TA Spectra Research Articles

Ta, Au and Th L X-ray energy shifts induced by 0.6, 1.2, 2.4 and 4 MeV/nucleon Kr ions

The L X-ray spectra of Ta, Au and Th induced by 0.6 to 4 MeV/nucleon (MeV/n) 84Kr ions have been studied by means of a LiF(100) equipped crystal spectrometer with an overall resolution of 30 eV at 8.146 keV. The mean energy shifts of the main lines have been measured with accuracies ranging from 5 eV for well separated lines, to 30 eV for poorly resolved lines. Mean energy shifts per M and N spectator electron vacancies were deduced and compared with predictions of different models. The mean number of these vacancies at the various beam energies were estimated. Diagram lines were also observed and assigned to the internal conversion of transition energies between nuclear Coulomb excited levels. In the case of the Lα lines of Ta and Au and at the lower beam energies (0.6 and 1.2 MeV/n) the full width at half maximum of these lines was used to deconvolute the shifted peaks in the individual corresponding satellite lines.

Mutation spectra of TA*, the major photoproduct of thymidylyl-(3'5')-deoxyadenosine, in Escherichia coli under SOS conditions.

The biological activity of TA*, the major photoproduct of thymidylyl-(3',5')-deoxyadenosine, has remained speculative since it was identified a decade ago. To determine the mutagenicity of TA* in Escherichia coli, we constructed the replicative form of an M13mp18-derived phage containing TA* in the (-)-strand by polymerase-catalyzed elongation of a TA*-containing 49mer opposite a uracil-containing (+)-strand of the phage. The in vitro synthesis mixture was transfected into an ung+, phr- E.coli host and the progeny were screened with a hybridization probe unique for the (-)-strand. TA* was found to block DNA replication substantially in the absence of SOS, but under SOS, TA* was bypassed more efficiently and was highly mutagenic. Among 56 analyzed (-)-strand progeny from two transfections, 46 (82%) were mutants, including six (11%) tandem mutants. The most abundant mutation was a 3'A-->T substitution (31/46, 56%). The possible biological consequences of TA* formation in the highly conserved TATA box consensus sequence on gene expression are discussed in light of the mutagenicity of TA*.

Photochromic centers and impurities in nominally pure KTaO3 and K1-xLixTaO3.

Investigations of photochromic centers and impurities in nominally pure single crystals of ${\mathrm{KTaO}}_{3}$ and ${\mathrm{K}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{TaO}}_{3}$ (KTL) have been carried out by the ESR method. In all investigated KTL samples (x=0.016, 0.02, 0.03) the axial symmetry spectrum of ${\mathrm{Fe}}^{3+}$ substituted for ${\mathrm{K}}^{+}$ [${\mathrm{Fe}}_{\mathrm{ax}}^{3+}$(K)], cubic symmetry spectrum of ${\mathrm{Fe}}^{3+}$ substituted for ${\mathrm{K}}^{+}$ [${\mathrm{Fe}}_{\mathit{c}}^{3+}$(K)], and that of ${\mathrm{Ni}}^{3+}$ substituted for ${\mathrm{Ta}}^{5+}$ [${\mathrm{Ni}}_{\mathit{c}}^{3+}$(Ta)] have been observed at T\ensuremath{\le}60 K. After light illumination with 365\ensuremath{\le}\ensuremath{\lambda}\ensuremath{\le}436 nm, several ESR spectra appeared, each one with the characteristic temperature of its annealing in the region 25\ensuremath{\le}T\ensuremath{\le}120 K. In KTL samples simultaneously the ${\mathrm{Fe}}_{\mathrm{ax}}^{3+}$(K) spectrum disappeared and intensities of ${\mathrm{Fe}}_{\mathit{c}}^{3+}$(K) and ${\mathrm{Ni}}_{\mathit{c}}^{3+}$(Ta) spectra decreased. In nominally pure ${\mathrm{KTaO}}_{3}$ the spectra ${\mathrm{Fe}}_{\mathrm{ax}}^{3+}$(K) and ${\mathrm{Fe}}_{\mathrm{ax}}^{+}$(K), observed before illumination, disappeared after it. The gradual restoration of these aforementioned spectra in both ${\mathrm{KTaO}}_{3}$ and KTL was observed at T>25 K when some of the photoinduced spectra disappeared. One of the photoinduced spectra was shown to be that of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ in both ${\mathrm{KTaO}}_{3}$ and KTL. A scheme of local electronic levels of impurities in the band gap of nominally pure ${\mathrm{KTaO}}_{3}$ and KTL was proposed. This scheme and especially the shallow ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ level in it made it possible to explain some peculiarities of photosensitive phenomena observed in nominally pure ${\mathrm{KTaO}}_{3}$ and KTL in recent years.

Q-Deformed Euclidean algebras and their representations

A new q-deformed Euclidean algebra Uq(ison), based on a definition of the algebra Uq(son) different from the Drinfeld-Jimbo definition, is given. Infinite-dimensional representations Ta of this algebra, characterized by one complex number, is described. Explicit formulas for operators of these representations in an orthonormal basis are derived. The spectrum of the operator Ta(In) corresponding to a q-analogue of the infinitesimal operator of shifts along the n-th axis is given. Contrary to the case of the classical Euclidean algebraison, this spectrum is discrete and the spectrum points have one point of accumulation.

Parameter correlations and constraints in least-squares analysis of photoemission data

Methods used to obtain information from data with unresolved lines are described and applied to core-level photoemission spectra. It is shown that the parameter correlation matrix provides an invaluable guide to least-squares optimization. Difficulties, signaled by large elements in this matrix, can be overcome by introducing physically motivated constraints. This approach yields a surface-atom core-level shift of 195 meV from the 3p spectrum of the K(110) surface, as well as surface and subsurface shifts of 375 and 60 meV from the 4f spectrum of Ta(110).

Kα X-ray yield from alpha particle bombardment of thick targets

Abstract Numerical and experimental investigations have been conducted on the generation of intense sources of monochromatic X-rays via bombardment of thick targets of pure elemental composition by alpha particle beams with energy of tens of MeV. The present results extend a previous study made with protons. The thick target K α yield has been calculated for 62 elements, for energies of the incident ions ranging from 1 MeV to 100 MeV and polynomial approximations for the yield functions have been deduced. Experimental measurements on Mo, Cd and Ta for bombarding energies between 15 and 36 MeV have been compared with the numerical predictions. Satellite X-ray lines have been observed in Cd and Ta spectra, which are particularly intense in the case of the latter element. Measurements on Ta and Pb made with protons of the same energy have shown that the phenomenon is much weaker than in alpha induced reactions. It is concluded that alpha particles are advantageous over protons in producing very intense monochromatic sources with targets of low-medium atomic number.

A comparison of the surface electronic structure of Ta(100) and TaC(111) using Auger‐photoelectron coincidence spectroscopy

We present N7VV (4f7/25d5d) Auger spectra from TaC(111) taken in time coincidence with photoemitted bulk and surface‐shifted 4f7/2 core‐level electrons, and compare these data with self‐convolutions of calculated valence band d densities of states (SCDOS) for the surface (first) and bulk (exponential sum of second, fourth, and sixth subsurface) atomic layers of a 13‐layer slab. The Ta 4f levels of TaC(111) and Ta(100) both exhibit large surface core level shifts, but of opposite sign. Therefore, we compare the TaC(111) data and calculations with analogous N7VV Auger coincidence spectra and calculated SCDOS that we have previously published for Ta(100). The bulk Ta(100) and surface TaC(111) spectra show favorable agreement with the calculations; the surface Ta(100) and bulk TaC(111) spectra, on the other hand, exhibit marked disagreement with the calculations. These discrepancies are consistent with and provide added support for the presence of our proposed1 surface‐to‐bulk (bulk‐to‐surface) interatomic Auger decay mode which adds a bulk (surface) contribution to the surface (bulk) Auger line shape of Ta(100) (TaC(111)). These conclusions are supported by comparison of our data to incoherent sums of bulk and surface SCDOS.

Low-frequency phonon spectra of periodic metallic superlattices

A Raman scattering experimental study of low-frequency acoustic phonons in one-dimensional periodic metallic Cu/Nb superlattices is presented. The approximations of elastic continuum and Thomas-Fermi are first used to interpret observed the phonon spectra of TA and LA modes in reduced Brillouin zone. One can find that the effective sound velocity in metallic superlattice is probably relevant to electron-phonon interaction.

Breakdown of jj coupling in spin–orbit-split atomic transition arrays

The global effects of configuration mixing on the total intensity of a transition array have been interpreted. The changes of the subarray intensities by the mixing of pure jj subconfigurations (breakdown of jj coupling) in heavy atomic ions can be calculated in the same way. The perturbed relative intensities are deduced from the eigenvectors of a 2 × 2 energy matrix. This matrix is not changed if the subarrays contain spectator electrons. The calculated values give the interpretation of some conspicuous features appearing, for instance, in highly ionized Ta, Pt and Hg x-ray spectra.

Raman scattering in a-Ge 1−xSi x:H alloys

Raman scattering measurements have been performed on a-Ge 1−xSi x:H alloys (x=0.5−1.0) prepared by glow discharge deposition. The widths of high frequency optic-like bands are found to be narrowed relative to studies of r.f. sputtered films, indicating increased bond angle order about both Si as well as Ge sites. The SiSi TO band width changes provide a means of estimating contributions of structural and compositional disorder. The low frequency TA spectra exhibit fine structure in x= 0.7 and 0.8 alloys that is not predicted for an average mass determined band shape.

Unoccupied surface states on Ta(100) observed with inverse photoemission.

In an attempt to understand the interrelation of surface electronic structure and surface geometry, we have used inverse photoemission to measure the unoccupied surface states on the (100) face of tantalum. Tantalum has one fewer electron than tungsten and the Ta(100) surface is bulk terminated at room temperature, while the W(100) surface is reconstructed. We observe several well-defined features within 2 eV of the Fermi level in the Ta(100) spectra that show strong sensitivity to surface contamination and are therefore associated with surface states and/or resonances. It is difficult to relate the observed features to the W surface states in an empirical way. The two-dimensional dispersion of these states is rather flat, as expected for d-derived states. The results are compared in detail to a recent slab calculation. The spectral intensity of the surface states is significantly weaker than for states on Cu and Ag, a fact that we relate to the higher density of unoccupied states immediately above the Fermi level for the 5d metals as compared to the 3d metals. A peak seen near the Fermi level for the normal-incidence spectrum is related to a strong surface state or resonance below the Fermi level broadened so that it spans the Fermi level. The most likely origin of this broadening is coupling to bulk bands.

Systematics of the pygmy resonance in keV neutron capture γ-ray spectra of nuclei with N ≈ 82–126

We have measured capture gamma-ray spectra of Pr, Tb, Ho, Lu, Ta and Au at neutron energies of 10 to 800 keV with an anti-Compton NaI(Tl) detector, employing a time-of-flight technique. An anomalous bump, so-called the pygmy resonance, was observed in all these spectra. Remarkable features of the pygmy resonance were found to be that the resonance energy and the electric-dipole strength exhausted in the resonance increase with neutron number but these quantities decrease precipitously around the neutron magic number of N = 82. Comparison with theoretical calculations suggests that the pygmy resonance is mainly made up of neutron particle-hole states decoupled from the giant electric-dipole resonance and is excited in a collective mode.

Self-trapped electrons in lithium tantalate

The electron spin resonance spectrum of Ta 4+ ions has been investigated in LiTaO 3. These defects were produced either by irradiating as-grown crystals with X-rays or by optically bleaching crystals that had been previously reduced. In both cases, the Ta 4+ ions were stable only at temperatures near or below 77 K. The g and hyperfine matrix parameters for the Ta 4+ spectrum are g ‖ = 1.503, g ⊥ = 1.172, A ‖ = ∼0, and A ⊥ = 699 MHz. These parameters describe a 5d 1 electron on the tantalum ion, and we conclude that the defect represents a “self-trapped” electron at the normal Ta 5+ site.

On the trail of metathesis catalysts

It was in the fall of 1973 that Geoffrey Wilkinson made one of his too-infrequent visits to the Central Research Department at E.I. duPont de Nemours and Company. On this occasion he announced that he had prepared hexamethyltungsten. At the time I was interested in adding the dianion of cyclooctatetraene to early transition metals in order to provide complexes of potential catalytic interest [I], and in the process had rediscovered Juvinall’s preparation of TaMqCl, [2]. Since the C,H, complexes I obtained did not appear to be potentially interesting catalysts, I decided to try to prepare pentamethyltantalum by adding methyllithium to TaMe,Cl,. Pentamethyltantalum, like hexamethyltungsten, turns out to be an unpredictable and potentially vicious material when isolated as a solid, owing to its ready decomposition to give methane and (under some conditions) hydrogen 131. Since Zr(CH,Pli), and Ti(CH~Ph)~ had been known for several years [4], I was not surprised to find that pentabenzyltantalum could be prepared, and that it was a relatively stable red crystalline species [3b,S]. I was surprised when I attempted to extend the list of pentaalkyls to pentaneopentyltantalum. Trineopentyltantalum dichloride was prepared straightforwardly, and upon addition of 2 equivaIents of neopentyllit~um to it in pentane a lovely orange solution was produced, from which a low melting orange crystalline product could be isolated virtually quantitatively [6a]. Its mass spectrum revealed that it was not pentaneopentyltantalum, but a compound whose mass was consistent with the formulation T~(CHB~‘)(CH,BU’)~. Paul Meakin was kind enough to run a 13C NMR spectrum of Ta(CHBut)(CHzBu’), that same night; I asked him to be sure to extend the window to 300 ppm downfield of TMS. He called the next morning to tell me that there was indeed a most peculiar doublet located at 250 ppm with a coupling constant of only 90 Hz. Since this was the first “carbene” complex that did not contain some sort of stabilizing substituent(s) (usually 0 or N) [7], I did not think the low “olefinic” carbon-hydrogen coupling constant was especially significant. Extensive studies did not result in any firm conclusions regarding the mechanism of formation of TatCHBut)(CH*But)~, and it is still uncertain whether Ta{CH,Bu’),Cl spontaneously loses neopentane or is dehydrohalogenat~ by a fifth equivalent of a lithium reagent to give the final product [6b]. The salient feature of the reaction in either case is removal of an a-hydrogen from one neopentyl group by another

Calculation of Gamma-Ray Production Cross Sections at Neutron Energies of 1–20 MeV

In order to prepare the p-ray data library requested in a design of fission and fusion reactors, γ-ray production cross sections and spectra of Al, Si, Ca, Fe, Ni, Cu, Nb, Ta, Au and Pb have been obtained at the neutron energies of 1–20 MeV, using a spin-dependent multi-step evaporation model. Calculations include dipole and quadrupole transition without the distinction between electric and magnetic process, and take explicit account of the role of yrast levels. The effects of the yrast levels and γ-ray strength function upon γ-ray production are also investigated in relation to particle emission. At the incident neutron energies where (n,nγ ) and/or (n,2nγ ) reactions are dominant, the present model is shown to be able to predict the production of secondary γ-rays (<9.0 MeV) from medium-heavy to heavy nuclei with reasonable accuracy.

Calculation of gamma-ray production cross sections at neutron energies of 1-20 MeV.

In order to prepare the p-ray data library requested in a design of fission and fusion reactors, γ-ray production cross sections and spectra of Al, Si, Ca, Fe, Ni, Cu, Nb, Ta, Au and Pb have been obtained at the neutron energies of 1–20 MeV, using a spin-dependent multi-step evaporation model. Calculations include dipole and quadrupole transition without the distinction between electric and magnetic process, and take explicit account of the role of yrast levels. The effects of the yrast levels and γ-ray strength function upon γ-ray production are also investigated in relation to particle emission. At the incident neutron energies where (n,nγ ) and/or (n,2nγ ) reactions are dominant, the present model is shown to be able to predict the production of secondary γ-rays (<9.0 MeV) from medium-heavy to heavy nuclei with reasonable accuracy.

Disorder-induced TA Raman lines in mixed Cu-halide crystals

The Raman spectra of $\mathrm{Cu}{\mathrm{Cl}}_{x}{\mathrm{Br}}_{1\ensuremath{-}x}$ and $\mathrm{Cu}{\mathrm{Br}}_{x}{\mathrm{I}}_{1\ensuremath{-}x}$ solid solutions are shown to contain extremely intense disorder-induced first-order TA phonons. The intensity of these lines is a function of the concentration $x$, and is highest in Cu${\mathrm{Cl}}_{0.60}$${\mathrm{Br}}_{0.40}$. The TA spectrum, well resolved at low temperature, fits well the density of states (DOS) calculated from inelastic-neutron-scattering data. Assuming a disorder model in which ${\mathrm{Cu}}^{+}$ populates the central site or off-center sites, the high-intensity of this line and its temperature and pressure dependence are explained. The Gr\uneisen parameters of the TA lines were measured in various mixed crystals, and fit well other related thermodynamic parameters. The low-temperature 2TA lines of both pure and mixed crystals fit the calculated two-phonon DOS. The spectral analysis at high temperature is discussed, and it is found that ${\ensuremath{\omega}}_{2\mathrm{TA}}(T)$ follows nicely $2{\ensuremath{\omega}}_{\mathrm{TA}}(T)$.

“Stage III” in tantalum deformed at 4.2°K

This paper reports a study on the electric resistivity spectrum of polycrystalline Ta severely deformed at 4.2/sup 0/K, particularly on the stage showing up at approximately 275/sup 0/K. Results are compared with those after irradiation at low temperatures. The activation energy of 0.67 +- 0.07 eV for the 275/sup 0/K recovery stage compares well with that for vacancy migration. (DLC)

Spectrum and energy levels of five-times ionized tantalum (Ta vi)

The spectrum of Ta vi produced with a sliding-spark discharge was photographed in the region of ~200–2000 A utilizing grazing-incidence and normal-incidence spectrographs. A system of seventy excited energy levels was deduced from 228 spectral lines. These arise from excitations out of both the 4f and 5p closed shells of the 4f145s25p6 ground configuration, giving the observed configurations 4f13nl with nl = 5d, 6s, 6p, 6d, 7s and 5p5nl with nl = 5d, 6s, 6p. Radial integrals were fitted for all of these configurations. A value of 785 130 ± 400 cm−1 for the ionization energy was derived. A graph of lowest excited levels of 4f135d and 5p55d from Lu iv to Re viii predicts the crossing of these configurations after W vii.

The structure of the spectrum of Ta V

Thirteen lines of the spectrum of Ta, observed by means of a 6.650 m vacuum spectrograph with a hollow cathode and a sliding spark as light sources are identified as belonging to the spectrum of four times ionized Ta. These lines are classified in a level scheme consisting of 10 levels with the 5d 2D 3 2 as ground level. The ionization potential was calculated to be 390400 (1000) cm -1 or 48.4 (0.1) V.