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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Shakeup and shakeoff spectra in the electron capture decay of atomic 7 Be

The most stringent laboratory-based experimental limits on the existence of submegaelectronvolt sterile neutrinos are currently set by decay spectroscopy of radioactive 7 Be embedded into superconducting sensors. The systematic uncertainties are dominated by the modeling of the electron shakeup and shakeoff spectra that are not based on state-of-the-art atomic theory and do not include electron correlations or relativistic effects. We have used the multiconfiguration Dirac-Fock formalism to obtain correlated wave functions ab initio and compute all single and double shake processes in the electron capture decay of atomic 7 Be . The simulations can explain some but not all of the observed spectral features, likely because the wave functions are modified by the Ta sensor material that the 7 Be is embedded into. The new models also show that the 𝐿/𝐾 electron capture ratio of 7 Be in Ta has previously been slightly underestimated revising the previous value of 0.070(7) to a new value of 0.0756(20).

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS2007

Asymptotic Energies and QED Shifts for Rydberg States of Helium

This paper reviews progress that has been made in obtaining essentially exact solutions to the nonrelativistic three-body problem for helium by a combination of variational and asymptotic expansion methods. The calculation of relativistic and quantum electrodynamic corrections by perturbation theory is discussed, and in particular, methods for the accurate calculation of the Bethe logarithm part of the electron self energy are presented. As an example, the results are applied to the calculation of isotope shifts for the short-lived 'halo' nucleus He-6 relative to He-4 in order to determine the nuclear charge radius of He-6 from high precision spectroscopic measurements carried out at the Argonne National Laboratory. The results demonstrate that the high precision that is now available from atomic theory is creating new opportunities to create novel measurement tools, and helium, along with hydrogen, can be regarded as a fundamental atomic system whose spectrum is well understood for all practical purposes.

Drake, G.W.F.↗
NASA NTRS1986

Hot hydrogen atom reactions moderated by H2 and He

Photolysis experiments were performed on the H2-CD4-NH3 and He-CD4-NH3 systems. The photolysis (1849 A) involved only NH3. Mixtures of H2:CD4:NH3 included all combinations of the ratios (200,400,800):(10,20,40):4. Two He:CD4:NH3 mixtures were examined where the ratios equalled the combinations 100:(10,20):4. Abstraction of a D from CD4 by the photolytically produced hot hydrogen from ammonia was monitored by mass spectrometric determination of HD. Both experiment and semiempirical hot-atom theory show that H2 is a very poor thermalizer of hot hydrogens with excess kinetic energy of about 2 eV. Applications of the hard-sphere collision model to the H2-CD4-NH3 system resulted in predicted ratios of net HD production to NH3 decomposition that were two orders of magnitude smaller than the experimental ratios. On the other hand, helium is found to be a very efficient thermalizer; here, the classical model yields reasonable agreement with experiments. Application of a semiempirical hot-atom program gave quantitative agreement with experiment for either system.

Aronowitz, S.↗
DOE OSTI2020

Origin of core-to-core x-ray emission spectroscopy sensitivity to structural dynamics

Recently, coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of Kα x-ray emission spectroscopy (XES). Understanding the origin of the unexpected sensitivity of core-to-core transitions to structural dynamics is important for further development of femtosecond time-resolved XES methods and, we believe, generally necessary for interpretation of XES signals from highly non-equilibrium structures that are ubiquitous in photophysics and photochemistry. Here, we use multiconfigurational wavefunction calculations combined with atomic theory to analyze the emission process in detail. The sensitivity of core-to-core transitions to structural dynamics is due to a shift of the minimum energy metal–ligand bond distance between 1s and 2p core-hole states. A key effect is the additional contraction of the non-bonding 3s and 3p orbitals in 1s core-hole states, which decreases electron– electron repulsion and increases overlap in the metal–ligand bonds. The effect is believed to be general and especially pronounced for systems with strong bonds. The important role of 3s and 3p orbitals is consistent with the analysis of radial charge and spin densities and can be connected to the negative chemical shift observed for many transition metal complexes. The XES sensitivity to structural dynamics can be optimized by tuning the emission energy spectrometer, with oscillations up to ±4% of the maximum intensity for the current system. The theoretical predictions can be used to design experiments that separate electronic and nuclear degrees of freedom in ultrafast excited state dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

ThAu 2 – , ThAu 2 O – , and ThAuOH – anions: Photoelectron spectroscopic and theoretical characterization

The thorium–gold negative ions ThAu 2 – , ThAu 2 O – , and ThAuOH – have been observed and experimentally characterized by anion photoelectron spectroscopy. These experiments are accompanied by extensive ab initio electronic structure calculations using a relativistic composite methodology based primarily on coupled cluster singles and doubles with perturbative triples calculations. The theoretical electron affinities (EAs) at 0 K agree with the experimental adiabatic EAs to within 0.02 eV for all species. Two separate isomers were located in the calculations for ThAuOH – , and detachment from both of these appears to be present in the photoelectron spectrum. Excited electronic states of the neutral molecules are reported at the equation of motion-coupled cluster singles and doubles level of theory. Atomization energies and heats of formation are also calculated for each neutral species and have expected uncertainties of 3 and 4 kcal/mol, respectively. The σ bonds between Th and Au are determined by natural bond orbital analysis to consist of predominately sd hybrids on Th bonding with the Au 6s orbital. In order to investigate the correspondence between the bonding in Th–Au and Th–F molecules, a limited number of calculations were also carried out on most of the F-analogs of this study. These results demonstrate that Au does behave like F in these cases, although the Th–F σ bonds are much more ionic compared to Th–Au. This results in an EA for ThF 2 that is 10 kcal/mol smaller than that of ThAu 2 . The EA values for the Th(IV) species, i.e., ThX 2 O and ThXOH, only differed, however, by 3–4 kcal/mol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1983

Low-temperature Positron Annihilation

For a satisfactory understanding of astrophysical annihilation radiation, especially that observed from the galactic center direction, the interaction of positrons with the ambient medium must be carefully investigated. Although hot, ionized regions may be important sources of annihilation radiation, the simpler processes occurring in low-temperature neutral hydrogen gas are mainly addressed. The goal is to set limits on conditions in the annihilation region by using the predictions of atomic theory compared with the observed gamma-ray line width, continuum strength and time dependence.

Drachman, R. J.↗
NASA NTRS1983

Low-temperature positron annihilation

For a satisfactory understanding of astrophysical annihilation radiation, especially that observed from the Galactic center direction, the interaction of positrons with the ambient medium must be carefully investigated. Although hot, ionized regions may be important sources of annihilation radiation, the simpler processes occurring in low-temperature neutral hydrogen gas are mainly addressed. The goal is to set limits on conditions in the annihilation region by using the predictions of atomic theory compared with the observed gamma-ray line width, continuum strength and time dependence.

Drachman, R. J.↗
NASA NTRS1988

Effect of mass-velocity on liquid jet atomization in Mach 1 gasflow

Interacting two-phase flow in four differently sized pneumatic two-fluid atomizers was investigated to determine the effect of gas mass-velocity on the Sauter mean diameter of sprays produced by small diameter liquid jets breaking up in high velocity gas flow. Tests were conducted primarily in the acceleration-wave regime for liquid jet atomization, where it was found that the loss of droplets due to vaporization had a marked effect on drop size measurements. A scattered-light scanner, developed at NASA Lewis Research Center, was used to measure the Sauter mean diameter, D sub 32, which was correlated with nitrogen gas mass-velocity to give the following expression: D (sup -1)(sub 32) = 11.7(rho (sub n) V (sub n)) (sup 1.33). The exponent 1.33 for the gas mass-velocity is identical to that predicted by atomization theory for liquid jet breakup in the acceleration-wave regime.

Ingebo, Robert D.↗
NASA NTRS1988

Small-droplet spray measurements with a scattered-light scanner

Interacting two-phase flow through pneumatic two-fluid nozzles was investigated to determine the effect of nitrogen gas mass-velocity on the Sauter mean diameter of water sprays produced by the breakup of small diameter liquid-jets in high velocity gas streams. Tests were conducted primarily in the aerodynamic-stripping regime of liquid-jet atomization. It was found that the loss of droplets due to vaporization and dispersion had a marked effect on drop size measurements. A scattered-light scanner, developed at NASA Lewis Research Center was used to measure Sauter mean diameters, D sub 32, as small as 5 microns, which were correlated with nitrogen gas mass-velocity to give the following expression: D(sup -1)(sub 32) = 11.7(rho (sub n) V(sub n))(sup 1.33) where D sub 32 and p(sub n)V(sub n) are given in centimeters and g/sq cm-sec, respectively. The exponent 1.33 is the same as that predicted by atomizing theory for liquid-jet breakup in high velocity gas streams.

Ingebo, Robert D.↗
NASA NTRS1989

Gas density effect on dropsize of simulated fuel sprays

Two-phase flow in pneumatic two-fluid fuel nozzles was investigated experimentally to determine the effect of atomizing-gas density and gas mass-flux on liquid-jet breakup in sonic-velocity gas-flow. Dropsize data were obtained for the following atomizing-gases: nitrogen; argon; carbon dioxide; and helium. They were selected to cover a gas molecular-weight range of 4 to 44. Atomizing-gas mass-flux ranged from 6 to 50 g/sq cm-sec and four differently sized two-fluid fuel nozzles were used having orifice diameters that varied from 0.32 to 0.56 cm. The ratio of liquid-jet diameter to SMD, D sub o/D sub 32, was correlated with aerodynamic and liquid-surface forces based on the product of the Weber and Reynolds number, We*Re, and gas-to-liquid density ratio, rho sub g/rho sub l. To correlate spray dropsize with breakup forces produced by using different atomizing-gases, a new molecular-scale dimensionless group was derived. The derived dimensionless group was used to obtain an expression for the ratio of liquid-jet diameter to SMD, D sub o/D sub 32. The mathematical expression of this phenomenon incorporates the product of the Weber and Reynolds number, liquid viscosity, surface tension, acoustic gas velocity, the RMS velocity of gas molecules, the acceleration of gas molecules due to gravity, and gas viscosity. The mathematical expression encompassing these parameters agrees well with the atomization theory for liquid-jet breakup in high velocity gas flow. Also, it was found that at the same gas mass-flux, helium was considerably more effective than nitrogen in producing small droplet sprays with SMD's in the order of 5 micrometers.

Ingebo, Robert D.↗
NASA NTRS1989

Gas density effect on dropsize of simulated fuel sprays

Two-phase flow in pneumatic two-fluid fuel nozzles was investigated experimentally to determine the effect of atomizing-gas density and gas mass-flux on liquid-jet breakup in sonic-velocity gas-flow. Dropsize data were obtained for the following atomizing-gases: nitrogen; argon; carbon dioxide; and helium. They were selected to cover a gas molecular-weight range of 4 to 44. Atomizing-gas mass-flux ranged from 6 to 50 g/sq cm-sec and four differently sized two-fluid fuel nozzles were used having orifice diameters that varied from 0.32 to 0.56 cm. The ratio of liquid-jet diameter to SMD, D sub o/D sub 32, was correlated with aerodynamic and liquid-surface forces based on the product of the Weber and Reynolds number, and gas-to-liquid density ratio, rho sub g/rho sub 1. To correlate spray dropsize with breakup forces produced by using different atomizing-gases, a new molecular-scale dimensionless group was derived. The derived dimensionless group was used to obtain an expression for the ratio of liquid-jet diameter to SMD, D sub o/D sub 32. The mathematical expression of this phenomenon incorporates the product of the Weber and Reynolds number, liquid viscosity, surface tension, acoustic gas velocity, the RMS velocity of gas molecules, the acceleration of gas molecules due to gravity, and gas viscosity. The mathematical expression encompassing these parameters agrees well with the atomization theory for liquid-jet breakup in high velocity gas flow. Also, it was found that at the same gas mass-flux, helium was considerably more effective than nitrogen in producing small droplet sprays with SMD's in the order of 5 micrometers.

Ingebo, Robert D.↗
NASA NTRS1991

Spray measurements of aerothermodynamic effect on disintegrating liquid jets

An experimental investigation was made to determine the effect of atomizing gas mass flux and temperature on liquid jet breakup in sonic velocity gas flow. Characteristic drop size data were obtained by using the following atomizing gases: nitrogen, argon, and helium to breakup water jets in high velocity gas flow. A scattered light scanning instrument developed at Lewis Research Center was used to measure Sauter mean diameter (SMD). The three gases gave a molecular weight range of 4 to 40 and atomizing gas mass flux and temperature were varied from 6 to 50 g/sq cm and 275-400 K, respectively. The ratio of liquid jet diameter to SMD, D(sub 0)/D(sub 32), was correlated with aerodynamic and liquid-surface force ratios, i.e., the product of the Weber and Reynolds number, We Re, the gas to liquid density ratio, rho(sub g)/rho(sub 1) g and also the molecular scale dimensionless group, rho(sub 1)(Vm exp 3)/ mu(sub 1) g, to give the following expression: D(sub 0)/D(sub 32) = 0.90 x 10(exp -8) x (We Re rho sub g/rho sub 1)exp 0.44 x (rho sub 1 Vm exp 3/mu sub 1 g)exp 0.67 where We Re = ((rho sub g)exp 2(D sub 0)exp 2(V sub C)exp3))/ mu sub 1 sigma, mu sub 1 is liquid viscosity, sigma is surface tension, V sub C is the acoustic gas velocity, V sub m is the RMS velocity of gas molecules, and g is the acceleration of gas molecules due to gravity. Good agreement was obtained with atomization theory for liquid-jet breakup in the regime of aerodynamic stripping. Also, due to its low molecular weight and high acoustic velocity, helium was considerably more effective than nitrogen or argon in producing small-droplet sprays with values of D(sub 32) on the order of 5 microns.

Ingebo, Robert D.↗
NASA NTRS1991

A statistical study of high coronal densities from X-ray line-ratios of Mg XI

An X-ray line-ratio density diagnostic was applied to 50 Mg XI spectra of flaring active regions on the sun recorded by the Flat Crystal Spectrometer on the SMM. The plasma density is derived from R, the flux ratio of the forbidden to intercombination lines of the He-like ion, Mg XI. The R ratio for Mg XI is only density sensitive when the electron density exceeds a critical value (about 10 to the 12th/cu cm), the low-density limit (LDL). This theoretical value of the low-density limit is uncertain as it depends on complex atomic theory. Reported coronal densities above 10 to the 12th/cu cm are uncommon. In this study, the distribution of R ratio values about the LDL is estimated and the empirical values are derived for the 1st and 2nd moments of this distribution from 50 Mg XI spectra. From these derived parameters, the percentage of observations is derived which indicated densities above this limit.

Linford, G. A.↗
NASA NTRS2018

Laboratory Measurements of X-ray Emission from Highly Charged Argon Ions

Uncertainties in atomic models will introduce noticeable additional systematics in calculating the flux of weak dielectronic recombination (DR) satellite lines, affecting the detection and flux measurements of other weak spectral lines. One important example is the Ar xvii Heβ DR, which is expected to be present in emission from the hot intracluster medium of galaxy clusters and could impact measurements of the flux of the 3.5 keV line that has been suggested as a secondary emission from a dark matter interaction. We perform a set of experiments using the Lawrence Livermore National Laboratory's electron beam ion trap (EBIT-I) and the X-ray Spectrometer quantum calorimeter (XRS/EBIT) to test the Ar xvii Heβ DR origin of the 3.5 keV line. We measured the X-ray emission following resonant DR onto helium-like and lithium-like Argon using EBIT-I's Maxwellian simulator mode at a simulated electron temperature of T(e) = 1.74 keV. The measured flux of the Ar xvii Heβ DR lined is too weak to account for the flux in the 3.5 keV line, assuming reasonable plasma parameters. We, therefore, rule out Ar xvii Heβ DR as a significant contributor to the 3.5 keV line. A comprehensive comparison between the atomic theory and the EBIT experiment results is also provided.

Esra Bulbul↗
NASA NTRS1993

Wigner functions for nonclassical states of a collection of two-level atoms

The general theory of atomic angular momentum states is used to derive the Wigner distribution function for atomic angular momentum number states, coherent states, and squeezed states. These Wigner functions W(theta,phi) are represented as a pseudo-probability distribution in spherical coordinates theta and phi on the surface of a sphere of radius the square root of j(j +1) where j is the total angular momentum.

Agarwal, G. S.↗
NASA NTRS1973

Polarization phenomena in multiphoton ionization of atoms

The theory of multiphoton ionization for an atomic system of arbitrary complexity is developed using a density matrix formalism. An expression is obtained which determines the differential N-photon ionization cross section as a function of the polarization states of the target atom and the incident radiation. The parameters which characterize the photoelectron angular distribution are related to the general reduced matrix elements for the N-photon transition. Two-photon ionization of unpolarized atoms is treated as an illustration of the use of the theory. The dependence of the multiphoton ionization cross section on the polarization state of the incident radiation, which has been observed in two- and three-photon ionization of Cs, is accounted for by the theory. Finally, the photoelectron spin polarization produced by the multiphoton ionization of unpolarized atoms, like the analogous polarization resulting from single-photon ionization, is found to depend on the circular polarization of the incident radiation.

Jacobs, V. L.↗