-flip cross sections for collision energies lower than Δ∈.

Bound States of Positron with Molecules

A study for electron binding proves that positrons should be able to form an infinite number of bound states with molecules in the Born-Oppenheimer approximation if the molecular dipole moments are greater than 1.625 Debye (D) (the critical dipole moment), although there is no comprehensive theoretical nor experimental observation. Characteristic features of positron binding with molecules obtained in the Hartree-Fock calculation are discussed in terms of the relationship with the dipole moment of the molecules. It is shown that the current level of the precision of theoretical calculations based on the SCF approach has not verified the theoretical critical dipole moment for the positron binding, and further significant improvement is required. However, the present results, though qualitative, provide much insight of underlying physics of the positron binding and suggest the future direction of research.