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At least 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Second-order Born approximation for the ionization of molecules by electron and positron impact

Second-order Born approximation is applied to study the ionization of molecules. The initial and final states are described by single-center wave functions. For the initial state a Gaussian wave function is used while for the ejected electron it is a distorted wave. Results of the present model are compared with recent (e,2e) experiments on the water molecule. Preliminary results are also presented for the ionization of the thymine molecule by electrons and positrons.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Electron-impact study of the S{sub 2} molecule using the R-matrix method

The present study deals with the calculation of elastic [integrated and differential cross section (DCS)], momentum-transfer, excitation, and ionization cross sections for electron impact on S{sub 2} molecules using the R-matrix method. The target states are represented by including correlations via a configuration-interaction technique. We used a double zeta plus polarization Gaussian basis set contracted as (12,8,1)/(6,4,1) for S atoms. The results of the static exchange, correlated one-state, and 20-state close-coupling approximations are presented. We have detected a stable anionic bound state {sup 2}{Pi}{sub g} of S{sub 2}{sup -} having the configuration 1{sigma}{sub g}{sup 2}{center_dot}{center_dot}{center_dot}5{sigma}{sub g}{sup 2} 1{sigma}{sub u}{sup 2}{center_dot}{center_dot}{center_dot}4{sigma}{sub u}{sup 2} 1{pi}{sub u}{sup 4} 1{pi}{sub g}{sup 4} 2{pi}{sub u}{sup 4} 2{pi}{sub g}{sup 3}. The vertical electron affinity value is 1.42 eV, which is comparable with the experimental value of 1.67{+-}0.015 eV. We detected two shape resonances, both of {sup 2}{Pi}{sub u} symmetry in the excitation cross sections of the {sup 1}{Delta}{sub g} and {sup 1}{Sigma}{sub g}{sup +} excited states. The dissociative nature of these resonances is explored by performing scattering calculations in which the S-S bond is stretched. These resonances support dissociative attachment, yielding S and S{sup -}. We have also predicted six resonances of various symmetries ({sup 2}A{sub u}, {sup 2}B{sub 1g},{sup 4}A{sub u}, {sup 4}B{sub 1g}) in the X {sup 3}{Sigma}{sub g}{sup -} {yields} B {sup 3}{Sigma}{sub u}{sup -} transition. We have calculated the DCS, in a correlated one-state model, by using the polydcs program of Sanna and Gianturco. The data from the momentum-transfer cross section, generated from DCS, are used to compute effective collision frequencies over a wide electron temperature range (200-30 000 K). The ionization cross sections are calculated in the binary-encounter Bethe model in which Hartree-Fock molecular orbitals at a self-consistent level are used to calculate kinetic and binding energies of the occupied molecular orbitals. We have included up to g-partial wave (l=4) in the scattering calculations. For this molecule we have used a Born-closure top-up procedure to account for the higher partial waves for the convergence of the cross section for the dipole-allowed excitation from the ground state. We have also evaluated the scattering length of the S{sub 2} molecule, which is equal to 2.615a{sub 0}.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Magnetic-field effects in transitions of X Li molecules (X: even isotopes of group II atoms)

We analyze the Zeeman shift in the (v,N)=(0,0){yields}(1,0) transition frequency of X Li molecules (X: even isotopes of group II atoms), which is of interest in metrology. The Zeeman shift in the transition frequency between stretching states is found to be less than 1 mHz with a magnetic field of 1 G. X {sup 6}Li molecules are more advantageous than X {sup 7}Li molecules for measuring the transition frequency without the Zeeman shift because of the smaller g factor of the Li nuclear spin.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Spectroscopic prescription for optimal stimulated Raman transfer of ultracold heteronuclear molecules to the lowest rovibronic level

We show that the multiplicative product of a molecular-beam excitation spectrum and an ultracold-molecule excitation spectrum, with a frequency offset appropriate to the initial ultracold-molecule level, provides the relative rate of stimulated Raman transfer (SRT) from a given high rovibrational level to the lowest rovibronic level, i.e., the v{sup ''}=0,J{sup ''}=0 level of the ground electronic state for photoassociated (and magnetoassociated) ultracold molecules. This product spectrum clearly indicates the optimal pathways for SRT, even when the two component spectra are completely unassigned. We illustrate this specifically for the case of KRb.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Prospects for rapid deceleration of small molecules by optical bichromatic forces

We examine the prospects for utilizing the optical bichromatic force (BCF) to greatly enhance laser deceleration and cooling for near-cycling transitions in small molecules. We discuss the expected behavior of the BCF in near-cycling transitions with internal degeneracies, then consider the specific example of decelerating a beam of calcium monofluoride molecules. We have selected CaF as a prototype molecule both because it has an easily accessible near-cycling transition and because it is well suited to studies of ultracold molecular physics and chemistry. We also report experimental verification of one of the key requirements, the production of large bichromatic forces in a multilevel system with significant energy level splittings, by performing tests in an atomic beam of metastable helium.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Thermalization of atom-molecule Bose gases in a double-well potential

We study the nonequiliribium dynamics of atom-molecule Bose gases in a double-well potential. In this system, the internal atom-molecule tunneling has significant influence on the dynamics. We investigate the regularity of dynamics by studying the level statistics of the quantum system. We find that chaotic energy eigenstates arise from the competition between the interwell and the atom-molecule internal tunnelings. Furthermore, we show that the physical quantities relax to the microcanonical averages in the full-quantum dynamics when the system is chaotic. This thermalization is caused by the verification of the eigenstate thermalization hypothesis (ETH). We show numerically that the onset of ETH occurs simultaneously with that of chaos. In addition, we show that the energy eigenstates become exponentially localized states simultaneously with the onset of chaos.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

d-wave superfluidity in optical lattices of ultracold polar molecules

Recent work on ultracold polar molecules, governed by a generalization of the t-J Hamiltonian, suggests that molecules may be better suited than atoms for studying d-wave superfluidity due to stronger interactions and larger tunability of the system. We compute the phase diagram for polar molecules in a checkerboard lattice consisting of weakly coupled square plaquettes. In the simplest experimentally realizable case where there is only tunneling and an XX-type spin-spin interaction, we identify the parameter regime where d-wave superfluidity occurs. We also find that the inclusion of a density-density interaction destroys the superfluid phase and that the inclusion of a spin-density or an Ising-type spin-spin interaction can enhance the superfluid phase. We also propose schemes for experimentally realizing the perturbative calculations exhibiting enhanced d-wave superfluidity.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Reply to 'Comment on 'Increase in specific heat and possible hindered rotation of interstitial C{sub 2} molecules in neutron-irradiated graphite''

In a recent paper [T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 (2010)], we have interpreted the irradiation-induced increase in the low-temperature specific heat of neutron-irradiated graphite as due to the hindered rotation of interstitial C{sub 2} molecules with a rotational frequency of 1.39x10{sup 12} s{sup -1} in the periodic potential with a height of 0.040 eV, and concluded that the C{sub 2} molecules do not form covalent bonds with atoms in the adjacent graphite layers. In their Comment [preceding paper, C. D. Latham et al., Phys. Rev. B 82, 056101 (2010)] on this paper, based on the recent first-principles theoretical calculations and related experiments, Latham et al. assert that self-interstitial atoms cannot exist as clusters of nearly free C{sub 2} molecules. In this Reply, based on the experiments, we address the issues raised by Latham et al. in their Comment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

{beta}-connectin studies by small-angle x-ray scattering and single-molecule force spectroscopy by atomic force microscopy

The three-dimensional structure and the mechanical properties of a {beta}-connectin fragment from human cardiac muscle, belonging to the I band, from I{sub 27} to I{sub 34}, were investigated by small-angle x-ray scattering (SAXS) and single-molecule force spectroscopy (SMFS). This molecule presents an entropic elasticity behavior, associated to globular domain unfolding, that has been widely studied in the last 10 years. In addition, atomic force microscopy based SMFS experiments suggest that this molecule has an additional elastic regime, for low forces, probably associated to tertiary structure remodeling. From a structural point of view, this behavior is a mark of the fact that the eight domains in the I{sub 27}-I{sub 34} fragment are not independent and they organize in solution, assuming a well-defined three-dimensional structure. This hypothesis has been confirmed by SAXS scattering, both on a diluted and a concentrated sample. Two different models were used to fit the SAXS curves: one assuming a globular shape and one corresponding to an elongated conformation, both coupled with a Coulomb repulsion potential to take into account the protein-protein interaction. Due to the predominance of the structure factor, the effective shape of the protein in solution could not be clearly disclosed. By performing SMFS by atomic force microscopy, mechanical unfolding properties were investigated. Typical sawtooth profiles were obtained and the rupture force of each unfolding domain was estimated. By fitting a wormlike chain model to each peak of the sawtooth profile, the entropic elasticity of octamer was described.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Structure of human farnesyl pyrophosphate synthase in complex with an aminopyridine bisphosphonate and two molecules of inorganic phosphate

A co-crystal structure of human farnesyl pyrophosphate synthase in complex with an aminopyridine bisphosphonate, YS0470, and two molecules of inorganic phosphate has been determined. The identity of the phosphate ligands was confirmed by anomalous diffraction data. Human farnesyl pyrophosphate synthase (hFPPS) produces farnesyl pyrophos@@phate, an isoprenoid essential for a variety of cellular processes. The enzyme has been well established as the molecular target of the nitrogen-containing bisphosphonates (N-BPs), which are best known for their antiresorptive effects in bone but are also known for their anticancer properties. Crystal structures of hFPPS in ternary complexes with a novel bisphosphonate, YS0470, and the secondary ligands inorganic phosphate (P{sub i}), inorganic pyrophosphate (PP{sub i}) and isopentenyl pyrophosphate (IPP) have recently been reported. Only the co-binding of the bisphosphonate with either PP{sub i} or IPP resulted in the full closure of the C-@@terminal tail of the enzyme, a conformational change that is required for catalysis and that is also responsible for the potent in vivo efficacy of N-BPs. In the present communication, a co-crystal structure of hFPPS in complex with YS0470 and two molecules of P{sub i} is reported. The unusually close proximity between these ligands, which was confirmed by anomalous diffraction data, suggests that they interact with one another, with their anionic charges neutralized in their bound state. The structure also showed the tail of the enzyme to be fully disordered, indicating that simultaneous binding of two P{sub i} molecules with a bisphosphonate cannot induce the tail-closing conformational change in hFPPS. Examination of homologous FPPSs suggested that this ligand-dependent tail closure is only conserved in the mammalian proteins. The prevalence of P{sub i}-bound hFPPS structures in the PDB raises a question regarding the in vivo relevance of P{sub i} binding to the function of the enzyme.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Plasma-free atomic layer deposition of Ru thin films using H{sub 2} molecules as a nonoxidizing reactant

The ruthenium (Ru) thin films were grown by atomic layer deposition (ALD) using a sequential supply of dicarbonyl-bis(5-methyl-2,4-hexanediketonato) Ru(II) (C{sub 16}H{sub 22}O{sub 6}Ru) and H{sub 2} as a reactant at a substrate temperature of 250 °C. Deposition was possible using H{sub 2} molecules without a plasma by increasing the chamber pressure to above 10 Torr. Specifically, high-quality Ru films with a low resistivity of ∼40 μΩ cm and few amount of oxygen (∼1.2 at. %) were obtained under a chamber pressure of 300 Torr though the oxygen was contained in the precursor. Under the optimized conditions, self-limited film growth with regard to the precursor and reactant pulsing times was confirmed under elevated chamber pressures. The ALD-Ru process proposed in this study showed one of the highest growth rates of 0.12 nm/cycle on a thermally grown SiO{sub 2} substrate, as well as a very low number of incubation cycles (approximately 12 cycles). Cross-sectional view transmission electron microscopy showed that no interfacial oxide had formed during the deposition of the ALD-Ru films on a W surface using H{sub 2} molecules, whereas ∼7 nm thick interfacial oxide was formed when O{sub 2} molecules were used as a reactant. The step coverage of the ALD-Ru film onto very small-sized trenches (aspect ratio: ∼4.5 and the top opening size of 25 nm) and holes (aspect ratio: ∼40 and top opening size of 40 nm) was excellent (∼100%).

36 MATERIALS SCIENCE↗
DOE OSTI2018

X-ray Crystallographic Characterization of the Swine MHC I Molecule SLA-3*0202 Complexed with IAV-HA Nonapeptide

The swine major histocompatibility complex (MHC) class I molecules are also called swine leukocyte antigen (SLA), and most of the highly polymorphic SLA genes are associated with swine diseases. However, the well documented structural reports on swine MHC I molecules remain quite limited. In order to clarify the structural characteristics of the Chinese heishan wild boar MHC class I molecule, SLA-3*0202 and swine β2-microglobulin (sβ2m) with a KMNTQFTAV nonapeptide derived from Influenza A virus Hemagglutinin protein (IAV-HA) were assembled and crystallized. The crystal diffracted at 1.55 Å resolution and belonged to the sp. gr. C121, with the unit-cell parameters a = 206.46 Å, b = 41.47 Å, c = 106.74 Å. The Matthews coefficient and solvent content were calculated to be 2.30 Å{sup 3} Da{sup –1} and 46.64%, respectively. The availability of the structure, which is being solved by molecular replacement, will provide new insights into swine MHC I presenting IAV peptides.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Dispersion of Lifetimes of Excited States of Single Molecules in Organic Matrices at Ultralow Temperatures

Fluorescence excitation spectra of single terrylene molecules in transparent naphthalene and polyethylene matrices at ultralow (30–100 mK) temperatures have been studied under the conditions where the widths of zero-phonon spectral lines are determined only by the lifetime T{sub 1} of the excited electronic state. The experimentally observed dispersion of T{sub 1} values for identical molecules is attributed to local field effects, which are responsible for the dependence of T{sub 1} on the effective value of refractive index n of the matrix, which is characteristic of the localization region of each molecule. It has been shown that the dependence T{sub 1}(n) for ensembles of point emitters in organic matrices is satisfactorily explained within the model of a “virtual cavity” around the emitter inside a continuous medium, as well as within the developed quantum kinetic approach including various contributions of the local environment to the lifetime T{sub 1}. The recalculation of average T{sub 1} values to the corresponding n values with the use of the expressions obtained for T{sub 1}(n) has shown that the difference of the calculated refractive indices of naphthalene and polyethylene from the well-known tabulated n values is less than 1%.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Non-self-sustained glow discharge with electrostatic confinement of electrons sustained by a fast neutral molecule beam

Experimental study of plasma produced at the nitrogen pressure 0.2-1 Pa in the chamber volume V Almost-Equal-To 0.12 m{sup 3} as a result of injection into the chamber of a broad nitrogen molecule beam with 1-4 keV energy and 0.1-1 A equivalent current is carried out, and the study results are presented. Dependences of the plasma density distribution on the beam equivalent current I{sub b}, energy E{sub b}, and gas pressure p indicate a crucial role of fast molecules in gas ionization, and the probe characteristics reveal two groups of plasma electrons with the temperatures T{sub e} {approx} 0.4 eV and T{sub e} {approx} 16 eV. Immersion in plasma of an electrode isolated from the chamber and application to the electrode of a positive voltage U result in non-self-sustained discharge. When U changes from {approx}0.5 to {approx}1.5 V, the discharge current I rapidly rises to a certain value I*, and after that the rate of rise dI/dU drops by an order of magnitude. At U {approx} 10 V, the current I rises to I{sub 0} Almost-Equal-To 1.5I*, and dI/dU once again drops by an order of magnitude. Current I{sub 0} specifies the number of electrons produced inside the chamber per second, and it grows up with E{sub b}, I{sub b}, and p. At U > 20 V, due to gas ionization by fast electrons emitted by the chamber and accelerated up to the energy {approx}eU in the sheath between the plasma and the chamber walls, the current I rises again. When U grows up to {approx}50 V, production of fast electrons with energies exceeding the ionization threshold begins inside the sheath, and the ionization intensity rises dramatically. At U > 150 V, contribution of fast electrons to gas ionization already exceeds the contribution of fast molecules, and the plasma density and its distribution homogeneity inside the chamber both grow up substantially. However, even in this case, the discharge is non-self-sustained, and only at U > 300 V it does not expire when the beam source is switched off.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Intercalation of C{sub 60} Fullerene Molecules under Single-Layer Graphene on Molybdenum Carbide

It is shown that heating of a fullerite film several monomolecular layers thick deposited onto single-layer graphene formed on a substrate of molybdenum carbide Mo{sub 2}C at T = 700–800 K leads to the intercalation of C{sub 60} molecules under the graphene layer. The direct deposition of C{sub 60} molecules at T = 650 K also leads to the intercalation of C{sub 60} molecules under graphene; the maximal amount of fullerene accumulated under graphene is one single layer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

The interaction of low-energy electrons with fructose molecules

Using a hypocycloidal electronic spectrometer, the interactions of low energy electrons (0–8.50 eV) with fructose molecules, namely, electron scattering and dissociative attachment, are studied. The results of these studies showed that the fragmentation of fructose molecules occurs effectively even at an electron energy close to zero. In the total electron-scattering cross section by molecules, resonance features (at energies ~3.10 and ~5.00 eV) were first observed near the formation thresholds of light ion fragments OH{sup –} and H{sup –}. The correlation of the features observed in the cross sections of electron scattering and dissociative attachment is analyzed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Electron-Impact-Induced Fragmentation of a Glutamine Molecule

The formation of ionized products upon single and dissociative ionization of a glutamine (C{sub 5}H{sub 10}N{sub 2}O{sub 3}) molecule by electron impact at low (70 eV) and high (11.5 MeV) energies has been studied by method of mass spectrometry. The mass spectra of glutamine molecules have been obtained upon electron irradiation to various doses (0, 5, 10, and 20 kGy) and for various threshold functions of ion fragment yield. The absolute values of ion fragment appearance energies are determined. Electron beams were generated by a three-electrode gun and electron accelerator (microtron). Comparative analysis of the measured mass spectra of unirradiated and irradiated glutamine molecules showed that high-energy irradiation produced irreversible changes in the molecular structure.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Diffusion dynamics of small molecules from mesoporous silicon films by real-time optical interferometry

Time-dependent laser reflectometry measurements are presented as a means to rigorously characterize analyte diffusion dynamics of small molecules from mesoporous silicon (PSi) films for drug delivery and membrane physics applications. Calculations based on inclusion of a spatially and temporally dependent solute concentration profile in a one-dimensional Fickian diffusion flow model are performed to determine the diffusion coefficients for the selected prototypical polar species, sucrose (340 Da), exiting from PSi films. The diffusion properties of the molecules depend on both PSi pore size and film thickness. For films with average pore diameters between 10-30 nm and film thicknesses between 300-900 nm, the sucrose diffusion coefficient can be tuned between approximately 100 and 550 {mu}m{sup 2}/s. Extensions of the real-time measurement and modeling approach for determining the diffusivity of small molecules that strongly interact with and corrode the internal surfaces of PSi films are also discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗