Using a kind of potential model wave function for alkali metal atoms, we nonperturbatively study the effect of fine structure on the Rydberg spectra of Cs atom in a strong magnetic field. Our numerical results reveal spectral structure dramatically different from the well-established Paschen-Back effect, and we argue that the fine structure of the Rydberg Cs atom cannot be neglected even in a magnetic field as strong as several teslas. We also give an error estimate of our results and a word on possible experimental verification. © 1996 The American Physical Society.

The relativistic coupled-cluster method is used to calculate ionization potentials and excitation energies of the barium and radium atoms and their monocations. Large basis sets are used, with l up to 5, the Dirac-Fock or Dirac-Fock-Breit orbitals found, and the external 28 electrons of barium or 42 electrons of radium are correlated by the coupled-cluster method with single and double excitations. Good agreement (within a few hundred wave numbers) is obtained for the ionization potentials and low excitation energies (up to 3 eV for Ba, 4 eV for Ra). The Breit interaction has little effect on the excitation energies, but it improves significantly the fine-structure splittings of Ra. Large relativistic effects on the energies are observed, up to 1 eV for barium and 2 eV for radium. The nonrelativistic ground states of ${\mathrm{Ba}}^{+}$ and ${\mathrm{Ra}}^{+}$ are (n-1)d ${}_{}{}^2{}_{}{}^{}$D rather than ns ${}_{}{}^2{}_{}{}^{}$S. © 1996 The American Physical Society.

${\mathrm{He}}_2$ is examined in the second state of the ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$ symmetry. This state is shown to be one of the two ‘‘parent’’ states of the lowest known stable negative ionic ${\mathrm{He}}_2^{\mathrm{-}}$ state of the ${}_{}{}^4{}_{}{}^{}\mathrm{\Phi }_{\mathit{g}}^{}{}_{}{}^{}$ symmetry. It is compared with the other parent neutral ${\mathrm{He}}_2$ state, which is the lowest of the ${}_{}{}^3{}_{}{}^{}\mathrm{\Delta }_{\mathit{u}}^{}{}_{}{}^{}$ symmetry, with a consequence that the negative ionic state be a closed channel resonance with respect to both of its neutral parent states. Furthermore the third ${\mathrm{He}}_2$ ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$ and the second ${\mathrm{He}}_2$ ${}_{}{}^3{}_{}{}^{}\mathrm{\Delta }_{\mathit{u}}^{}{}_{}{}^{}$ states are shown to be the parents of the second ${\mathrm{He}}_2^{\mathrm{-}}$ ${}_{}{}^4{}_{}{}^{}\mathrm{\Phi }_{\mathit{g}}^{}{}_{}{}^{}$ resonance. © 1996 The American Physical Society.

An alternate approach was used to study the reactions of muonic atoms and molecules in solid layers of hydrogen isotopes. Muons stopped in a protium layer with a small admixture of tritium producing μp atoms. Muon transfer from a proton to a triton generated μt atoms, which were emitted into a deuterium layer. Subsequent epithermal dtμ molecular formation followed by dt fusion gave a prompt signal for the μt emission. Fusion time spectra at various tritium concentrations were measured by detecting neutrons and α particles and the μp disappearance rates were determined. The rates of μp→μt transfer and ppμ formation were measured to be ${\lambda }_{\mathit{pt}}$=[5.86±(0.10${)}_{\mathit{stat}}$±(0.15${)}_{\mathit{syst}}$${(}_{\mathrm{-}0}^{+0.30\mathrm{}}$${)}_{\mathit{model}}$]×${10}^9$ ${\mathrm{s}}^{\mathrm{-}1}$ and ${\lambda }_{\mathit{pp}\mathrm{\mu }}$=[3.21 ±(0.10${)}_{\mathit{stat}}$±(0.14${)}_{\mathit{syst}}$${(}_{\mathrm{-}0.06}^{+0\mathrm{}}$${)}_{\mathit{model}(}$]×${10}^6$ ${\mathrm{s}}^{\mathrm{-}1}$, respectively. Comparisons are made with the most recent theoretical values. © 1996 The American Physical Society.

The differential scattering cross sections and the generalized oscillator strengths of the 1${\mathrm{}}^1$S→2${\mathrm{}}^1$S and 1${\mathrm{}}^1$S→2${\mathrm{}}^1$P transitions of helium have been measured by angle-resolved electron-energy-loss spectroscopy with an incident electron energy of 1500 eV and within the range 2°–11.5° of scattering angles. The corrections for angular factors and density effects have also been made for the experimental results. The differential cross sections and generalized oscillator strength values are absolute and are the first to be measured at such a high impact energy. The experimental results are compared with other measurements and theoretical calculations in the literature. © 1996 The American Physical Society.

Inhomogeneous broadening in vibrational Q lines for ${\mathrm{H}}_2$ broadened by heavy rare gases due to speed-changing collisions was previously demonstrated through the nonlinear behavior of the linewidths versus the concentration. In order to explore a possible inhomogeneous broadening (similar mechanism) for a pure rotational relaxation mechanism, the broadening and line shift of the pure rotational quadrupole transition ${\mathit{S}}_0$(3) of ${\mathrm{H}}_2$ have been measured to high precision with a frequency stabilized diode laser spectrometer. Foreign gas effects with helium and argon at various dilution ratios in ${\mathrm{H}}_2$ have been investigated. A nonlinear dependence of the broadening on the mixing ratio was detected in a pure rotational transition for the ${\mathrm{H}}_2$-Ar mixture. This is the spectral signature expected for an inhomogeneous broadening due to speed-changing collisions. © 1996 The American Physical Society.

We present spectra of hyperfine resolved vibrational levels of the ${\mathit{A}}^1$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$and 1${\mathrm{}}^3$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ states of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ obtained via photoassociation of colliding ultracold atoms in a magneto-optical trap. A simple first-order perturbation theory analysis accurately accounts for the frequency splittings and relative transition strengths of all observed hyperfine features. Assignment of the hyperfine structure allows accurate determination of a vibrational level center of gravity, which significantly decreases the experimental uncertainty of vibrational energies. Differences in the spectra of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ are attributed to quantum statistics. The 1${\mathrm{}}^3$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ series obeys Hund’s case ${\mathit{b}}_{\mathrm{\beta }\mathit{S}}$ coupling and the hyperfine constant is extracted for both isotopes. © 1996 The American Physical Society.

Direct experimental evidence is presented for the formation of aligned states of the linear triatomic molecules ${\mathrm{CO}}_2$ and ${\mathrm{CS}}_2$; manifestation of pendular motion is obtained in high-resolution measurements of the angular distributions of fragments produced by an intense laser-field–molecule interaction. Classical calculations of the dissociation dynamics on an excited-state potential-energy surface yield results that qualitatively reproduce our experimental observations. © 1996 The American Physical Society.

Lifetimes of 21 excited states in atomic Yb were measured using time-resolved fluorescence detection following pulsed laser excitation. The lifetime of the 4${\mathit{f}}^{14}$5d6${\mathit{s}}^{\mathrm{}3}$${\mathit{D}}_1$ state, which is of particular importance for a proposed study of parity nonconservation in atoms, was measured to be 380(30) ns. © 1996 The American Physical Society.

At energies below the threshold for direct electron impact excitation, resonant excitations can make a significant contribution to the total excitation rate of a given energy level. In this paper, the rates of resonant excitation into the levels of the 3${\mathit{d}}^9$4s and 3${\mathit{d}}^9$4p configurations of ${\mathrm{Mo}}^{14+}$ have been calculated using a fully relativistic, multiconfiguration atomic structure code and detailed accounting of energy levels. By including the effects of resonant excitations in collisional-radiative models for the spectrum of Ni I–like ${\mathrm{Mo}}^{14+}$ and (by isoelectronic scaling) ${\mathrm{Zr}}^{12+}$, the ratio of the emissivity of the 3${\mathit{d}}^{10}$-4${\mathit{d}}^9$4s E2 transitions to the emissivity of the 3${\mathit{d}}^{10}$-3${\mathit{d}}^9$4p E1 transitions is greatly enhanced, and sensitivity to electron temperature in the ratio is introduced. This ratio is density sensitive for ${\mathit{n}}_{\mathit{e}}$≥${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}3}$, and therefore, given knowledge of either local temperature or density conditions, the E2-E1 ratio can serve as a diagnostic for local conditions in magnetically confined fusion plasmas. The current work demonstrates the need to include resonant excitations in collisional-radiative models of the soft x-ray emission of nickel-like ions. Good agreement is found between measurements of E1 and E2 line brightness ratios made in a tokamak plasma, and the predictions of collisional-radiative models in the present work. © 1996 The American Physical Society.

It is argued that the most appropriate definition of the local temperature T(r) for the ground state of an electronic system is provided by the formula 3/2ρ(r)kT(r)=1/8${\sum }_{\mathit{i}}$[(∇${\mathrm{\rho }}_{\mathit{i}}$⋅∇${\mathrm{\rho }}_{\mathit{i}}$)/${\mathrm{\rho }}_{\mathit{i}}$], where ρ(r) is the total electron density and the ${\mathrm{\rho }}_{\mathit{i}}$ are Kohn-Sham orbital densities. T(r) is everywhere non-negative. For atoms, T(r) is nearly stepwise constant. T(r) behaves very much like the Politzer average local ionization energy index. Accordingly, T(r) measures reactivity toward attack by an electron-attracting reagent. Exchange energies and Compton profiles are calculated for several atoms using this definition of the local temperature. © 1996 The American Physical Society.

We have measured the relative line strengths for a fairly large number of lines of the 3s-3p and 3p-3d transition arrays of N II with a wall-stabilized arc source in order to test the quality of recent advanced atomic structure calculations. We have observed significant deviations from LS coupling for weaker 3p-3d lines and have found that our measurements agree much better with recent intermediate coupling results. Also, fairly strong intersystem, or spin-forbidden, lines occur for the N II ion due to a near coincidence of the 3s${\mathrm{}}^1$${\mathit{P}}^{\mathit{o}}$ and 3s${\mathrm{}}^3$${\mathit{P}}^{\mathit{o}}$ levels. We provide experimental comparison data for two recent calculations that differ by a factor of 2 for these intersystem lines. © 1996 The American Physical Society.

Delayed annihilation time spectra (DATS) of antiprotons in room-temperature helium gas have been studied as a function of the concentration of admixed noble gases (Ne,Ar,Kr,Xe) as well as molecular gases (${\mathrm{N}}_2$,${\mathrm{O}}_2$,${\mathrm{H}}_2$) at the low-energy antiproton ring at CERN. The DATS were a superposition of two exponential components, one with a lifetime of several 100 ns and the other with a lifetime 1–3 μs. They showed a shorter average lifetime (${\mathit{T}}_{\mathrm{av}}$) than DATS of pure helium. Ne, Ar, and Kr were found to affect ${\mathit{T}}_{\mathrm{av}}$ only slightly even in concentrations up to 20%, while Xe showed a much stronger influence. In the case of molecular gases, the presence of ${\mathrm{N}}_2$ influenced the DATS much less than ${\mathrm{O}}_2$ and ${\mathrm{H}}_2$, which destroyed the metastability almost completely in concentrations of 100 ppm and less. The decay rate of the slow component of DATS was found to exhibit a linear relation to the number density of the admixture. From this a collisional destruction (‘‘quenching’’) cross section ${\mathrm{\sigma }}_{\mathit{q}}$ was extracted. No evidence for a delayed annihilation of antiprotons in pure Ne, Kr, and Xe was observed. © 1996 The American Physical Society.

A density-functional exchange identity is derived in a very simple manner through coordinate scaling for the external potential within the coupling-constant approach. It is observed that this identity may be expressed as a recursion relation which is especially useful for testing approximations to the exchange functional, and to its corresponding exchange potential, as part of a full density-functional calculation involving correlation as well as exchange. © 1996 The American Physical Society.

We propose two simple, first-principles generalized gradient corrections for exchange, which are exact for the one- and two-electron oscillator and for the weakly perturbed electron gas in second order of the gradient and which are very close to Becke’s empirical correction. Also, the procedure is given for fulfilling additional exact constraints. The result is tested for the exchange energy of atoms and some exactly solved one- and two-electron problems in the external potentials -Z/r and 1/2${\mathrm{\omega }}^2$${\mathit{r}}^2$ for a wide range of the parameters Z and ω. © 1996 The American Physical Society.

The (2${\mathit{p}}^2$${)}^1$S state of beryllium is studied using the coupled-cluster approach, and found to lie about 0.0048 a.u. above the ionization limit, with a width of about 0.000 34 a.u., which is the best agreement to date between theoretical and experimental data for this state. The use of complex rotation in order to describe the autoionizing property of the state, and an appropriate potential to describe excited orbitals, thereby alleviating ‘‘intruder state’’ problems, were found to be crucial components for the success of the calculation. © 1996 The American Physical Society.

An exact solution to a quantum model for atomic S scattering with constant coupling of ionic and covalent states is found. The scattering cross-section anomaly near the ionization threshold is analyzed accurately. © 1996 The American Physical Society.

The method of complex-coordinate rotation is used to investigate resonances in ${\mathit{e}}^{+}$-${\mathrm{He}}^{+}$ scattering. By using Hylleraas-type wave functions we have obtained resonance parameters for two S- and two P-wave resonances. Our resonance positions for the two S-wave states are consistent with those obtained by Bhatia and Drachman [Phys. Rev. A 42, 5117 (1990)] who used the stabilization method. The total widths for the lowest S- and P-wave resonances are determined in the present work as 3.52 and 4.83 eV, respectively, raising the possibility that such resonances may be detectable in ${\mathit{e}}^{+}$-${\mathrm{He}}^{+}$ scattering experiments. © 1996 The American Physical Society.

A detailed investigation is performed for the photodetachment cross section of the ${\mathrm{He}}^{\mathrm{-}}$ from the metastable 1s2s2p${\mathrm{}}^4$${\mathit{P}}^{\mathit{o}}$ state, with photon energy up to 4 eV, which covers the whole energy region up to the He(n=4) (n is the principal quantum number for the outer electron) threshold. The B-spline basis and inverse iteration approach is employed to solve the multichannel process. In contrast to other earlier theoretical calculations, in addition to the 1s2${\mathit{p}}^2$${\mathrm{}}^4$P sharp resonance, we also find 1s3${\mathit{p}}^2$${\mathrm{}}^4$P, 1s3p4p${\mathrm{}}^4$P, 1s4${\mathit{p}}^2$${\mathrm{}}^4$P, 1s4p5p${\mathrm{}}^4$P, and 1s3s4s${\mathrm{}}^4$S Feshbach resonances in this energy region. Two Cooper minima are found in the 1s2p${(}^3$${\mathit{P}}^{\mathit{o}}$)kp${\mathrm{}}^4$P channel at the energies of 2.83 and 3.25 eV. The angular distribution of the photoelectrons is also studied; the asymmetry parameter β with respect to the He(2${,}^3$${\mathit{P}}^{\mathit{o}}$) target is quite different from earlier predictions due to the Feshbach resonances in the photodetachment to the final ${}_{}{}^4{}_{}{}^{}$P state. © 1996 The American Physical Society.

Detailed level-by-level calculations of the excitation-autoionization (EA) cross sections and rate coefficients were performed using the relativistic distorted-wave method along the Cu isoelectronic sequence for all the elements with 34≤Z≤92. While in a previous work only the 3d-4l inner-shell collisional excitations had been taken into account, the present extensive calculations include the 3${\mathit{d}}^{10}$4s-3${\mathit{d}}^9$4snl (n=4–7 and l=0–n-1) and the 3${\mathit{p}}^6$3${\mathit{d}}^{10}$4s-3${\mathit{p}}^5$3${\mathit{d}}^{10}$4snl (n=4,5 and l=0–n-1) excitations. An extrapolation method is used to evaluate the contribution for higher principal quantum numbers. Configuration mixing and secondary autoionization processes following radiative decay from autoionizing levels are also included. The results show that the EA processes give a dominant contribution compared to direct ionization, up to a factor of about 15 at Z=43. The additional inner-shell excitations for Z<55 produce an increase in the EA effect varying from 20% to a factor of 2, with respect to the previously predicted 3d-4l EA rates. The additional excitations are the most significant for heavy elements with Z≳55, since they open EA channels, resulting in an EA rate varying typically along the sequence from 3 to 1 times the direct ionization rate. © 1996 The American Physical Society.