High-energy behavior of the double photoionization of helium from 2 to 12 keV

Photon Momentum Transfer in Single-Photon Double Ionization of Helium

We theoretically and experimentally investigate the photon momentum transfer in single-photon double ionization of helium at various large photon energies. We find that the forward shifts of the momenta along the light propagation of the two photoelectrons are roughly proportional to their fraction of the excess energy. The mean value of the forward momentum is about 8/5 of the electron energy divided by the speed of light. This holds for fast and slow electrons despite the fact that the energy sharing is highly asymmetric and the slow electron is known to be ejected by secondary processes of shake off and knockout rather than directly taking its energy from the photon. The biggest deviations from this rule are found for the region of equal energy sharing where the quasifree mechanism dominates double ionization.

Ejection of quasi-free-electron pairs from the helium-atom ground state by single-photon absorption

We investigate the single-photon double ionization of helium at photon energies of 440 and 800 eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique fingerprint of an elusive quasifree photon double ionization mechanism predicted by Amusia et al. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.

Linear Scaling Constrained Density Functional Theory in CONQUEST

The constrained density functional theory (cDFT) formalism is implemented in the linear scaling density functional theory (DFT) code CONQUEST. This will enable the simulation of electron-transfer processes in large biologically and technologically relevant systems. The Becke weight population scheme is chosen to define the constraint, as it enables force components to be calculated both analytically and efficiently in a linear scaling code. It is demonstrated that the imposition of a constraint is not affected by the truncation of the density matrix. Demonstration calculations are performed on charge-separated excited states in small biphenyl molecules, and cDFT is found to produce accurate energy and geometry changes for this system. The capability of the method is shown in calculations on poly phenylene-vinylene oligomers and a hydrated DNA 10-mer.

Antisite-defect-induced surface segregation in ordered NiPt alloy

By means of first principles simulations we demonstrate that tiny deviations from stoichiometry in the bulk composition of the NiPt-L1(0) ordered alloy have a great impact on the atomic configuration of the (111) surface. We predict that at T=600 K the (111) surface of the Ni51Pt49 and Ni50Pt50 alloys corresponds to the (111) truncation of the bulk L1(0) ordered structure. However, the (111) surface of the nickel deficient Ni49Pt51 alloy is strongly enriched by Pt and should exhibit the pattern of the 2x2 structure. Such a drastic change in the segregation behavior is due to the presence of different antisite defects in the Ni- and Pt-rich alloys and is a manifestation of the so-called off-stoichiometric effect.

Double-to-single target ionization ratio for electron capture in fast p-He collisions

We have used the ion storage ring CRYRING and its internal gas-jet target and recoil-ion-momentum spectrometer to measure absolute cross sections for transfer ionization (TI: p+He-->H0+He2++e(-)) in 2.5-4.5 MeV p-He collisions with separate Thomas (TTI) and kinematic (KTI) TI contributions. The probability for electron emission in kinematical capture decreases with increasing velocity and appears to approach the photoionization shakeoff value (1.63%) [T. Aberg, Phys. Rev. A 2, 1726 (1970)]]. The velocity dependence of the TTI cross section is consistent with the theoretically predicted v(-11) scaling [J. S. Briggs and K. Taulbjerg, J. Phys. B 12, 2565 (1979)]].

Thermoelectric power of bismuth nanocomposites

Because of the increase in the electronic density of states in low-dimensional systems, semiconductor quantum wires constitute a most promising thermoelectric material. We report here the first experimental observation of a very large enhancement of the thermoelectric power of composites containing bismuth nanowires with diameters of 9 and 15 nm, embedded in porous alumina and porous silica. The temperature dependence of the electrical resistance shows that the samples are semiconductors with energy gaps between 0.17 and 0.4 eV, consistent with the theoretical predictions.

Experimental confirmation of the predicted shallow donor hydrogen state in zinc oxide

We confirm the recent prediction that interstitial protium may act as a shallow donor in zinc oxide, by direct spectroscopic observation of its muonium counterpart. On implantation into ZnO, positive muons--chemically analogous to protons in this context--form paramagnetic centers below about 40 K. The muon-electron contact hyperfine interaction, as well as the temperature and activation energy for ionization, imply a shallow level. Similar results for the cadmium chalcogenides suggest that such shallow donor states are generic to the II-VI compounds. The donor level depths should serve as a guide for the electrical activity of interstitial hydrogen.

Visualizing interfacial structure at non-common-atom heterojunctions with cross-sectional scanning tunneling microscopy

We describe how cross-sectional scanning tunneling microscopy (STM) may be used to image the interfacial bonding across the nearly lattice-matched, non-common-atom GaSb/InAs heterojunction with atomic-scale precision. The method, which takes advantage of the length difference between interfacial and bulk bonds, appears equally applicable to AlSb/InAs and suggests how one might recover the complete structure of either heterojunction from atomic-resolution STM data.

Spin-density and charge-density excitations in the paramagnetic phase of semiconductor double quantum well systems

The interplay of tunneling, Coulomb coupling, and many-body effects on the charge-density excitation (CDE) and spin-density excitation (SDE) of double quantum well systems has been analyzed. For increasing interwell distances, the system moves from the strong-tunneling regime (one-well limit) towards the zero tunneling but still strongly Coulomb-coupled regime, passing by the intermediate regime of a small but finite tunneling-induced gap ( less, similar1 meV). Important renormalizations due to many-body effects are found in the long-wavelength limit of the CDE and SDE, with the former exhibiting a logarithmic correction to the single-particle linear dependence on the tunneling-induced gap, and the latter exhibiting a soft mode in the weak-tunneling, low-density regime. The oscillator strength of the SDE soft mode has been calculated and found to be clearly measurable.

Computational method for general multicenter electronic structure calculations

Here a three-dimensional fully numerical (i.e., chemical basis-set free) method [P. F. Batcho, Phys. Rev. A 57, 6 (1998)], is formulated and applied to the calculation of the electronic structure of general multicenter Hamiltonian systems. The numerical method is presented and applied to the solution of Schrödinger-type operators, where a given number of nuclei point singularities is present in the potential field. The numerical method combines the rapid "exponential" convergence rates of modern spectral methods with the multiresolution flexibility of finite element methods, and can be viewed as an extension of the spectral element method. The approximation of cusps in the wave function and the formulation of multicenter nuclei singularities are efficiently dealt with by the combination of a coordinate transformation and a piecewise variational spectral approximation. The complete system can be efficiently inverted by established iterative methods for elliptical partial differential equations; an application of the method is presented for atomic, diatomic, and triatomic systems, and comparisons are made to the literature when possible. In particular, local density approximations are studied within the context of Kohn-Sham density functional theory, and are presented for selected subsets of atomic and diatomic molecules as well as the ozone molecule.

Novel behavior of bond-centered muonium in heavily doped n-type silicon: Curie-like spin susceptibility and charge screening

Bond-centered muonium ( Mu(0)(BC)) has been observed in very heavily doped n-type Si:P. It exhibits a Curie-like electronic spin susceptibility which leads to a giant negative shift in the muon spin precession frequency. At high dopant levels, the Mu(0)(BC) hyperfine parameters, deduced from a model involving spin exchange with free carriers, are significantly reduced from those in intrinsic Si. This indicates that the spin density distribution for Mu(0)(BC) in metallic Si:P is altered significantly by charge screening effects, likely a general phenomenon for deep impurities in materials with high carrier concentrations.