M. von Hartrott

Core polarization effects in the πh92 shell and g-factors of the 8+ states in 204Po and 206Po

Abstract The g- values of [(π h 9 2 ) 2 , 8 + ] isomeric levels in 204 Po and 206 Po have been measured with the stroboscopic method. The results obtained are: g( 204 Po ) = 0.923±0.013 and g( 206 Po ) = 0.919±0.013 . These and other known g- factors of (π h 9 2 ) n states in the 208 Pb region are compared with theoretical predictions in order to analyze the dependence of the h 9 2 proton g -value on the core constitution. The variation of the g -values is systematically smaller than expected from first-order configuration mixing theory, if pure shell-model wave functions are assumed.

Electric field fluctuations and bond character in liquid alloys

Abstract Electric field fluctuations within a liquid alloy can be measured by nuclear quadrupolar spin relaxation rate RQ of noble gas atoms imbedded in the liquid. The method is demonstrated using Xe as a prove in liquid Te alloys, viz. TeAg, Ga, Ge, In, Sn, Tl. It is proposed to use RQ in order to derive a measure of ionic contribution to bonding. Combining the RQ data with conductivity data, and applying a simple model approach, we estimate the fraction of metallic-covalent-ionic bonds for each of the alloys.

Local fluctuations and quadrupolar relaxation: liquid lead alloys

The nuclear spin relaxation rate R has been studied in liquid alloys Pb1-CXC with X=Li, As, Ag, In, Sn, Sb, Te, Au, Tl and Bi at various concentrations C, and for X=In, Sb, Bi as a function of temperature T at C=0.5. The 47 mu s isomer 207mPo, for which the quadrupolar contribution to R, RQ, is dominant, was used as the probe nucleus; it was produced and aligned by the nuclear reaction 206Pb ( alpha , 3n) and the relaxation rate was measured by the perturbed angular distribution technique. For most of the alloys studied here the contribution to RQ due to density fluctuations, RQNN, dominates. Thus these data complement the earlier measurements by the authors of alloys where the concentration fluctuation contribution, RQCC, dominated. They confirm the earlier findings that the magnitude of RQ in alloys is essentially determined by the valence difference between the partner elements and that the long-range components of the interaction are decisive for quadrupolar relaxation (i.e. distances larger than the nearest-neighbour distance).

Nuclear quadrupolar relaxation of Sb in liquid In-Sb alloys

In the In1-cSbc alloys the nuclear spin relaxation of Sb is measured using the TDPAD technique on the 340 mu s isomer 117mSb which is produced and aligned by the nuclear reaction 115In( alpha ,2n). The temperatures and compositions studied are: at T=900K, 0<or=c<or=0.8; at c=0, 430K<or=T<or=1000K; and at c=0.5, 770K<or=T<or=1100K. These measurements complement and extend existing NMR data for Sb in In1-cSbc. The quadrupolar contribution to the rate (TQ-1) is found to dominate for 117mSb. Its temperature dependence (at c=0.5) and concentration dependence (over the full range 0<or=c<or=1) can be described fairly well by the current theory of TQ-1 for a random substitutional metallic alloy. The parameters entering the theory are discussed. The quadrupole moment of the isomer 117mSb is determined as Q=(-)0.54(20) barn.

Association in strongly interacting liquid binary alloys and nuclear spin relaxation

An experimental quantity sensitive to the time development of the distance between two atoms in a liquid alloy, is the quadrupolar nuclear spin relaxation rate RQ. The existing material for s-p-alloys shows a systematic occurrence of string enhancements of RQ if there is an attractive interaction among the partner elements in alloys. Using a model of associates for the alloy, it is possible to connect RQ rather directly with the lifetime of the associate. For In---Hg at 260 K as an example, 10?11 sec is obtained.

Magnetic nuclear spin relaxation of bismuth in liquid metallic alloys of thallium

The magnetic part of the nuclear spin relaxation rate, RM, has been measured at constant T in liquid Tl1-cPbc, Tl1-cBic and Tl1-cAuc (c<0.6), for the Bi nuclear spin. The 182 mu s isomer 207mBi, produced and aligned by the Tl ( alpha , 2n) reaction, and the TDPAD technique were used. Together with data from the literature, RM is established for Tl-Pb and Tl-Bi over the whole concentration range. For each alloy, the temperature dependence was measured at one intermediate concentration (c=0.5 or 0.4). The results agree with the free-electron model, revealing that structural and impurity effects on RM are small.

On the minimum size of metallic clusters in a liquid semiconductor system

Use is made of the fact that the NMR Knight shift K can be used as a local, i.e. atomic scale, indicator of whether the electronic state of the atom is metallic or non-metallic. The authors have measured K for the probe atom Bi in the liquid semiconductor system Tl-Te, and find that KBi drops in a narrower range around Tl2Te than either KTe or the susceptibility. Adopting the quasichemical model for the analysis, it is concluded for this system that a Bi atom needs only a very small number of metallic neighbours (either Tl or Te) in order to show the metallic value of K.

Microscopic charge fluctuations in the liquid semiconducting alloys Ga-Te and In-Te

Measurements are presented of the nuclear quadrupolar spin relaxation rate on an inert probe atom, 123Xe (using the TDPAD technique), in GacTe1-c and IncTe1-c alloys. Despite the apparent similarities of these systems, the relaxation rate in In-Te alloys is larger than in Ga-Te alloys. A possible connection of this difference with a difference in bond type is discussed.

Electron Localization in the Liquid T1-Te-System as Seen by Bi-Impurity

Electron localization is observed in numerous liquid alloys of s-p-metals, e.g. in alloys of Tellurium, at stoichiometric compositions. There they show the property of strong decrease of the magnetic susceptibility X, electric conductivity σ, and Knight shift K as well as strong increase of the viscosity, Hall effect and of the magnetic nuclear spin relaxation rate RM /1, 2, 3, 5/.

ELECTRIC FIELD FLUCTUATIONS IN LIQUID TELLURIUM ALLOYS - A HINT TO BOND CHARACTER

Atomic scale electric field fluctuations in liquid tellurium alloys are detected as they induce nuclear spin relaxation rate RQ in noble gas impurity atoms, via quadrupolar interaction. Results for Xe in liquid Ag, Ga, In, Tl, Ge, Sn---Te alloys are discussed, assuming that bonding in these alloys may be described as a mixture of metallic, covalent and ionic bonding. Using an inhomogeneous model, and obtaining the metallic fraction from electrical conductivity, the Xe R0 data are used to derive estimates of the ionic bonding fraction in the alloys studied.