Zeev Vager

Spin Selectivity in Electron Transmission Through Self-Assembled Monolayers of Double-Stranded DNA

Photoelectrons emitted from a DNA-covered gold surface can have an unbalanced spin population of up to 60%. In electron-transfer processes, spin effects normally are seen either in magnetic materials or in systems containing heavy atoms that facilitate spin-orbit coupling. We report spin-selective transmission of electrons through self-assembled monolayers of double-stranded DNA on gold. By directly measuring the spin of the transmitted electrons with a Mott polarimeter, we found spin polarizations exceeding 60% at room temperature. The spin-polarized photoelectrons were observed even when the photoelectrons were generated with unpolarized light. The observed spin selectivity at room temperature was extremely high as compared with other known spin filters. The spin filtration efficiency depended on the length of the DNA in the monolayer and its organization.

Coulomb explosion imaging of small molecules.

Most of our knowledge of the structures of free molecules comes from indirect observations that focus on other properties (for example, energy or mass spectra) that depend on structure. Recently, a new method has been developed that instead yields geometrical images of individual molecules. This method takes advantage of the large Coulomb repulsion of the nuclei within molecules rapidly stripped of their electrons. The first experiments with this new technique have already revealed structural images of several important species, such as positively charged methane ions.

Spin Specific Electron Conduction through DNA Oligomers

Spin-based properties, applications, and devices are commonly related to magnetic effects and to magnetic materials. Most of the development in spintronics is currently based on inorganic materials. Despite the fact that the magnetoresistance effect has been observed in organic materials, until now spin selectivity of organic based spintronics devices originated from an inorganic ferromagnetic electrode and was not determined by the organic molecules themselves. Here we show that conduction through double-stranded DNA oligomers is spin selective, demonstrating a true organic spin filter. The selectivity exceeds that of any known system at room temperature. The spin dependent resistivity indicates that the effect cannot result solely from the atomic spin-orbit coupling and must relate to a special property resulting from the chirality symmetry. The results may reflect on the importance of spin in determining electron transfer rates through biological systems.

Imaging the Absolute Configuration of a Chiral Epoxide in the Gas Phase

Foil-Forged Images X-ray diffraction is widely used to determine molecular geometries and can often distinguish mirror image isomers (enantiomers), which generally requires well-ordered crystals. Herwig et al. (p. 1084) report an imaging technique to characterize enantiomers in the gas phase. A succession of ionization events were induced by passage through a carbon foil that culminated in a Coulomb explosion of mutually repelling nuclei. The trajectories of these nuclei precisely reflected the original molecular structure. Ultrafast electron stripping by a carbon foil enables precise elucidation of molecular geometries as the nuclei fly apart. In chemistry and biology, chirality, or handedness, refers to molecules that exist in two spatial configurations that are incongruent mirror images of one another. Almost all biologically active molecules are chiral, and the correct determination of their absolute configuration is essential for the understanding and the development of processes involving chiral molecules. Anomalous x-ray diffraction and vibrational optical activity measurements are broadly used to determine absolute configurations of solid or liquid samples. Determining absolute configurations of chiral molecules in the gas phase is still a formidable challenge. Here we demonstrate the determination of the absolute configuration of isotopically labeled (R,R)-2,3-dideuterooxirane by foil-induced Coulomb explosion imaging of individual molecules. Our technique provides unambiguous and direct access to the absolute configuration of small gas-phase species, including ions and molecular fragments.

Study of laser interaction with negative ions

Abstract Negative ions can be neutralized by detaching their additional electron through interaction with a laser beam. By properly choosing the laser wavelength, the process is highly selective; it can in principle enhance the discrimination power of an accelerator mass spectrometry system for ions of different elements (e.g. isobaric background) by allowing their separation prior to their injection into a tandem accelerator. We demonstrate this process in the case of the 59 Ni- 59 Co pair with the AMS system based on the Rehovot 14UD Pelletron accelerator and a pulsed Nd-YAG laser at the fundamental wavelength (1064 nm). A photodetachment cross section of (0.6 ± 0.3) × 10 −17 cm 2 was measured for 59 Co − and a suppression factor of 125 for the 59 Co isobaric background was achieved in a 59 Ni AMS measurement. The duty factor due to the pulsed laser was about 10 −4 . The laser-AMS system was also applied to the study of rare negative ions in the actinide region. Preliminary results on the laser interaction with uranium negative ions are reported.

Electron photodetachment cross sections of small carbon clusters: evidence for nonlinear isomers.

Absolute cross sections for photodetachment of negative carbon clusters are reported for Cn (n = 3, ..., 8). The results indicate that various neutral isomers exist, some with electron affinities as low as 1 electron volt. The method of production plays an important role in the characteristics of carbon clusters.

Electronic polarization in solids probed by dissociation of fast molecular ions

Abstract The interpretation of molecular ion interaction with solid through conventional stopping power theories is critised. Comparison of experimental data to simulations of proton velocity distributions after 11.2 MeV OH+ dissociation in carbon foils supports this criticism. A different description of the average potential around a moving ion in solid due to electronic polarization is given. The polarization of the Fermi sea of electrons is described by the coulomb distortion of the single electron wave functions due to scattering on the moving ion. The use of such potentials for the simulation of experiments agrees favourably with the measured data.

A system for Coulomb explosion imaging of small molecules at the Weizmann Institute

Abstract A description of a system for molecular structure imaging at the Weizmann Institute is presented. A novel method of controlled laser beam photo detachment inside the high voltage terminal of a tandem accelerator, enabling the study of neutral fast molecules by the Coulomb Explosion Imaging technique, is described. Also, a new type of three dimension multiparticle detector is presented.

An electron multiplier capable of working at low vacuum: The microsphere plate

A new type of electron multiplier that has recently been developed, the microsphere plate (MSP), does not have the problem of ion feedback, which limits the operation of the known microchannel plates. Due to its structure, the MSP can amplify charge current even at pressures as high as 0.1 mbar.

Coulomb Explosion Imaged Cryptochiral (R,R)-2,3-Dideuterooxirane: Unambiguous Access to the Absolute Configuration of (+)-Glyceraldehyde

The absolute configuration of (R,R)-2,3-dideuterooxirane, which has been independently determined using Coulomb explosion imaging, has been unambiguously chemically correlated with the stereochemical key reference (+)-glyceraldehyde. This puts the absolute configuration of D(+)-glyceraldehyde on firm experimental grounds.