Calibration of the isomer shift of119Sn from internal conversion measurement

Radioactive¹¹⁹Sb was implanted into 6 different host matrices, Y, Β-Sn, Pt, Au, Pb and CaSnO₃, and internal-conversion and Mössbauer spectra were measured for the same samples. The isomer-shift calibration constant of the Mössbauer transition of¹¹⁹Sn was derived as δR/R=(0.87±0.25)×10^?4 for R=1.2×A^1/3 fm or, δ〈r²〉=(3.6±1.0)×10^?3 fm².     

Interpretation of the isomer shift of interstitially implanted119Sn impurities in group IV semiconductors

Recent Mössbauer measurements of the isomer shift of interstitially implanted¹¹⁹Sn impurities in group IV semiconductors are interpreted in terms of the electron contact density of compressed Sn atoms and ions, respectively. The finite space allowed to the impurity atom in the host crystal is approximated by a Wigner-Seitz sphere. Using a calibration procedure, the dependence of the isomer shift on the Wigner-Seitz radiusR _A has been calculated for several electron configurations of the tin atom (ion). The isomer shift values for¹¹⁹Sn interstitials in diamond, silicon, germanium, and -tin are found to correspond to compressed, neutral tin atoms; furthermore, the relaxation of the host lattice about the impurity Sn atom is discussed.     

Covalency and compression effects on the isomer shift of substitutional119Sn impurities in group IV semiconductors

Since substitutional tin is an isovalent impurity in group IV semiconductors, it represents an almost ideal probe for the study of the variation of the chemical bond in these materials using the isomer shift measurements. To obtain a realistic formula for the isomer shifts in this case, the impurity problem has been formulated in such a way as to include both the band structure and compression effects on the¹¹⁹Sn impurity. The increase of the isomer shift values measured on the impurity¹¹⁹Sn when going from diamond to silicon and germanium host crystals, is interpreted in terms of increasing dehybridization of the homopolar bond in these materials. The lattice relaxation around the tin impurity, due to its large size, seems to be relatively small, if any, in silicon and germanium, while a shift of the neighbouring carbon atoms outwards by 18% of the nearest neighbour distance has to be assumed to explain the experimental value of the isomer shift of substitutional tin in diamond.     

The problem of the Mössbauer isomer shift calibration for the119Sn resonance from rare-gas matrix isolation experiments

Data of new rare-gas matrix isolation experiments are presented and discussed in connection with the problem of the Mössbauer isomer shift calibration for¹¹⁹Sn. These experiments are: (i) A Mössbauer source experiment with^119mSn in solid xenon yielding the isomer shift of Sn⁺ with the atomic configuration 4d¹⁰5s²5p; (ii) Mössbauer absorption studies of isolated Sn(II) halide molecules (SnX₂, X-F, Cl, Br, I) in argon matrices; and (iii) Mössbauer absorption experiments with isolated¹²⁵Te atoms and molecules in argon and krypton matrices.This work was supported in part by a grant from the National Science Foundation and from the Bundesministerium für Forschung und Technologie     

Temperature dependence of the Mössbauer isomer shift

The isomer shift for nuclei in solids is calculated taking into account the core electron charge density, the overlap effects, the charge transfer effects, and the crystal field effects. The importance of the dynamical electron phonon interaction is pointed out. It is found that the phonon-induced dynamical configurational mixing and the dynamical charge transfer play an important role and contribute to a dynamical isomer shift which enhances the temperature-dependent second-order Doppler shift. It is found that the internal conversion and the electron capture give information which is related to the isomer shift. In mixedvalence compounds such as EuCu₂Si₂, the center shift of the M?ssbauer lines has been explained as if arising from the phonon-induced dynamical isomer shift.     

Temperature dependence of the isomer shift in white tin

The OPW method is used to study the temperature-dependent isomer shift of β-Sn above the room temperature from the contributions including both the crystal pseudopotential with the Debye-Waller factor and the self-energy effect. It is shown that, although the self-energy contribution to the absolute value of the isomer shift in β-Sn is negligibly small, its rate of change with respect to temperature is at least of the same order, but with different sign, as that of the Debye-Waller factor term. We conclude that the increase in the isomer shift of β-Sn with temperature observed experimentally can be well accounted for by the effect of self-energy interactions.     

Calibration of the isomer shift of121Sb and119Sn by means of Mössbauer spectroscopy and molecular orbital calculation

The relationship between the isomer shifts in Mössbauer spectroscopy and the electron contact densities has been investigated for several antimony and tin compounds. Mössbauer spectra for¹²¹Sb in rapidly frozen solutions of antimony compounds were measured. The isomer shifts are compared with the valence electron densities at the antimony nucleus calculated with theab initio molecular orbital method. The relative difference of the nuclear charge radius ΔR/R could be obtained as ΔR/R =?(10.2±1.0)×10^?4 for the 37.15 keV M1 transition of¹²¹Sb. Further, some computations of the electron density at the tin nucleus for several tin compounds were performed. By comparing the valence electron contact densities with isomer shifts, which had been reported for several tin compounds in rare-gas matrix states, the value of ΔR/R for 23.87 keV M1 transition in¹¹⁹Sn was estimated to be (1.57±0.03)×10^?4.     

Electronic structure and Mössbauer isomer shifts of119Sn defects in semiconductors

Although the Mössbauer isomer shift reflects the electronic configuration of the Mössbauer atom, in general, theoretical calculations are needed to express the IS unambiguously in terms of the occupation numbers of the valence shells. It turns out that this can be done using a Green-function procedure based on the parametrized tight-binding approximation including relativistic wave functions. In this way recent Mössbauer measurements on¹¹⁹Sn impurities implanted in group IV semiconductors and A^IIIb^V semiconducting compounds have been interpreted.     

Temperature dependent isomer shift in Sn119 — Pseudopotential approach

The theory formulated by Inglesfield based on the pseudopotential method was modified by the thermal lattice vibrations to interpret the explicit temperature dependence of the isomer shift in Sn¹¹⁹. A temperature dependent conduction electron charge density at the nucleus was derived by this method and the change of the isomer shift ib Sn¹¹⁹ with temperature was calculated. We found that the results of Sn¹¹⁹ in SnMg₂ and SnTe were in good agreement with the experimental results of Lin and Rothberg.     

Interpretation of isomer shifts of substitutional119Sn and129I in group IV semiconductors

The measurements of the isomer shift in Mössbauer effect of substitutionally implanted¹¹⁹Sn and¹²⁹I atoms in group IV semiconductors show that the contact density of 5s electrons at the impurity ńucleus decreases when going from Ge to Si and diamond. A satisfactory interpretation of this dependence can be given based on a simple perturbation approach: Interaction of the host crystal with the bonding valence electrons of the impurity atom causes a charge redistribution of these electrons and results in decrease of occupancy of the 5s level at the impurity atom. Some general aspects of this problem with substitutional isoelectronic and donor impurities are discussed.     

Theoretical study on pressure dependence of isomer shift and hyperfine field

The pressure dependence of the isomer shift and the hyperfine field are theoretically investigated. By expanding the Gibbs free energy of the system under proper circumstances, the explicit form of the pressure dependence of the isomer shift is obtained. At low temperatures, by applying some well-known relations, the pressure dependence of the hyperfine field is approximately derived. Finally, the theoretical results are compared with the experimental data.     

Pressure dependence of knight shift in lithium

A simple explanation of the recent experimental observation of Kushida and Hanabusa on the volume dependence of the average spin density P_F at the nucleus for lithium metal is given. It is argued that the increase in the direct contribution to P_F associated with the decrease in volume (increase in pressure) is substantially reduced by an increase in the negative core polarization contribution to P_F.     

Inhibition of the radioactive decay of the isomer 119m Sn with the use of Mössbauer backscattering

In research on the production of beams of coherent γ rays (γ-ray lasers), conditions under which a substantial change Δλ/λ=?(0.114 ±0.027) in the radioactive decay constant λ (the isomeric level 89.53 keV ^119m Sn, T _1/2=293 days) can occur have been found experimentally for the first time. This is made possible by coherent Mo ssbauer (23.87 keV) backscattering from a resonant screen located nearby. An interpretation of the effect observed is proposed on the basis of the idea of dynamic synchronization of oscillations between a nuclear level and a standing wave of Mössbauer radiation. Possibilities for further increasing Δλ/λ up to 0.5 are found.     

Pressure dependence of the muon knight shift in antimony

The pressure dependence of the muon Knight shift in antimony has been measured at 30K using polycrystalline samples and at 10K using single crystals. A considerably stronger pressure dependence is observed with the field parallel to the c-axis than perpendicular. The deduced linear parts of the isotropic and axial pressure dependence are dK_iso/dP=−0.19(3)%/kbar and dK_ax/dP−0.24 (5)%/kbar. First principle molecular-cluster calculations show the origin, of the huge Knight shift and its pressure dependence.     

On the Mössbauer isomer shift studies of the electronic structure of Sn implanted AIIIBV compounds

Recent Mössbauer isomer shift (IS) measurements on¹¹⁹Sn impurities substitutionally implanted in several A^IIIB^V compounds are interpreted in terms of their electronic structure. Since tin can replace both the A and the B atoms, two different IS lines arise corresponding to the donor and acceptor Sn impurities. To calculate the electronic configuration of ionized tin donors and acceptors and the relevant electron contact densities, a Green-function procedure is used based on the parametrized tight-binding approximation including relativistic wave functions. It turns out that with ionized Sn acceptors, the impurity is formed by a small cluster containing the tin atom in its neutral configuration rather than by a single negative Sn ion as might be anticipated at first sight. On the other hand, in the donor case the positive Sn ion plays the dominant role.     

Electron densities and isomer shift

Some of the problems concerning the absolute determination of electron density changes from Mössbauer isomer shift data are discussed. Three promissing experimental methods based on conversion electron spectroscopy, lifetime measurements, and muonic isomer shift measurements are presented.     

Calibration of the isomer shift of83Kr

Radioactive⁸³Rb was implanted in 6 different cubic metals (Al, W, Au, Ir, Pt, Pb) and both internal conversion electron spectra and Mössbauer spectra of the daughter nucleus⁸³Kr were measured. A value (r ²)=+5.1(9)×10^–3 fm² (corresponding to R/R=+1.6(3)×10^–4) was derived for the change of the mean square charge radius during the 9.40 keV transition in⁸³Kr. The systematics of the isomer shifts of krypton and other sp-elements substitutionally dissolved in metals are discussed.