Nonradiative decay from 5d states of rare earths in crystals

Nonradiative decay from 4f^n?1 5d states was investigated for trivalent rare earths in Y₃Al₅O₁₂. The rates of both 5d→4f and 5d→5d transitions were determined from measurements of the lifetimes and intensities of 5d fluorescence from Ce³⁺ and Pr³⁺. Because of the stronger ion-lattice coupling, nonradiative decay rates for transitions involving 5d states are much faster than those between 4f states. Decay rates are dependent upon the temperature and the energy gap to the next-lower level. The temperature dependences of the 5d fluorescence lifetimes from 77 to 700°K are reported.     

Effect of spin crossovers on the Mott-Hubbard transition at high pressures

The effect of spin crossovers in d _n terms on the effective Hubbard parameter (U _eff) determining the gap between the lower and upper Hubbard bands is analyzed using a many-electron approach to describe the electron structure of Mott insulators. A new mechanism of the insulator-metal transition is established for d ⁵ ions, which is related to a decrease in U _eff caused by spin crossover. For other ions, the U _eff value is either independent of pressure (d ², d ⁴, d ⁷) or it exhibits nonmonotonic growth (d ³, d ⁶, d ⁸).     

Absolute oscillator strengths of 5f-5f transitions of U4+ in ThBr4 and in hydrobromic acid solutions

Absolute oscillator strengths of 5f² transitions of U⁴⁺ in ThBr₄ and in hydrobromic acid solutions are found to be about 10^-4 which is two orders of magnitude larger than those for 4f-4f transitions (lanthanide series) and one order of magnitude larger than 5f³ ones (U³⁺). Applicability of Judd's theory to 5f² transitions is discussed with particular attention to the roles played by reduced matrix elements of odd rank tensor U^λ and by the proximity of the 6d configuration.     

Resonant photoemission from SmS(100)

A strong, sharp resonance enhancement of 4f photoemission has been observed on SmS(100) surfaces for photon energies in the region of the 4d-4f transitions at about 126 eV. The discrete final state reached via the excitation hν+4d¹⁰4f⁶→4d⁹4f⁷ autoionizes primarely via a super Coster-Kronig transition of the type 4d⁹4f⁷→4d¹⁰4f⁵ + unbound electron. Other decay channels, e.g, Sm 5p emission, as well as a surface induced binding energy shift in the Sm³⁺ final state are identified and discussed.     

Isotopieverschiebungskonstanten von Th,U, Pu und Am

The isotope shifts of unperturbed electron configurations have been determined from isotope shift measurements in the spectra of Th, U, Pu, and Am. The screening of the 7s electron charge density at the nucleus by 6d, 7s, and 7p electrons is discussed. It turns out that the same screening factors as for the 6s electron in lighter elements can be used. The screening of the 7s electron charge density at the nucleus by one 5f electron amounts to about 25%: [δT(f ⁿ s)?δT(f ⁿ )]/[δT(f ⁿ }s)?δ(f ^n} )]=0.7₅. The charge density at the nucleus due to the filleds (andp _1/2) shells is considerabely screened by anf electron. The isotope shiftδT(f^n}-1 d ^m +2)?δT(f ⁿ d ^m ) produced by this effect is of the same order of magnitude as the isotope shiftδT(f ⁿ d ^m s)-δT(f ⁿ d ^m ) due to ans electron. The experimental isotope shift constants are found to be:Β C _exp(Th²³⁰–Th²³²)=880±120;Β C _exp(U²³³–U²³⁵)=1000±180;Β C _exp(U²³⁴–U²³⁶)=1070±200;Β C _exp(U²³⁶–U²³⁸)=1080±180;Β C _exp(Pu²³⁸–Pu²⁴⁰)=1200±120;Β C _exp(Pu²³⁹–Pu²⁴¹)=1060±100;Β C _exp(Pu²⁴⁰–Pu242)=900 ±90;Β C _exp(Am²⁴¹–Am²⁴³)=890±50 [10^?3cm^?1]. The ratiosΒ C _exp/C _th are discussed.     

Evidence for a valence transition in UxY1-xSb

Electrical, optical and lattice-constant measurements on single crystalline U_xY_1-xSb (x=0, 0.01, 0.03, 0.08, 0.15, 0.20, 0.30, 0.70 and 1) reveal a nonmagnetic U 5f² singlet groundstate for x ? 0.15, a lowering of the f³→f²d¹ transition energy with decreasing x for x > 0.15 and a marked anomaly of the lattice parameter for x ? 0.15. These results as well as the anomalous concentration dependence of the U moment can be described by a valence transition of U at a concentration of about 15% U (x = 0.15).     

Binding energy correlations in lanthanide 4d auger and autoionization spectra

The correlations of both Auger transition energies and electron binding energies with electronegativity values have permitted the calculation of both hole-hole and atomic reorganization energies for each of the lanthanide 4d Auger transitions. A similar correlation for autoionization energies has shown that, for each of the lanthanides, the 4d–4f direct recombination is associated with a coupled Auger process. A simple equation, together with the hole-hole and reorganization energy data derived herein, is used to predict the energy differences between autoionization and Auger energies; reasonable agreement with experiment is obtained.     

The probability of prompt and delayed fission of muonic237Np

Fission fragments from the reaction²³⁷Np(μ ^?,γ,f) have been measured in coincidence with muonic X-rays. The efficiency of the fission fragment detector is determined from (μ ^?,γ,f)-data of the same experiment. The total fission probability perμ-stopP _t has been measured as well as the fission probabilities P_f of the non-radiative muonic (3d→1s)- and (2p→1s)-transitions; the latter has been divided into two parts leading to different mean excitation energiesE:P _t =(54±17)%,P _f (3d→1s)=(41±21)%,P _f (2p→1s,E=6.218 MeV)=(61±19)%, andP _f (2p→1s,E=6.525 MeV)=(57±18)%. The influence of the muon on the fission barrier is discussed. The fission probability after muon capture is compared with a calculated value using a distribution of nuclear excitation energies following muon capture and the fission probability as measured in a²³⁸U(³He,αf)-reaction.     

Review of Ni-like ion X-ray laser research at Lawrence Livermore National Laboratory

We have demonstrated saturated output on several nickel-like ion X-ray lasers ranging from niobium to silver by using a psec, high-power, chirped-pulse-amplification, tabletop laser. These results have been achieved at wavelengths from 20.3 to 13.9 nm on the Ni-like 3d⁹4d¹S₀→3d⁹4p¹P₁ laser line using a total of 5 to 7 J of energy in a traveling wave excitation scheme. Strong amplification is also observed for Ni-like Sn at 11.9 nm. Gain of 41 cm⁻¹, gain-length product of 18, and output energy of 12 μJ are measured for the Ni-like Pd line at 14.7 nm. For Ni-like Mo, experiments are done using multilayer mirrors to obtain two-dimensional images of the output aperture of the laser and to measure the total laser energy as a function of various parameters such as the delay between the short and long pulses and the energy of the two pulses. For Mo we measure an output energy of 2 μJ and a gain-length product of 16.6. To model the Mo experiments, the LASNEX code is used to calculate the hydrodynamic evolution of the plasma and provide the temperatures and densities to the XRASER code, which then does the kinetics calculations to determine the gain. The temporal and spatial evolution of the plasma is studied both with and without radiation transport included for the 4f and 4p→3d Ni-like Mo resonance lines. High gains are predicted and observed for both the 3d⁹4d¹S₀→3d⁹4p¹P₁ laser line at 18.9 nm and the 3d⁹4f¹P₁→3d⁹4d¹P₁ photopumped line at 22.6 nm.     

X-ray M4,5 resonant Raman scattering from Gd with final 4p hole: calculations with 4p–4d–4f configuration interaction in the final state and comparison with the experiment

We present calculations of resonant Raman scattering (RRS) at the M_4,5 thresholds of Gd in the scattering channel 3d¹⁰4f⁷→3d⁹4f⁸→[4p⁵4f⁸↔4d⁸4f⁹]. We have included in the final state the interaction between the two configurations within the brackets, having one 4p and two 4d holes, respectively. The influence of the configuration interaction on the scattering spectra is shown to be important. The calculations are made within a purely ionic model including only the spectral features dispersing with the incident photon energy and do not account for the M₄ to M₅ Coster-Kronig conversion. The calculations are compared with recent experimental results on Gd metal. The agreement is excellent when choosing the excitation energy in the M₅ region. In the M₄ region the calculations agree with the measurements by assuming that the Coster-Kronig contribution is approximated in shape by the RRS spectrum measured with direct M₅ excitation. The implications of the results are discussed.     

Radiative constants in the spectra of ions of the erbium isoelectronic sequence

The probabilities of the 4f ¹³6p→4f ¹³5d lectric dipole transitions and the lifetimes of levels of the 4f ¹³6p and 4f ¹³5d configurations are calculated for spectra of ions Yb III, Lu IV, Hf V, and Ta VI of the erbium isoelectronic sequence. The wave functions of the intermediate coupling scheme, necessary for calculating the relative line strengths, are obtained semiempirically from experimentally measured energy intervals between fine-structure levels. To pass to the absolute values, the radial integrals of transitions, evaluated with the Hartree-Fock functions, are used.     

Radiationless transition probabilities in muonic208Pb,232Th,and238U

The probabilities for non-radiative (n.r.) excitationsP _n.r. in the muonic nuclides²⁰⁸Pb,²³²Th, and²³⁸U have been determined from (μ^?,γγ)-measurements by comparing the intensities of muonic X-ray transitions in single and coincidence spectra. The value ofP _n.r. (3p→1s), measured for the first time, is about 90% for the actinides²³²Th and²³⁸U, but only about 8% for²⁰⁸Pb. The value ofP _n.r. (3d→1s) is found to be 10 % for²³³Th, 13% for²³⁸U, and about 4% for²⁰⁸Pb. For²⁰⁸Pb a vanishing strength of the n.r. decay of the 2p-level is found, while for²³²Th and for²³⁸U n.r. strengths of about 20% and 26%, respectively, are observed. By regarding two subcomplexes of the 2p→1s transitions leading to different mean excitation energies the n.r. transition probabilities were found to be different for²³⁸U only, 21.6% and 31.1.%, respectively.     

Many-electron model of band structure and metal-insulator transition under pressure in FeBO<Subscript>3</Subscript>

A many-electron model is proposed for the band structure of FeBO₃ with regard to strong electron correlations in the d⁴, d⁵, and d⁶ configurations. Under normal conditions, FeBO₃ is characterized by a dielectric charge-transfer gap in the strong correlation regime U?W. With increasing pressure, not only does the d-band W width grow but simultaneously the effective Hubbard parameter U_eff sharply drops, which is due to the crossover of high-spin and low-spin ground state terms of the Fe²⁺, Fe³⁺, and Fe⁴⁺ ions. It is predicted that a transition from the semiconducting antiferromagnetic state to the metallic paramagnetic state will occur in the high-pressure phase with increasing temperature.     

Analysis of the spectra of In XII–XIV and Sn XIII–XV in the far-VUV region

The 4p ⁶4d?(4p ⁶5p + 4p ⁶4f + 4p ⁵4d ²) transitions in the spectra of In XIII-Sn XIV and the 4p ⁶4d ²?(4p ⁶4d5p + 4p ⁶4d4f + 4p ⁵4d ³) transitions in the spectra of In XII-Sn XIII are investigated. About 150 spectral lines in the wavelength range 120–160 Å, which is important for projection vacuum UV lithography, have been classified for the first time. The fine structure intervals of the ground state 4p ⁶4d ² D and the energies of eight levels of the 4p ⁶5p, 4p ⁶4f, and 4p ⁵4d ² configurations are measured in each spectrum of In XIII and Sn XIV. The energies of all levels of the ground configuration 4p ⁶4d ² and the energies of 35 levels of the interacting configurations (4p ⁶5p + 4p ⁶4f + 4p ⁵4d ³) are determined in the spectra of In XII and Sn XIII. The resonant transitions 4p ^{6 1} S ₀?4p ⁵4d ¹ P ₁ in the spectra of In XIV and Sn XV are also identified. The semiempirical parameters of the configurations under study determined by mean-square fitting of measured level energies are in agreement along both isoelectronic and isonuclear sequences of the spectra.     

Crossover from high-temperature vortex-liquid to low-temperature vortex-glass behavior in single phase HgBa2Ca2Cu3O8+δ superconductor

The ρ-T curves in our single phase HgBa₂Ca₂Cu₃O_8+δ superconductor were measured as a function of temperature and magnetic field, ρ=ρ₀exp(−U_eff/κ_BT). It can be transformed to another form d(lnρ)/d(1/T)=−U_eff+TdU_eff/dT, then this becomes a plot of the activation energy U_eff as a function of temperature. Our data plotted in these ways show a clear crossover from high-temperature two-dimensional vortex-liquid to a critical region associated with the low-temperature three-dimensional vortex-glass phase transition. The critical exponents v(z−1)=3.9±1.9 in this system are little different with previous measurements in BSCCO and YBCO systems.     

Cross sections and angular distributions of three-jet final states ine + e − annihilation for heavy quarks

The cross sectionsdσ _U ,dσ _L ,dσ _T anddσ _I fore ⁺ e ^?→q \(\bar q\) g are calculated for heavy quarks, and the resultant angular distribution is discussed.     

Analysis of the sequence of insulator-metal phase transitions at high pressure in systems with spin crossovers

Possible variants of the Mott-Hubbard phase transitions at high pressure in systems with spin crossovers are considered. Owing to the universal character of the dependence of the effective Hubbard parameter U _eff(d ⁿ ) on the average number of d electrons, which is determined by the presence of spin crossovers, cascades of insulator-metal-insulator phase transitions in systems with d ³, d ⁶, and d ⁸ configurations become possible. Moreover, the systems with d ⁶ configuration can exhibit transitions from a metal in the absence of external pressure to an insulator at high pressure.     

Spectral and kinetic characteristics of trivalent praseodymium in LaF3-LiF and SrAl12O19

Recent data on cascade transitions in the 4f shell of the Pr³⁺ ion in various matrices are analyzed. Spectral and kinetic characteristics of LaF₃-LiF:Pr and SrAl₁₂O₁₉:Pr phosphors, which show photon cascade emission, were investigated. The emission intensities in the first (¹ S→¹ I ₆ transitions) and the second (transitions from the ³ P ₀ level to the ³ H and ³ F multiplets) cascade stages were measured and the temperature dependences of the intensities of the main emission lines and their kinetic characteristics were determined. The following parameters of SrAl₁₂O₁₉:Pr were found: the band gap width (7.5 eV), the energy gap between the ¹ S ₀, 4f and 5d levels (0.24 eV), and the characteristics of the 4f→5d band (6.0–7.5 eV) of the Pr³⁺ luminescence excitation. It is shown that the LaF₃-LiF:Pr compound has a number of specific features in comparison with other Pr³⁺-doped phosphors.     

Fragment angular distributions from fission of238U,209Bi and197Au by 140-MeV4He Ions

The fission fragment angular distributions from reactions of 140-MeV⁴He ions with²³⁸U,²⁰⁹Bi and¹⁹⁷Au have been studied. From the anisotropies in the angular distributions, values for? _eff, the effective moment of inertia at the fission saddle point, have been estimated and compared with results obtained at lower bombarding energies. The derivation of? _eff values has included corrections for the effects of incomplete fusion mechanisms on the orbital angular momentum distribution of the fissioning nuclei and for neutron evaporation prior to fission. The results are also compared with heavy-ion-induced fission data for systems of similar fissility. Also, examination of the forward-backward symmetry of the²³⁸U angular distribution substantiates other results which show that the fraction of fission reactions which follow complete fusion of the target and projectile is less than 0.5 for 140-MeV⁴He-ion bombardment of²³⁸U.