Search Results (38 total)

Two oxygen species, which are constituents of the active centers for ethylene epoxidation over silver, have been characterized by a number of physical methods sensitive to adsorbate electronic structure such as x-ray photoelectron, ultraviolet photoelectron, Auger, and x-ray-absorption near-edge structure spectroscopy. One of the species denoted as nucleophilic oxygen due to its activity in total oxidation only exhibits spectroscopic characteristics close to those of bulk Ag₂O. This allows us to describe this species as atomically adsorbed oxygen in the structure of surface silver(I) oxide. The considerable extent of the covalency in bonding of this oxidelike oxygen with the silver surface due to hybridization of O2p levels with Ag4d and Ag5sp orbitals should be also emphasized. Contrary to this, only 5sp orbitals of silver hybridize with 2p orbitals of oxygen as the other oxygen species forms. As a consequence, this species being also atomic oxygen is characterized by a lower oxygen-silver bonding interaction and a lower charge on the oxygen. The latter causes the activity of this electrophilic species in epoxidation. Possible models of adsorption centers for these oxygen species are discussed.

The magnetic moment of the 2₁⁺ and 4₁⁺ states in ¹⁵²Gd have been measured by the transient field technique. Heavy ion beams of ⁵⁸Ni and ³²S were used to Coulomb excite the 2₁⁺, 0₂⁺, 4₁⁺, 2₂⁺, and 6₁⁺ states in ¹⁵²Gd. A ratio of g factors, g(4₁⁺)/g(2₁⁺)=1.10(24) was obtained. This result is consistent with a collective model description and does not support large single particle contributions to the wave function of low-lying states as were found for the isotone ¹⁵⁰Sm.

A Reply to the Comment by Faris Gel'mukhanov and Hans Agren.

The average g factors of the high-energy states of the three superdeformed bands in ¹⁹⁴Hg were determined directly in a transient field experiment. The reaction ¹⁵⁰Nd(⁴⁸Ca,4n)¹⁹⁴Hg at a beam energy of 203 MeV was used to provide recoiling reaction product nuclei with sufficient velocity to traverse a gadolinium ferromagnetic layer. The resulting g factors g(SD1)=0.36(10), g(SD2)=0.41(20), and g(SD3)=0.71(26) are in agreement with cranked Hartree-Fock calculations as well as with the picture of a rigid rotation for which g=Z/A.

Projectile excitation and the transient field technique have been used to measure the g factors of the 2₁⁺, 2₂⁺, and 4₁⁺ states in ^74–82Se in order to study the influence of the N=50 shell closure at low excitation energies. The states of interest were populated by Coulomb exciting beams of the appropriate isotope by the same natural Si target. The results demonstrate that inverse kinematics provides a very powerful technique, highly suitable for measurements with radioactive beams. The g factors obtained for ^74–82Se are compared with results from IBM-II calculations.

Experimental and theoretical studies are reported on the inelastic (Raman) scattering of wavelength-selected polarized x rays from the K edge of gas-phase chlorine molecules. The polarized emission spectra exhibit prominent nondipole features consequent of phase variations of the incident and emitted radiation over molecular dimensions, as predicted by the Kramers-Heisenberg scattering formalism. Issues pursuant to the detection of core-hole localization by resonant Raman scattering from homonuclear diatomic molecules are critically examined.

Single and multinucleon transfer yields for the ¹³⁶Xe+⁶⁴Ni reaction at a scattering energy ≈5% above the Coulomb barrier energy are studied using particle–γ-ray coincidence data. Q-value and scattering-angle distributions are extracted for the stronger channels. A fast transfer mechanism dominates the yields to these channels over an extended Q-value range, leading to a concentration of the cross section near the grazing angle. Analysis of the angular distributions based on a semiclassical barrier penetration model suggests that the single-nucleon and two-neutron exchange channels are dominated, respectively, by direct and two-step sequential transfer from the ground or low-lying excited states of the participating nuclei. The multiproton transfer channels have angular distributions that indicate a more complex mechanism, although direct cluster transfer from an excited configuration cannot be fully discounted. In a separate analysis, the relative population of different mass channels is found to be in general agreement with the expectations of a “random walk” model of particle exchange.

The decays of seniority isomers in the N=82 nuclei ¹⁵⁰Er and ¹⁵²Yb and in their respective N=81 isotopes ¹⁴⁹Er and ¹⁵¹Yb were studied following mass separation by the Argonne Fragment Mass Analyzer. Conversion electrons were detected with Si p-i-n diodes operated at room temperature. The low-energy isomeric transitions in ^151,152Yb have been observed for the first time in the electron spectra. Multipolarity assignments were made for many of the decay γ rays of the four nuclei.

A superdeformed band has been found in the ¹⁵⁴Dy (N=88) nucleus. The dynamic moment of inertia is identical to that of the yrast superdeformed band of ¹⁵²Dy and the transition energies are similar to those of an excited superdeformed band in ¹⁵³Dy. It is proposed that the two valence neutrons above the N=86 shell gap occupy the deformation-driving [514]9/2 orbital.

The ⁶⁸Ni nucleus has been identified among the products of deep-inelastic reactions of ⁶⁴Ni projectiles bombarding ¹³⁰Te and ²⁰⁸Pb targets. Three new states, including the high-lying 2⁺ (2033 keV) and the 0.86 ms 5^- isomer, indicate a substantial subshell closure at neutron number N=40. The level structure and the observed very slow E3 transition speed are discussed within the shell model.

The lifetimes of states in two of the ΔI=1 bands in ¹⁹⁶Pb have been measured using the Doppler shift attenuation method, and a new ΔI=1 band has been observed in this nucleus. States in the bands were populated in the reaction ¹⁰⁷Er(³⁰Si,4n) at a beam energy of 142 MeV. Individual level lifetimes were extracted from the data by a analysis of the Doppler broadened γ-ray lineshapes. Under the assumption of pure M1 radiation, average reduced transition strengths B(M1)∼1.5 W.u. were obtained. The B(M1) values in both the ‘‘regular’’ and ‘‘irregular’’ bands exhibit similar dependence on spin. The neutron and proton configurations and nature of the collectivity in these bands is discussed.

Reanalysis of ichproductsofbinaryreactio nsof$³⁴S, ³⁶S, and ³⁷Cl beams on ¹⁶⁰Gd targets. Gates set on known amma$raysin$A∼160 products selected individual react ion channels and identified coincident amma$raysin$A∼36 partner products. Transfers of protons from projectile to target and of neutrons from target to projectile were generally favored, leading to excited neutron-rich light nuclei, including some difficult to reach by other means. Notable results include the observation of amma$raycascadesup^thehigh e stknownyraststatesinfour$N=20 isotones. In two N=19 nuclei, ³³Si and ³⁴P, the two N=22 nuclei, ³⁸S and ³⁹Cl, previously unknown yrast states were identified.

The superdeformed band in ¹⁹⁶Pb has been studied extensively using the reaction ¹⁷⁰Er(³⁰Si,4n) at beam energies of 142, 146, and 151 MeV. New transitions have been added at the top and bottom of the previously known band. Gamma-ray directionsl correlations were measured for most of the transitions in the band verifying the expected stretched E2 character. The collectivity of the band has been measured using the Doppler shift attenuation method yielding an intrinsic quadrupole moment Q₀ of 18.3±3.0 e b, in good agreement with theoretical predictions. The variations of the dynamic moment of inertia scrI⁽²⁾ as a function of the rotational frequency ħω have been studied and compared with cranked shell model calculations. The dependence of scrI⁽²⁾ on mass for superdeformed bands in the Pb isotopes is also investigated.

The multiplicities of p and α particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV ¹³⁶Xe+⁴⁸Ti have been measured for different exit channel mass asymmetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from ∼400 MeV to 110 MeV as the fragment mass asymmetry, A_H/A_L, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from ∼5×10^-22 s to ∼1×10^-20 s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.

Product recoils from the reaction ⁹⁶Ru +255 MeV ⁵⁸Ni → ¹⁵⁴Hf^* were analyzed using the Argonne Fragment Mass Analyzer and transported to a collector foil behind the focal plane, where decay of μs isomers could be studied under low background conditions. The recoils were dispersed in A/q as they traversed the position-sensitive focal-plane detector, and recoil–γ-ray coincidences could thus be used to assign masses for specific γ-ray cascades. By use of these methods, yrast isomers with half-lives of 2.6(7) μs and 20(1) μs have been firmly assigned to the exotic N=81 nucleus ¹⁵¹Yb, and the main features of the isomeric decays have been established. The yrast level structure of ¹⁵¹Yb and lighter N=81 isotones are compared.

Evidence for collective oblate behavior in ¹⁹⁶Pb is presented. One irregular and two regular bands of M1 transitions have been observed following the ¹⁷⁰Er(³⁰Si,4n) and ¹⁷⁶Yb(²⁶Mg,6n) reactions. Transitions linking the most intense regular band to the low-lying negative-parity yrast levels are observed, establishing excitation energies, spins, and probable parities of the band members. In contrast, no such transitions have been found for the irregular band and the weaker regular band. The bands are interpreted as corresponding to collective oblate rotation, arising mainly from deformation-aligned high-j, shape-driving quasiproton excitations across the Z=82 shell gap, coupled to rotation-aligned quasineutrons.

We present results from an experimental study of reabsorption effects in subthreshold π⁰ production in the reaction ¹²⁹Xe+¹⁹⁷Au at 44 MeV/nucleon. Within the picture of pion generation in nucleon-nucleon scattering we deduce, from our data and from a comparison with the systematics of production cross sections available for lighter reaction systems, information on the π⁰ absorption length in nuclear matter. For the π⁰ kinetic-energy range ≃5–100 MeV the energy-averaged λ_abs and its momentum dependence are obtained, and compared with optical-model calculations.

We report results of an experiment to search for a resonance enhancement of the e⁺e^- scattering cross section in a metallic Li target utilizing an energy-tunable monoenergetic positron beam. Within statistical uncertainties (0.27%) no evidence has been observed for deviations from Bhabha scattering over the entire invariant-mass region 1560 keV/c²<M_X⁰<1860 keV/c² that can be associated with the e⁺e^- sum-peak energies in GSI heavy-ion experiments. Under the assumptions that the e⁺e^- channel dominates, we deduce lower limits on lifetimes τ>3.3×10^-13 sec (J=0) and τ>8.2×10^-13 sec (J=1), at 90% C.L.

High-energy γ rays have been measured in coincidence with heavy fragents in deeply inelastic reactions of ¹³⁶Xe+⁴⁸Ti at 18.5 MeV/nucleon. The giant dipole resonance (GDR) strength function is deduced from an analysis of the photon spectra within the statistical model. The GDR width Γ is studied as a function of the fragment excitation energy E^*. A saturation at about Γ=10 MeV is observed for E^*/A≥1.0 MeV/nucleon.

Long-lived 10⁺ isomers in ¹²²Sn and ¹²⁴Sn have been identified among the products of ¹²⁴Sn+325 MeV ⁷⁶Ge collisions. The measurements virtually complete a series of B(E2) determinations for (νh_11/2)ⁿ states in ^116-130Sn, which pinpoint half filling of the νh_11/2 subshell very close to N=73. Results for these Z=50 isotopes and for the N=82 isotones are contrasted, and an enlightening comparison between the effective E2 charges observec in tin and lead isotopes is developed.

Strongly anisotropic, polarized Cl K-V x-ray emission from gas-phase CF₃Cl has been observed following resonant excitation with a linearly polarized x-ray beam. Distinctively different angular distributions are observed for x-ray emission involving molecular orbitals of different symmetries. A classical model of the x-ray absorption-emission process accurately describes the observed radiation patterns.

X-ray emission from the molecule H₂S is strongly polarized following excitation of a sulfur K-shell electron to an unoccupied subthreshold molecular orbital with a polarized x-ray beam. Changes in the polarization of the emission spectrum are observed as the incident beam’s energy is swept across the subthreshold absorption resonance. The previously unresolved absorption resonance is shown experimentally to be primarily associated with a molecular orbital of b₂ symmetry, but with a high-excitation-energy component due to an orbital with a₁ symmetry. Satellite emission intensity is shown to depend on the primary photon energy and is therefore associated with multivacancy effects and not with contamination, as previously suggested.

We report the experimental observation of the temperature dependence of the intensity of low-energy positron-annihilation-induced Auger-electron emission spectroscopy (PAES) from Cu(100). These studies show that the mechanism for stimulating Auger electrons is found to compete with positronium (Ps) emission from a surface. The positrons that induce Auger-electron emission therefore originate from the same surface state from which Ps is thermally desorbed. Hence, PAES should have higher surface sensitivity (∼1 Å) relative to conventional methods for generating Auger-electron emission from surfaces (∼5–10 Å).

Positron- and electron-induced secondary-electron-energy distributions have been measured from MgO(100) and Ni(110) crystals. The energy distributions for positron- and electron-induced secondary electrons from MgO and Ni and for the slow-positron emission from the MgO target have been fit by the same analytic function. This analytic function fails to fit the slow-positron energy emission spectrum from Ni. The similarity of positron-induced secondary electron and reemitted positron energy spectra suggests that the slow-positron emission process in ionic insulators may be analogous to secondary-electron generation.

The present study is based on a sample of 407 775 single muons and 12 559 muon bundles with zenith angles smaller than 60°, observed in the 12.3m×6m×6m Fréjus proton-decay detector, at a depth of 4850 hg/cm². The variation of the vertical muon intensity with depth is given. Using a maximum-likelihood method, the muon lateral distribution of underground bundles is investigated as a function of muon multiplicity and zenith angle. The same method yields the true multiplicity distribution at the site depth, corrected for all detection effects.