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W. R. Johnson

Publications and source records attributed to W. R. Johnson.

At least 19 recordsLinked to original sources

Determination of the scalar and vector polarizabilities of the cesium $6s \ ^2S_{1/2} \rightarrow 7s \ ^2S_{1/2}$ transition and implications for atomic parity non-conservation

Using recent high-precision measurements of electric dipole matrix elements of atomic cesium, we make an improved determination of the scalar ($α$) and vector ($β$) polarizabilities of the cesium $6s \ ^2S_{1/2} \rightarrow 7s \ ^2S_{1/2} $ transition calculated through a sum-over-states method. We report values of $α= -268.82 \ (30) \ a_0^3$ and $β= 27.139 \ (42) \ a_0^3$ with the highest precision to date. We find a discrepancy between our value of $β$ and the past preferred value, resulting in a significant shift in the value of the weak charge $Q_w$ of the cesium nucleus. Future work to resolve the differences in the polarizability will be critical for interpretation of parity non-conservation measurements in cesium, which have implications for physics beyond the Standard Model.

physics.atom-ph

Opacity of Shock-Heated Boron Plasmas

Standard measures of opacity, the imaginary part of the atomic scattering factor $f_2$ and the x-ray mass attenuation coefficient $μ/ρ$, are evaluated in shock-heated boron, boron carbide and boron nitride plasmas. The Hugoniot equation, relating the temperature $T$ behind a shock wave to the compression ratio $ρ/ρ_0$ across the shock front, is used in connection with the plasma equation of state to determine the pressure $p$, effective plasma charge $Z^\ast$ and the K-shell occupation in terms of $ρ/ρ_0$. Solutions of the Hugoniot equation (determined within the framework of the generalized Thomas-Fermi theory) reveal that the K-shell occupation in low-Z ions decreases rapidly from 2 to 0.1 as the temperature increases from 20eV to 500eV; a temperature range in which the shock compression ratio is near 4. The average-atom model (a quantum mechanical version of the generalized Thomas-Fermi theory) is used to determine K-shell and continuum wave functions and the photoionization cross section for x-rays in the energy range $ω=1$eV to 10keV, where the opacity is dominated by the atomic photoionization process. For an uncompressed boron plasma at $T=10$eV, where the K-shell is filled, the average-atom cross section, the atomic scattering factor and the mass attenuation coefficient are all shown to agree closely with previous (cold matter) tabulations. For shock-compressed plasmas, the dependence of the $μ/ρ$ on temperature can be approximated by scaling previously tabulated cold-matter values by the relative K-shell occupation, however, there is a relatively small residual dependence arising from the photoionization cross section. Attenuation coefficients $μ$ for a 9 keV x-ray are given as functions of $T$ along the Hugoniot for B, C, B$_4$C and BN plasmas.

physics.plasm-ph

X-ray line coincidence photopumping in a solar flare

Line coincidence photopumping is a process where the electrons of an atomic or molecular species are radiatively excited through the absorption of line emission from another species at a coincident wavelength. There are many instances of line coincidence photopumping in astrophysical sources at optical and ultraviolet wavelengths, with the most famous example being Bowen fluorescence (pumping of O III 303.80 A by He II), but none to our knowledge in X-rays. However, here we report on a scheme where a He-like line of Ne IX at 11.000 A is photopumped by He-like Na X at 11.003 A, which predicts significant intensity enhancement in the Ne IX 82.76 A transition under physical conditions found in solar flare plasmas. A comparison of our theoretical models with published X-ray observations of a solar flare obtained during a rocket flight provides evidence for line enhancement, with the measured degree of enhancement being consistent with that expected from theory, a truly surprising result. Observations of this enhancement during flares on stars other than the Sun would provide a powerful new diagnostic tool for determining the sizes of flare loops in these distant, spatially-unresolved, astronomical sources.

astro-ph.SR

Forbidden M1 and E2 transitions in monovalent atoms and ions

We carried out a systematic high-precision relativistic study of the forbidden magnetic-dipole and electric-quadrupole transitions in Ca+, Rb, Sr+, Cs, Ba+, Fr, Ra+, Ac2+, and Th3+. This work is motivated by the importance of these transitions for tests of fundamental physics and precision measurements. The relative importance of the relativistic, correlation, Breit correction, and contributions of negative-energy states is investigated. Recommended values of reduced matrix elements are presented together with their uncertainties. The matrix elements and resulting lifetimes are compared with other theoretical values and with experiment where available.

physics.atom-ph

Relativistic many-body calculation of energies, lifetimes, polarizabilities, blackbody radiative shift and hyperfine constants in Lu2+

Energy levels of 30 low-lying states of Lu2+ and allowed electric-dipole matrix elements between these states are evaluated using a relativistic all-order method in which all single, double and partial triple excitations of Dirac-Fock wave functions are included to all orders of perturbation theory. Matrix elements are critically evaluated for their accuracy and recommended values of the matrix elements are given together with uncertainty estimates. Line strengths, transition rates and lifetimes of the metastable 5d(3/2) and 5d(5/2) states are calculated. Recommended values are given for static polarizabilities of the 6s, 5d and 6p states and tensor polarizabilities of the 5d and 6p(3/2) states. Uncertainties of the polarizability values are estimated in all cases. The blackbody radiation shift of the 6s(1/2)-5d(5/2) transition frequency of the Lu2+ ion is calculated with the aid of the recommended scalar polarizabilities of the 6s(1/2) and 5d(5/2) states. Finally, A and B hyperfine constants are determined for states of 175Lu2+ with n <= 9. This work provides recommended values of transition matrix elements, polarizabilities and hyperfine constants of Lu2+, critically evaluated for accuracy, for benchmark tests of high-precision theoretical methodology and planning of future experiments.

physics.atom-ph

X-ray Thomson scattering for partially ionized plasmas including the effect of bound levels

X-ray Thomson scattering is being developed as a method to measure the temperature, electron density, and ionization state of high energy density plasmas such as those used in inertial confinement fusion. Most experiments are currently done at large laser facilities that can create bright X-ray sources, however the advent of the X-ray free electron laser (X-FEL) provides a new bright source to use in these experiments. One challenge with X-ray Thomson scattering experiments is understanding how to model the scattering for partially ionized plasmas in order to include the contributions of the bound electrons in the scattered intensity. In this work we take the existing models of Thomson scattering that include elastic ion-ion scattering and the electron-electron plasmon scattering and add the contribution of the bound electrons in the partially ionized plasmas. We validated our model by analyzing existing beryllium experimental data. We then consider several higher Z materials such as Cr and predict the existence of additional peaks in the scattering spectrum that requires new computational tools to understand. We also show examples of experiments in CH and Al that have bound contributions that change the shape of the scattered spectra.

physics.plasm-ph

Resonant Bound-Free Contributions to Thomson Scattering of X-rays by Warm Dense Matter

Recent calculations [Nilsen et al. arXiv:1212.5972] predict that contributions to the scattered photon spectrum from 3s and 3p bound states in chromium (Z=24) at metallic density and T=12 eV resonate below the respective bound-state thresholds. These resonances are shown to be closely related to continuum lowering, where 3d bound states in the free atom dissolve into a resonant l=2 partial wave in the continuum. The resulting d-state resonance dominates contributions to the bound-free dynamic structure function, leading to the predicted resonances in the scattered X-ray spectrum. Similar resonant features are shown to occur in all elements in the periodic table between Ca and Mn (20 <= Z <= 25).

physics.plasm-ph

The effect of bound states on X-ray Thomson scattering for partially ionized plasmas

X-ray Thomson scattering is being developed as a method to measure the temperature, electron density, and ionization state of high energy density plasmas such as those used in inertial confinement fusion. X-ray laser sources have always been of interest because of the need to have a bright monochromatic x-ray source to overcome plasma emission and eliminate other lines in the background that complicate the analysis. With the advent of the xray free electron laser (X-FEL) at the SLAC Linac Coherent Light Source (LCLS) and other facilities coming online worldwide, we now have such a source available in the keV regime. Most Thomson scattering codes used to model experimental data greatly simplify or neglect the contributions of the bound electrons to the scattered intensity. In this work we take the existing models of Thomson scattering that include elastic ion-ion scattering and inelastic electron-electron scattering and add the contribution of bound electrons in the partially ionized plasmas. To date, most experiments have studied hydrogen or beryllium plasmas. We first analyze existing experimental data for beryllium to validate the code. We then consider several higher Z materials such as Cr and predict the existence of additional peaks in the scattering spectrum that requires new computational tools to understand. For a Sn plasma, we show that bound contributions change the shape of the scattered spectrum in a way that would change the plasma temperature and density inferred from experiment.

physics.plasm-ph

Average-Atom Model for X-ray Scattering from Warm Dense Matter

A scheme for analyzing Thomson scattering of x-rays by warm dense matter, based on the average-atom model, is developed. Emphasis is given to x-ray scattering by bound electrons. Contributions to the scattered x-ray spectrum from elastic scattering by electrons moving with the ions and from inelastic scattering by free and bound electrons are evaluated using parameters (chemical potential, average ionic charge, free electron density, bound and continuum wave functions, and occupation numbers) taken from the average-atom model. The resulting scheme provides a relatively simple diagnostic for use in connection with x-ray scattering measurements. Applications are given to dense hydrogen, beryllium, aluminum, titanium, and tin plasmas. At high momentum transfer, contributions from inelastic scattering by bound electrons are dominant features of the scattered x-ray spectrum for aluminum, titanium, and tin.

physics.plasm-ph

Using the X-FEL to understand X-ray Thomson scattering for partially ionized plasmas

For the last decade numerous researchers have been trying to develop experimental techniques to use X-ray Thomson scattering as a method to measure the temperature, electron density, and ionization state of high energy density plasmas such as those used in inertial confinement fusion. With the advent of the X-ray free electron laser (X-FEL) at the SLAC Linac Coherent Light Source (LCLS) we now have such a source available in the keV regime. One challenge with X-ray Thomson scattering experiments is understanding how to model the scattering for partially ionized plasmas. Most Thomson scattering codes used to model experimental data greatly simplify or neglect the contributions of the bound electrons to the scattered intensity. In this work we take the existing models of Thomson scattering that include elastic ion-ion scattering and the electron-electron plasmon scattering and add the contribution of the bound electrons in the partially ionized plasmas. Except for hydrogen plasmas almost every plasma that is studied today has bound electrons and it is important to understand their contribution to the Thomson scattering, especially as new X-ray sources such as the X-FEL will allow us to study much higher Z plasmas. Currently most experiments have looked at hydrogen or beryllium. We will first look at the bound electron contributions to beryllium by analysing existing experimental data. We then consider several higher Z materials such as Cr and predict the existence of additional peaks in the scattering spectrum that requires new computational tools to understand. For a Sn plasma we show that the bound contributions changes the shape of the scattered spectrum in a way that would change the plasma temperature and density inferred by the experiment.

physics.plasm-ph

Thomson scattering in the average-atom approximation

The average-atom model is applied to study Thomson scattering of x-rays from warm-dense matter with emphasis on scattering by bound electrons. Parameters needed to evaluate the dynamic structure function (chemical potential, average ionic charge, free electron density, bound and continuum wave-functions and occupation numbers) are obtained from the average-atom model. The resulting analysis provides a relatively simple diagnostic for use in connection with x-ray scattering measurements. Applications are given to dense hydrogen, beryllium, aluminum and titanium plasmas. In the case of titanium, bound states are predicted to modify the spectrum significantly.

physics.plasm-ph

Development of a configuration-interaction + all-order method for atomic calculations

We develop a theoretical method within the framework of relativistic many-body theory to accurately treat correlation corrections in atoms with few valence electrons. This method combines the all-order approach currently used in precision calculations of properties of monovalent atoms with the configuration-interaction approach that is applicable for many-electron systems. The method is applied to Mg, Ca, Sr, Zn, Cd, Ba, and Hg to evaluate ionization energies and low-lying energy levels.

physics.atom-ph

Observation of the Nuclear Magnetic Octupole Moment of 87Rb from Spectroscopic Measurements of Hyperfine Intervals

The magnetic octupole moment of 87Rb is determined from hyperfine intervals in the 5p 2P[3/2] state measured by Ye et al. [Opt. Lett. 21, 1280 (1996)]. Hyperfine constants A = 84.7189(22) MHz, B = 12.4942(43) MHz, and C = -0.12(09) kHz are obtained from the published measurements. The existence of a significant value for C indicates the presence of a nuclear magnetic octupole moment Omega. Combining the hyperfine constants with atomic structure calculations, we obtain Omega = -0.58(39) (b muN). Second-order corrections arising from interaction with the nearby 5p 2P[1/2] state are found to be insignificant.

physics.atom-ph

Low-frequency plasma conductivity in the average-atom approximation

Low-frequency properties of a plasma are examined within the average-atom approximation, which presumes that scattering of a conducting electron on each atom takes place independently of other atoms. The relaxation time tau distinguishes a high-frequency region omega tau > 1, where the single-atom approximation is applicable explicitly, from extreme low frequencies omega tau < 1, where, naively, the single-atom approximation is invalid. A proposed generalization of the formalism, which takes into account many-atom collisions, is found to be accurate in all frequency regions, from omega =0 to omega tau >1, reproducing the Ziman formula in the static limit, results based on the Kubo-Greenwood formula for high frequencies, and satisfying the conductivity sum-rule precisely. The correspondence between physical processes leading to the conventional Ohm's law and the infrared properties of QED is discussed. The suggested average-atom approach to frequency-dependent conductivity is illustrated by numerical calculations for the an aluminum plasma in the temperature range 2--10 eV.

physics.plasm-ph

Hyperfine structure of the metastable 3P2 state of alkaline earth atoms as an accurate probe of nuclear magnetic octupole moments

Measuring the hyperfine structure (HFS) of long-lived $^3P_2$ states of divalent atoms may offer the opportunity of extracting relatively unexplored nuclear magnetic octupole and electric hexadecapole moments. Here, using relativistic many-body methods of atomic structure and the nuclear shell model, we evaluate the effect of these higher nuclear moments on the hyperfine structure. We find that the sensitivity of HFS interval measurements in $^{87}$Sr needed to reveal the perturbation caused by the nuclear octupole moment is on the order of kHz. Results of similar analyses for $^{9}$Be, $^{25}$Mg, and $^{43}$Ca are also reported.

physics.atom-ph

Muonic atoms with extreme nuclear charge

Bound muons (also pions, kaons, etc) increase the fission barrier and produce some stabilizing effects for highly charged nuclei. If the binding energy of the muon exceeds $mc^2$, it becomes stable. The $1s$ state of a muon inside an exotic nucleus with atomic number $A= 5Z/2$ and such large charge $Z$ that the $1s$ energy $E$ is in the range $0\le E<-mc^2$ is considered.

physics.atom-ph

Coupled-cluster single-double calculations of the relativistic energy shifts in C IV, Na I, Mg II, Al III, Si IV, Ca II and Zn II

The relativistic coupled-cluster single-double method is used to calculate the dependence of frequencies of strong $E1$-transitions in many monovalent atoms and ions on the fine-structure constant $α$. These transitions are used in the search for manifestations of the variation of the fine-structure constant in quasar absorption spectra. Results of the present calculations are in good agreement with previous calculations but are more accurate.

physics.atom-ph

Excitation energies, polarizabilities, multipole transition rates, and lifetimes of ions along the francium isoelectronic sequence

Relativistic many-body perturbation theory is applied to study properties of ions of the francium isoelectronic sequence. Specifically, energies of the 7s, 7p, 6d, and 5f states of Fr-like ions with nuclear charges Z = 87 - 100 are calculated through third order; reduced matrix elements, oscillator strengths, transition rates, and lifetimes are determined for 7s - 7p, 7p - 6d, and 6d - 5f electric-dipole transitions; and 7s - 6d, 7s - 5f, and 5f_5/2 - 5f_7/2 multipole matrix elements are evaluated to obtain the lifetimes of low-lying excited states. Moreover, for the ions Z = 87 - 92 calculations are also carried out using the relativistic all-order single-double method, in which single and double excitations of Dirac-Fock wave functions are included to all orders in perturbation theory. With the aid of the SD wave functions, we obtain accurate values of energies, transition rates, oscillator strengths, and the lifetimes of these six ions. Ground state scalar polarizabilities in Fr I, Ra II, Ac III, and Th IV are calculated using relativistic third-order and all-order methods. Ground state scalar polarizabilities for other Fr-like ions are calculated using a relativistic second-order method. These calculations provide a theoretical benchmark for comparison with experiment and theory.

physics.atom-ph