Vacancy trapping at111In in tungsten

TDPAC measurements were carried out in tungsten foils and single crystals using implanted¹¹¹In as a probe nucleus. After damaging the bcc tungsten lattice by heavy ion implantation, electron- and proton-irradiation the trapping of three different defects in the temperature range 293–1,000 K was observed. They were characterized by quadrupole interaction frequencies of v _Q ¹ = 142(2) MHz, v _Q ² = 181(5) MHz and v _Q ³ = 263(5) MHz. The largest fraction of In nuclei experiencing electric field gradients corresponding to v _Q ¹ and v _Q ² was reached at app. 630 K whereas for v _Q ³ this maximum appeared at app. 850 K. The defect configurations producing the different electric field gradients could be identified as a monovacancy trapped in a nearest neighbour 111 position (v _Q ¹ ), a double vacancy consisting of two adjacent vacancies in the nearest neighbour shell of the probe atom (v _Q ² ) and possibly a larger cluster (v _Q ³ ).     

Charged cores in ionized He clusters

Experimental and theoretical studies in large ionic helium clusters have suggested the presence of a diatomic (and occasionally triatomic) charged molecular core surrounded by the other atoms which are bound to it by weaker interactions [1-3]. The understanding of the interactions between the system He ₂ ⁺ and an additional He atom of the cluster is therefore important in order to start modelling the full cluster interaction potential. In the present work we carry out a new set of calculations on the full potential and on the bound states supported by the He ₂ ⁺ isolated ion and further extend them to generate a Rigid Rotor (RR) potential energy surface (PES) for the triatomic system with He ₂ ⁺ kept at its equilibrium geometry (2.0 a.u.). The 13 bound states which were found and the overall angular anisotropy that exists for this Potential Energy Surface (PES) are discussed in detail. We additionally show results of calculations on the surface vibrational extension to nine different values of the He ₂ ⁺ interatomic distance, thereby generating a fuller, three-dimensional interaction potential. A simpler modelling of the latter via “Pseudo Rigid Rotor” calculations for the bound states with a vibrationally excited core is also presented and discussed.     

Bubble formation and decay in 3He and 4He clusters

The energy transfer in 3He and 4He clusters electronically excited by monochromatic synchrotron radiation is investigated by luminescence spectroscopy. Depending on the cluster size and the isotopic constitution, either sharp, broadened, or shifted emission bands of single He molecules are observed. The spectral features show that He molecules emit light either within a bubble inside the cluster or in the vacuum after desorption from the cluster. From the luminescence intensity, the cluster diameter, and the radiative lifetime, an average velocity of approximately 7 m/s of bubbles in 4He clusters could be deduced. In the nonsuperfluid 3He clusters this velocity was observed to be significantly lower.     

Cubic defect clusters in fcc metals

The number of cubic defects in In-implanted copper, silver and platinum has been determined by decorating the cubic defects with He atoms. For this, the samples were post-implanted with He⁺ ions at subthreshold energies. Our results, obtained by DPAC measurements on¹¹¹In, are compared with data from Mössbauer spectroscopy and channeling experiments.     

Au/In bilayers studied with 111In deposited at the interface

Au/In bilayer samples of different stoichiometries were studied using the time-differential perturbed angular correlation technique. The ¹¹¹In probe atoms were deposited at the gold/indium interface to follow the formation of the intermetallic compounds at an early stage. Au-In phases of different composition (AuIn₂, AuIn, Au₇In₃, Au₃In) were created at the interfaces upon annealing. In addition a metastable Au₇In₃(m) phase, observed only after quenching in the bulk Au_0.7In_0.3 alloy, was found. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stability of small mixed clusters of 4He and 3He atoms

The existence of small helium clusters containing a variable number of 4He and 3He atoms is studied within a variational Monte Carlo calculation employing the Aziz HFD-B(HE) pair interaction. The clusters 4He(2) support one and two 3He atoms; however, the system with three 3He atoms is metastable, and the next bound system requires at least 18 fermions. All clusters obtained by adding 3He atoms to the trimer 4He(3) and the tetramer 4He(4) are bound, but the clusters 4He(3)-3He(3,4,5) and 4He(3,4)-3He(9) are metastable. All remaining clusters with three or more bosons and any number of fermions are stable.     

Observation of Andreev-Pushkarov vacancy clusters in phase-separated solid solutions of 4He in 3He

New features are observed for the pressure in a phase-separated dilute solid solution of ⁴He in ³He subjected to multiple temperature cycling within the phase-separation region. The results are explained within the framework of the hypothesis of A.F. Andreev and D.I. Pushkarov that the vacancies in a crystal without ideal periodicity are surrounded by clusters with a periodic structure. The equation for determining the radius of a cluster of pure ⁴He in a solution of ⁴He in ³He is refined. This hypothesis is shown to provide quantitative agreement between the calculated and experimental data under the assumption that the homogenization of the phase-separated solution is accompanied by the formation of metastable vacancies with a concentration of ～(4–5)×10^?5.