Electron paramagnetic resonance studies of the neutral nitrogen vacancy in diamond

Despite the numerous experimental and theoretical studies on the negatively charged nitrogen vacancy center (NV-) in diamond and the predictions that the neutral nitrogen vacancy center (NV0) should have an S=1/2 ground state, NV0 has not previously been detected by electron paramagnetic resonance (EPR). We report new EPR data on a trigonal nitrogen-containing defect in diamond with an S=3/2 excited state populated via optical excitation. Analysis of the spin Hamiltonian parameters and the wavelength dependence of the optical excitation leads to assignment of this S=3/2 state to the (4)A(2) excited state of NV0. This identification, together with an examination of the electronic structure of the NV centers in diamond, provides a plausible explanation for the lack of observation (to date) of an EPR signal from the NV0 ground state.

Ferroelectric Vortices and Related Configurations

This chapter discusses that the theoretical studies, using both atomistic and phenomenological approaches, have made clear predictions about the existence and behaviour of ferroelectric (FE) vortices. Effective Hamiltonians can be implemented within both Monte Carlo (MC) and molecular dynamics (MD) simulations. In contrast to the effective Hamiltonian method, which is atomistic in nature, the phase field method employs a continuum approach, in which the polarization field is the order parameter. Properties of FE nanostructures are largely governed by the existence of a depolarization field, which is much stronger than the demagnetization field in magnetic nanosystems. The topological patterns seen in rare earth manganites are often referred to as vortices and yet this claim never seems to be explicitly justified. By inspection, the form of a vortex structure is such that there is a continuous rotation in the orientation of dipole vectors around the singularity at the centre of the vortex.

Unexpected higher-order effects in charged particle impact ionization at high energies

Most of the experimental and theoretical studies of electron-impact ionization of atoms, referred to as (e, 2e), have concentrated on the scattering plane. The assumption has been that all the important physical effects will be observable in the scattering plane. However, very recently it has been shown that, for C6+-helium ionization, experiment and theory are in nice agreement in the scattering plane and in very bad agreement out of the scattering plane. This lack of agreement between experiment and theory has been explained in terms of higher-order scattering effects between the projectile and target ion. We have examined electron-impact ionization of magnesium and have observed similar higher-order effects. The results of the electron-impact ionization of magnesium indicate the possible deficiencies in the calculation of fully differential cross sections in previous heavy particle ionization work.

Nonharmonic transverse vibration of the H-bonded molecules and the THz spectra in ice and water

A nonlinear equation of motion is found for the dimer comprising two charged H2O molecules. The THz dielectric response to nonharmonic vibration of a nonrigid dipole, forming the hydrogen bond (HB), is found in the direction transverse to this bond. An explicit expression is derived for the autocorrelator that governs the spectrum generated by transverse vibration (TV) of such a dipole. This expression is obtained by analytical solution of the truncated set of recurrence equations. The far infrared (FIR) spectra of ice at the temperature - 7 degrees C are calculated. The wideband, in the wavenumber (frequency) v range 0... 100.0 cm(-1), spectra are obtained for liquid water at room temperature and for supercooled water at -5.6 degrees C. All spectra are represented in terms of the complex permittivity epsilon(v) and the absorption coefficient alpha(v). The obtained analytical formula for epsilon comprises the term epsilon(perpendicular to) pertinent to the studied TV mechanism with three additional terms Delta epsilon(q), Delta epsilon(mu), and epsilon(or) arising, respectively, from: elastic harmonic vibration of charged molecules along the H-bond; elastic reorientation of HB permanent dipoles; and rather free libration of permanent dipoles in 'defects' of water/ice structure. The suggested TV-dielectric relaxation mechanism allows us: (a) to remove the THz 'deficit' of loss epsilon" inherent in previous theoretical studies; (b) to explain the THz loss and absorption spectra in supercooled (SC) water; and (c) to describe, in agreement with the experiment, the low- and high-frequency tails of the two bands of ice H2O located in the range 10...300 cm(-1). Specific THz dielectric properties of SC water are ascribed to association of water molecules, revealed in our study by transverse vibration of HB charged molecules. (C) 2006 Published by Elsevier B.V.

Charge exchange and dissociative processes in collisions of slow He2+ ions with H2O molecules

Experimental and theoretical studies of one-electron capture in collisions of He2+ ions with H2O molecules have been carried out in the range 0.025-12 keV amu(-1) corresponding to typical solar wind velocities of 70-1523 km s(-1). Translational energy spectroscopy (TES), photon emission spectroscopy (PES), and fragment ion spectroscopy were employed to identify and quantify the collision mechanisms involved. Cross sections for selective single electron capture into n=1, 2, and 3 states of the He+ ion were obtained using TES while PES provided cross sections for capture into the He+(2p) and He+(3p) states. Our model calculations show that He+(n=2) and He+(n=3) formation proceeds via a single-electron process governed by the nucleus-electron interaction. In contrast, the He+(1s) formation mechanism involves an exothermic two-electron process driven by the electron-electron interaction, where the potential energy released by the electron capture is used to remove a second electron thereby resulting in fragmentation of the H2O molecule. This process is found to become increasingly important as the collision energy decreases. The experimental cross sections are found to be in reasonable agreement with cross sections calculated using the Demkov and Landau-Zener models.

State-selective one-electron capture by slow state-prepared N2+(P-2) ground-state ions in collisions with hydrogen atoms

The technique of double translational energy spectroscopy.(DTES), recently successfully developed in this laboratory for use with targets of atomic hydrogen, has been used to study one-electron capture by ground-state N2+(2s22p)(2)p(0) ions in collisions with hydrogen atoms at energies within the range 0.8-6.0 keV. Cross sections for the formation of the main excited product channels have been determined. The measurements allow a re-evaluation of our previous TES measurements carried out with N2+ primary beams containing an admixture of metastable N2+(2s2p2)(4)p ions. The main findings of these earlier measurements are confirmed and the DTES measurements now remove any ambiguity in interpretation of the experimental data. While recent theoretical studies correctly predict the two main N+ D-3(0) and P-3(0) product channels, the quantitative agreement with experiment is only partially satisfactory.

Atmospheric parameters and rotational velocities for a sample of Galactic B-type supergiants

High-resolution optical spectra of 57 Galactic B-type supergiant stars have been analysed to determine their rotational and macroturbulent velocities. In addition, their atmospheric parameters (effective temperature, surface gravity and microturbulent velocity) and surface nitrogen abundances have been estimated using a non-local thermodynamic equilibrium grid of model atmospheres. Comparisons of the projected rotational velocities have been made with the predictions of stellar evolutionary models and in general good agreement was found. However, for a small number of targets, their observed rotational velocities were significantly larger than predicted, although their nitrogen abundances were consistent with the rest of the sample. We conclude that binarity may have played a role in generating their large rotational velocities. No correlation was found between nitrogen abundances and the current projected rotational velocities. However, a correlation was found with the inferred projected rotational velocities of the main-sequence precursors of our supergiant sample. This correlation is again in agreement with the predictions of single star evolutionary models that incorporate rotational mixing. The origin of the macroturbulence and microturbulent velocity fields is discussed and our results support previous theoretical studies that link the former to subphotospheric convection and the latter to non-radial gravity mode oscillations. In addition, we have attempted to identify differential rotation in our most rapidly rotating targets.

Positronium formation from valence and inner shells in noble gas atoms

When recent experimental positronium (Ps) formation cross sections in noble gases have been compared with the most up-to date theoretical studies, the agreement is qualitative, but not quantitative. In this paper we re-examine this process and show that at low energies Ps formation must be treated non-perturbatively. We also look at Ps formation with inner shell electrons.

Kinematic modeling of Exechon parallel kinematic machine

The studies on PKMs have attracted a great attention to robotics community. By deploying a parallel kinematic structure, a parallel kinematic machine (PKM) is expected to possess the advantages of heavier working load, higher speed, and higher precision. Hundreds of new PKMs have been proposed. However, due to the considerable gaps between the desired and actual performances, the majorities of the developed PKMs were the prototypes in research laboratories and only a few of them have been practically applied for various applications; among the successful PKMs, the Exechon machine tool is recently developed. The Exechon adopts unique over-constrained structure, and it has been improved based on the success of the Tricept parallel kinematic machine. Note that the quantifiable theoretical studies have yet been conducted to validate its superior performances, and its kinematic model is not publically available. In this paper, the kinematic characteristics of this new machine tool is investigated, the concise models of forward and inverse kinematics have been developed. These models can be used to evaluate the performances of an existing Exechon machine tool and to optimize new structures of an Exechon machine to accomplish some specific tasks.