Ital Al Research Articles

Electronic excitation of H2 by electron impact using soft norm-conserving pseudopotentials.

We calculate electronic excitation cross sections for the {ital b}{sup 3}{Sigma}{sup +}{sub {ital u}} {ital a}{sup 3}{Sigma}{sup +}{sub {ital g}} {ital c}{sup 3}{Pi}{sub {ital u}}, and {ital d}{sup 3}{Pi}{sub {ital u}} states of H{sub 2} by electron impact. Our results were obtained with the Schwinger multichannel method with pseudopotentials and real potentials at the two-channel level of approximation. Pseudo-H atoms are used to generate H{sub 2} molecules with almost the same low-energy spectrum as the real molecules. We show that the dynamics of the electronic excitation process of the pseudomolecules by electron impact is very similar to the real case. Our results support the idea that pseudopotentials can be used to obtain reliable molecular electronic excitation cross sections by low-energy electron impact, confirming the expectations of previous studies with CH{sub 2}O [Bettega {ital et} {ital al}., Phys. Rev. A {bold 25}, 1111 (1993)] and HBr [Rescigno, J. Chem. Phys. {bold 104}, 125 (1996)]. {copyright} {ital 1996 The American Physical Society.}

Parametrization of pion-nucleon phase shifts and effects upon pion-nucleus scattering calculations.

Pi meson interactions with free nucleons as known from recent experiments have been fit by Arndt {ital et} {ital al}. in the form of phase shifts. In the present work these phase shifts are fit by a simple parametrization, and observables computed from the resulting parametrized phase shifts are compared successfully to important examples of recent measurements. The parameters of this work are readily suited to pion-nucleus optical model codes built upon the impulse approximation. Some examples of the effect of the new pion-nucleon data upon these pion-nucleus calculations are shown, relative to older results based upon far fewer pion-nucleon data. {copyright} {ital 1996 The American Physical Society.}

Internal bremsstrahlung spectrum of 139Ce.

The internal bremsstrahlung spectrum which accompanies the electron capture decay of {sup 139}Ce to the first excited state of {sup 139}La has been measured in coincidence with the nuclear gamma ray which deexcites that state. The measured intensity above 42 keV is found to be (1.070{plus_minus}0.024) times that predicted by the recent calculations of Suri{acute c} {ital et} {ital al}. The {ital Q}{sub EC} value is found to be 264.6{plus_minus}2.0 keV. {copyright} {ital 1996 The American Physical Society.}

Comment on "Superconductivity and Madelung potential of YBa2Cu3O6+x ordered superstructures"

We note discrepancies in numerical results in our calculations of the Coulomb potentials in YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} ({ital x} = 0, 0.5, 1) with those given by Wang {ital et} {ital al}. in {open_quote}{open_quote}Superconductivity and Madelung potential of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} ordered superstructures{close_quote}{close_quote} [Phys. Rev. B {bold 45}, 10834 (1992)]. {copyright} {ital 1996 The American Physical Society.}

High-Dispersion Spectroscopy of Giants in Metal-poor Globular Clusters. II. Oxygen and Sodium Abundances

Oxygen and sodium abundances or upper limits have been determined for 17 red giant members of the metal-poor Galactic globular clusters M68 (two stars), M30 (two stars), M15 (two stars), M55 (two stars), and NGC 6397 (five stars), as well as for the moderately metal-poor clusters NGC 4833 (one star), NGC 6144 (one star), and NGC 6752 (three stars). The behavior of [O/Fe] with respect to luminosity and metallicity is investigated. As found in previous works, the oxygen-to-iron ratios show an upper envelope, in agreement with the mean level of field halo stars of similar metallicities ([O/Fe]=+0.4{plus_minus}0.15). Moreover, as is true of the more metal-rich giants in globular clusters, about one-third of the stars do not have enhanced oxygen with respect to iron, in contrast to field giants of similar metallicity, which are almost invariably oxygen-rich. Several explanations for this deficiency are examined, including emission possibly associated with mass-loss filling in the oxygen lines and evolutionary mixing effects. In particular, the abundances of oxygen and sodium are seen to be anticorrelated, in agreement with previous results (Kraft {ital et} {ital al}. 1995). The enhancement of oxygen in these extremely metal-poor clusters appears similar to that of less metal-deficient clusters, anmore » important datum for determinations of their relative ages. {copyright} {ital 1996 The American Astronomical Society.}« less

Yu et al. Reply.

A Reply to the Comment by R. A. Klemm {ital et} {ital al}. {copyright} {ital 1996 The American Physical Society.}

Observed changes in the diurnal temperature and dewpoint cycles across the United States

We analyzed hourly coterminous U.S. airport data, beginning in 1948, for changes in daily temperature and dewpoint regimes. We found an ubiquitous phase shift in the daytime cycle of warming, towards later in the day. Overall day and night temperature changes were very consistent with the results of Karl et al. [1993] even though this is an entirely different source of data, and one that is not specifically deurbanized, although we attempted to use stations that were not from downtown airports. We divided our results into eastern and western subsets as a first approximation for high and low sulfate aerosol conditions, and found evidence that was consistent with a sulfate effect on overall temperature, but inconsistent with modeled estimates of the effect of sulfates on the intradiurnal regime. It is difficult to establish the causes of the observed intradiurnal phase shift of warming time although urbanization and anthropogenic emissions are likely to be involved. The nature of our finding is subtle, and is consistent with other results that show very modest (and nonobvious) responses to greenhouse changes. Further research will ultimately clarify its causes and effects.

Supersymmetry between deep and shallow optical potentials for 16O+16O scattering.

Pairs of supersymmetric transformations allow one to remove square-integrable solutions from a complex potential without modifying its complex phase shifts. This technique is applied to {sup 16}O+{sup 16}O scattering where deep and shallow optical potentials provide essentially similar fits of excitation functions over the 10{endash}35 MeV range in the center-of-mass system. After removing complex normalizable solutions from the deep optical potential of Kond{bar o} {ital et} {ital al}., the resulting potential resembles the shallow potential of Chatwin {ital et} {ital al}., except for the fact that it is singular at the origin. The transformation succeeds in removing bound states and also narrow resonances from the real part of the deep potential. Different theoretical aspects of scattering with complex potentials are also discussed such as the behavior of complex phase shifts in the vicinity of a resonant or normalizable state, and the Levinson theorem. In order to explain phase-shift behaviors, resonance locations are calculated by the complex rotation method, combined with numerical methods valid for determining square-integrable solutions of a Schr{umlt o}dinger equation involving a complex potential. In phase-equivalent pairs of supersymmetric transformations, the apparent contradiction between removing a square-integrable solution which corresponds to a pole of the scattering matrix, andmore » keeping this {ital S} matrix unchanged is explained with the Jost function. {copyright} {ital 1996 The American Physical Society.}« less

Theory of slow-atom collisions.

A general theory of slow-atom collisions is presented with special emphasis on the effects of nuclear statistics and atomic fine and/or hyperfine structures. Symmetry properties of the collision complex and correlations between the molecular states and the separated-atom states are carefully examined. The frame transformations between various angular momentum coupling schemes are derived, which, in combination with the multichannel quantum defect theory, provides a solid foundation for the computation and the physical interpretation of slow-atom collision processes. The theory reduces to those of Stoof {ital et} {ital al}. [Phys. Rev. B {bold 38}, 4688 (1988)] and Zygelman {ital et} {ital al}. [Phys. Rev. A {bold 49}, 2587 (1994); {bold 50}, 3920 (1994)] in their respective ranges of validity. {copyright} {ital 1996 The American Physical Society.}

Temperature and baryon-chemical-potential-dependent bag pressure for a deconfining phase transition.

We explore the consequences of a bag model developed by Leonidov {ital et} {ital al}. for the deconfining phase transition in which the bag pressure is made to depend on the temperature and baryon chemical potential in order to ensure the entropy and baryon number conservation at the phase boundary together with the Gibbs construction for an equilibrium phase transition. We show that the bag pressure thus obtained yields an anomalous increasing behavior with the increasing baryon chemical potential at a fixed temperature which defies a physical interpretation. We demonstrate that the inclusion of the perturbative interactions in the QGP phase removes this difficulty. Further consequences of the modified bag pressure are discussed. {copyright} {ital 1996 The American Physical Society.}

Microwave instability beyond threshold.

Stability of the steady-state bunch distribution described by the Haissinski solution [J. Haissinski, Nuovo Cimento {bold 18B}, 72 (1973)] is studied above the threshold of microwave instability. It is shown that instability may lead to a new self-consistent state corresponding to particles trapped in a separatrix of an unstable mode. The free energies of the two solutions are compared. The relaxation oscillations between the new and Haissinski solutions are possible and may be related to the sawtooth instability observed recently in the experiments [P. Krejcik {ital et} {ital al}., (unpublished)]. {copyright} {ital 1996 The American Physical Society.}

Calculation of double photoionization of helium using the convergent close-coupling method.

We use the convergent close-coupling formalism to calculate the double-to-single-photoionization cross-section ratio in helium for photon energies from threshold up to 400 eV. Our results for the velocity and acceleration forms of the electromagnetic interaction operator are very close, and lie in between the measurements of D{umlt o}rner {ital et} {ital al}. [Phys. Rev. Lett. {bold 76}, 2654 (1996)] and Levin {ital et} {ital al}. [Phys. Rev. Lett. {bold 76}, 1220 (1996)]. {copyright} {ital 1996 The American Physical Society.}

Contribution of excitation autoionization to the electron-impact ionization of Ar7+

We report a detailed calculation of the contribution of excitation autoionization to the electron-impact ionization of Ar{sup 7+} by a distorted-wave Born approximation with exchange. Inner-shell excitations of the type 2{ital s}{sup 2}2{ital p}{sup 6}3{ital s}{r_arrow}2{ital s}{sup 2}2{ital p}{sup 5}3{ital snl} ({ital n}=3,4,5; {ital l}=0,1,2,3) and 2{ital s}{sup 2}2{ital p}{sup 6}3{ital s}{r_arrow}2{ital s}2{ital p}{sup 6}3{ital snl} ({ital n}=3,4; {ital l}=0,1,2,3) are included. The level-to-level excitation cross sections and autoionization branching ratios are calculated in detail. The present results demonstrate that the use of unit branching ratios will overestimate the excitation-autoionization cross section by about 20{endash}40{percent}. Furthermore, the calculation shows the largest contribution is from 2{ital p}{r_arrow}3{ital p} and 2{ital p}{r_arrow}3{ital d} excitations and the latter dominates the contribution of excitation autoionization to electron-impact ionization. The calculated results are also compared with recent experimental measurements and theoretical calculations. Being different from the results of Griffin, Bottcher, and Pindzola [Phys. Rev. A {bold 45}, 2929 (1992); {bold 24}, 2273 (1981)], our results for the branching ratio are closer to the measurement of Rachafi {ital et} {ital al}. [J. Phys. B {bold 24}, 1037 (1991)] and our values are smaller than the experimental one of Zhang {ital et} {ital al}. [Phys. Rev. Amore » {bold 45}, 2929 (1992)], while the results for the unit branching ratio are very close to the calculation of Tayal [Phys. Rev. A {bold 49}, 2561 (1994)]. In addition, we find that the configuration interaction changes the autoionization branching ratios of individual levels, but has little effect on the total excitation-autoionization cross sections. {copyright} {ital 1996 The American Physical Society.}« less

Two-dimensional R-matrix propagator: Application to electron-hydrogen scattering.

The two-dimensional {ital R}-matrix propagator of Le Dourneuf {ital et} {ital al}. [J. Phys. B {bold 23}, L559 (1990)] is generalized to arbitrary angular momenta and applied to the study of electron-hydrogen collisions at energies up to the {ital n}=5 threshold. Results are presented for phase shifts, resonance positions and widths, as well as cross sections for partial waves of total angular momenta {ital L}{le}3. The stability and efficiency of the method are established. A comparison is made below the {ital n}=3 threshold with results from other theories. Agreement is very good, in particular with the results of a finite-difference numerical integration of the Schr{umlt o}dinger equation by Wang and Callaway [Phys. Rev. A {bold 50}, 2327 (1994)]. Partial cross sections for {sup 1}{ital S} between the {ital n}=3 and 5 thresholds illustrate the potential of the method for studying excitation into high Rydberg states. {copyright} {ital 1996 The American Physical Society.}

Erratum: Isotopic effect of the mean lifetimes of the NeAr2+ doubly charged rare-gas dimer

It has been suggested recently by Chen {ital et} {ital al}. [Phys. Rev. A {bold 49}, 3472 (1994)] that the measured long-lived NeAr{sup 2+} formed in fast NeAr{sup +} + Ar charge-stripping collisions is mostly in its {ital v}=12 vibrational state bound to the electronic ground state, and that this molecular ion decays by tunneling through the potential barrier. Such a decay rate is expected to depend strongly on the reduced mass of the molecular ion leading to large isotopic effects. We have measured the mean lifetimes of the {sup 20}Ne{sup 40}Ar{sup 2+} and {sup 22}Ne{sup 40}Ar{sup 2+} isotopes in order to see this isotopic effect. Surprisingly, the mean lifetimes of both isotopes are similar to each other. Thus, it is suggested that the observed NeAr{sup 2+} molecular ions do not decay via a tunneling mechanism, which would indicate that they reside in the metastable electronic ground state. Rather, electronic transitions from bound or metastable excited states into other repulsive states are the origin for the experimentally observed decay. Qualitative estimates for the shapes and ordering of these states in the electronic spectrum of NeAr{sup 2+} are given.

Normal-state Nernst effect of a high-critical-temperature superconductor.

We have analyzed the data of Clayhold {ital et} {ital al}. for the Nernst effect in the normal state of a high critical superconductor, i.e., Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8+{delta}}. This requested to derive a kinetic expression for the Nernst effect, an expression able to take into account inelastic scattering and magnetic-field dependence. This was done along a relaxation time formalism for the solution of the Boltzmann equation but leaving a {ital background} {ital term} which can be calculated by the most appropriate method. The final calculation leads to the evaluation of the {ital background} {ital term} resulting from the thermoelectric field-free effect. In order to do this we have considered a model of Livanov and Sergeev. The Nernst effect is explained by a simple two band model for electrons and holes with different mobilities. The resulting fit to the experimental data looks rather convincing. Several predictions are made thereafter. {copyright} {ital 1996 The American Physical Society.}

Müller et al. reply.

A Reply to the Comment by A. P. Baddorf, A. K. Swan, and J. F. Wendelken and I. K. Robinson {ital et} {ital al}. {copyright} {ital 1996 The American Physical Society.}

Alpha -decay properties of 181Pb.

The isotope {sup 181}Pb was produced in {sup 92}Mo bombardments of {sup 90}Zr and, together with other reaction products, was passed through a recoil mass separator and implanted in a double-sided silicon strip detector for {alpha}-particle assay. The half-life and energy of the main {sup 181}Pb {alpha} transition were determined to be 45 (20) ms and 7065 (20) keV, respectively. This {sup 181}Pb {ital E}{sub {alpha}} agrees with one previously measured value [7044 (15) keV], but not with the one [7211 (10) keV] used as input to the 1993 Atomic Mass Evaluation. The 6180-keV {alpha} transition assigned to {sup 181}Tl by Bolshakov {ital et} {ital al}. was observed, but the 6566-keV {alpha} particles ascribed to a 2.7-ms level in {sup 181}Tl by Schneider were not. {copyright} {ital 1996 The American Physical Society.}

Interacting boson model basis and Hamiltonian for Delta L=4 staggering.

It is shown that there exists a basis and a Hamiltonian within interacting boson model including {ital g} bosons that reproduce for even-even nuclei the geometric model results of Macchiavelli {ital et} {ital al}. [Phys. Rev. C {bold 51}, R1 (1995)] for {Delta}{ital L}=4 staggering. {copyright} {ital 1996 The American Physical Society.}

Total atomic energies using indirect-path methods.

Indirect path methods, viz., the {ital staircase} and the {ital ladder} methods of Levy {ital et} {ital al}. [Phys. Rev. A {bold 36}, R249 (1987)], have been employed to estimate the nonrelativistic ground-state energies of neutral atoms He-Ar, within the local-exchange prescription of Harbola and Sahni. We demonstrate that the total atomic energies thus estimated are comparable to their Hartree-Fock staircase counterparts and become superior to the latter for {ital Z}{ge}8. {copyright} {ital 1996 The American Physical Society.}