Direction Of Momentum Transfer Research Articles

Twisted electron impact single ionization coincidence cross-sections for noble gas atoms

We present the angular profiles of the triple differential cross-section (TDCS) for the (e, 2e) process on the noble gas atoms, namely He (1s), Ne (2s and 2p), and Ar (3p), for the plane wave and the twisted electron impact. We develop the theoretical formalism in the first Born approximation (FBA). The present study compares the TDCS for different values of orbital angular momentum number m and opening angles θ p of the twisted electron beam with that of the plane wave beam in co-planar asymmetric geometry. In addition, we also investigate the TDCS for macroscopic targets to explore the influence of the opening angle θ p of the twisted electron beam on the TDCS. Our results show that the peaks in binary and recoil regions shift from the momentum transfer direction. The results also show that for larger opening angles, the peaks for p-type orbitals split into double-peak structures, which are not observed in the plane wave results for the given kinematics. The angular profiles for averaged cross-section show the dependence of TDCS on the opening angles, especially for the larger opening angles, which are significantly different from the plane wave TDCS.

Unusual Breathing Behavior of Optically Excited Barium Titanate Nanocrystals

Coherent X-ray diffraction patterns were recorded by using an X-ray free-electron laser to illuminate barium titanate nanocrystals as a function of time delay after laser excitation. Rather than seeing any significant thermal expansion effects, the diffraction peaks were found to move perpendicular to the momentum transfer direction. This suggests a laser driven rotation of the crystal lattice, which is delayed by the aggregated state of the crystals. Internal deformations associated with crystal contacts were also observed.

Triple differential cross sections for electron-impact ionization of methane at intermediate energy.

We report an experimental and theoretical investigation of electron-impact single ionization of the highest occupied molecular orbital 1t2 and the next highest occupied molecular orbital 2a1 states of CH4 at an incident electron energy of 250 eV. Triple differential cross sections measured in two different laboratories were compared with results calculated within the molecular 3-body distorted wave and generalized Sturmian function theoretical models. For ionization of the 1t2 state, the binary peak was observed to have a single maximum near the momentum transfer direction that evolved into a double peak for increasing projectile scattering angles, as has been seen for ionization of atomic p-states. A detailed investigation of this evolution was performed. As expected because of its s-type character, for ionization of the 2a1 state, only a single binary peak was observed. Overall, good agreement was found between experiment and theory.

Probing directionality of local electronic structure by momentum-selected STEM-EELS

Electron energy-loss spectroscopy (EELS) using a scanning transmission electron microscope (STEM) provides information on the local electronic structure around an atomic column irradiated by the incident electron probe. In this study, we present a method for detecting the directionality of chemical bonds with atomic resolution by selecting a specific atomic column and the direction of momentum transfer with an off-axis collection aperture in the STEM-EELS measurement. It is shown that the oxygen K-edge fine structures in SrTiO3 measured from the crystallographically equivalent oxygen sites, which are in TiO-planes perpendicular to the electron probe direction, show different structures depending on the directionality of the σ-bond between the oxygen and titanium atoms. This method opens up a way to explore the electronic states in anisotropic materials and interfaces with atomic resolution.

Out-of-plane ( e,2e ) measurements and calculations on He autoionizing levels as a function of incident-electron energy

Out-of-scattering-plane $(e,2e)$ measurements and calculations are reported for the three singlet helium $2\ensuremath{\ell}2{\ensuremath{\ell}}^{\ensuremath{'}}$ autoionizing levels, with 80, 100, 120, 150, and 488 eV incident-electron energies, and scattering angles ${60}^{\ensuremath{\circ}}, 50.{8}^{\ensuremath{\circ}}, {45}^{\ensuremath{\circ}}, 39.{2}^{\ensuremath{\circ}}$, and $20.{5}^{\ensuremath{\circ}}$, respectively. The kinematics are the same in all cases: the momentum transfer is $K=2.1$ a.u., and ejected electrons are detected in a plane that contains the momentum-transfer direction and is perpendicular to the scattering plane. The results are presented as $(e,2e)$ angular distributions energy integrated over each level. They are compared with fully nonperturbative $B$-spline $R$-matrix and hybrid second-order distorted-wave + $R$-matrix calculations.

Projectile-charge dependence of the differential cross section for the ionization of argon atoms at 1 keV

The single ionization triple differential cross sections (TDCS) of the Ar ($3p$) atoms are reported for the positron and electron impact at 1 keV. The calculated cross sections have been obtained using distorted wave Born approximation (DWBA) approach for the average ejected electron energies 13 and 26 eV at different momentum transfer conditions. The present attempt is helpful to probe the information on the TDCS trends for the particle-matter and antiparticle-matter interactions and to analyze the recent measurements [Phy. Rev. A 95, 062703 (2017)]. The binary electron emission is enhanced while the recoil emission is decreased for the positron impact relative to the electron impact in the DWBA calculation results. Systematic shift of peaks, shifting away from the momentum transfer direction for positron impact and shifting towards each other for electron impact, is observed with increasing momentum transfer.

Free Scattering Theory in Circularly Polarized Laser Field

<p class="Default">In the present study, we have investigated scattering of an electron by hydrogen atoms in the presence of the Circularly Polarized (CP) laser field. We have discussed the polarization effect of laser field on hydrogen atom and effect of the resulted polarized potential on differential scattering cross section is studied. We assumed the scattered electrons having kinetic energy 100 eV because it permitted to treat the scattering process in first order Born Approximation. The scattering electron was described by Volkov wave function. We found the differential scattering cross section decreases with the increase in scattering angle, for a fixed value of a laser parameters and kinetic energy of an incident electron. From this study we found that, the differential scattering cross section for the electric field perpendicular to the direction of momentum transfer depends on the elastic scattering amplitude. Finally, we concluded that the differential scattering cross section greatly depends upon the polarization of the laser field.</p><p><strong>Journal of Nepal Physical Society</strong><br />Volume 4, Issue 1, February 2017, Page: 78-87</p>

Beam-target double-spin asymmetry in quasielastic electron scattering off the deuteron with CLAS

Quasi-elastic electron scattering on the deuteron is a benchmark reaction to test our understanding of deuteron structure and the properties and interactions of the two nucleons bound in the deuteron. The experimental data presented here can be used to test state-of-the-art models of the deuteron and the two-nucleon interaction in the final state after two-body breakup of the deuteron. Focusing on polarization degrees of freedom, we gain information on spin-momentum correlations in the deuteron ground state (due to the D-state admixture) and on the limits of the Impulse Approximation (IA) picture as it applies to measurements of spin-dependent observables like spin structure functions for bound nucleons. We measured the beam-target double spin asymmetry for quasi-elastic electron scattering off the deuteron at several beam energies using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. The deuterons were polarized along (or opposite to) the beam direction. The double spin asymmetries were measured as a function of photon virtuality , missing momentum, and the angle between the (inferred) "spectator" neutron and the momentum transfer direction. The results are compared with a recent model that includes Final State Interactions (FSI) using a complete parameterization of nucleon-nucleon scattering, as well as a simplified model using the Plane Wave Impulse Approximation (PWIA). We find overall good agreement with both the PWIA and FSI expectations at low to medium missing momenta, including the change of the asymmetry due to the contribution of the deuteron D-state at higher momenta. At the highest missing momenta, our data clearly agree better with the calculations including FSI.

Coulomb drag in anisotropic systems: a theoretical study on a double-layer phosphorene

We theoretically study the Coulomb drag resistivity in a double-layer electron system with highly anisotropic parabolic band structure using Boltzmann transport theory. As an example, we consider a double-layer phosphorene on which we apply our formalism. This approach, in principle, can be tuned for other double-layered systems with paraboloidal band structures. Our calculations show the rotation of one layer with respect to another layer can be considered a way of controlling the drag resistivity in such systems. As a result of rotation, the off-diagonal elements of the drag resistivity tensor have non-zero values at any temperature. In addition, we show that the anisotropic drag resistivity is very sensitive to the direction of momentum transfer between two layers due to highly anisotropic inter-layer electron–electron interaction and also the plasmon modes. In particular, the drag anisotropy ratio, , can reach up to 3 by changing the temperature. Furthermore, our calculations suggest that including the local field correction in the dielectric function changes the results significantly. Finally, We examine the dependence of drag resistivity and its anisotropy ratio on various parameters like inter-layer separation, electron density, short-range interaction and insulating substrate/spacer.

Polarization-dependent DANES study on vertically-aligned ZnO nanorods

The local structural and local density of states of vertically-aligned ZnO nanorods are examined by using polarization-dependent diffraction anomalous near edge structure (DANES) measurements from c-oriented ZnO nanorods at the Zn K edge at the geometry of the incident x-ray electric field parallel and perpendicular to the x-ray momentum transfer direction. Orientation-dependent local structures determined by DANES are comparable with polarization- dependent EXAFS results. Unlike other techniques, polarization-dependent DANES can uniquely describe the orientation-dependent local structural properties and the local density of states of a selected element in selected-phased crystals of compounds or mixed-phased structures.

Effects of polarization direction on laser-assisted free–free scattering

This work will detail the effects of laser polarization direction (relative to the momentum transfer direction) on laser-assisted free–free scattering. Such processes play a role in the gas breakdown that occurs in electric discharges as well as providing a method for the laser heating of a plasma (Musa et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 175201, Mason 1993 Rep. Prog. Phys. 56 1275). Experimental results will be presented for electron-helium scattering in the presence of an Nd:YAG laser field ( eV) where the polarization direction was varied in a plane that is perpendicular to the scattering plane. To date, all of our experimental results are well described by the Kroll–Watson approximation (KWA) (Kroll and Watson 1973 Phys. Rev. A 8 804). The good agreement between our experiments and calculations using the KWA includes the case where the polarization is perpendicular to the momentum transfer direction, for which the KWA predicts vanishing cross section; other workers have found that the KWA tends to be inaccurate for cases where it predicts small cross sections (e.g. Musa et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 175201). We also present simulations of the effects that multiple scattering might have on experimental measurements. In particular, we examine conditions that are expected to be similar to those of the experiments reported by Wallbank and Holmes (Wallbank and Holmes 1993 Phys. Rev. A 48 R2515).

Test of target independence for free-free scattering in a Nd:YAG laser field

We report measurements of one-, two-, and three-photon processes during the elastic scattering of electrons through ${90}^{\ensuremath{\circ}}$ by helium, argon, and molecular-nitrogen targets, in the presence of 1.17-eV photons from a Nd:YAG laser. The incident energy of the electrons was 200 and 350 eV, and the linear polarization direction of the laser was parallel to the momentum transfer direction. Our measured free-free count rates for the three processes are target independent within the experimental uncertainties, perfectly consistent with the Kroll-Watson approximation, which assumes no interaction of the laser radiation with the target.

Investigation on Influences of Bubble Location and Momentum Transfer Direction on Liquid Turbulence Modification for the Dilute Bubbly Flow

Investigation on Influences of Bubble Location and Momentum Transfer Direction on Liquid Turbulence Modification for the Dilute Bubbly Flow

Threshold displacement energies in graphene and single-walled carbon nanotubes

The threshold displacement energy ${E}_{d}$ has been determined for graphene and 216 different ($n, m$) single-walled carbon nanotube chiralities, with $5\ensuremath{\le}n\ensuremath{\le}20$ and $0\ensuremath{\le}m\ensuremath{\le}n$, under several model conditions using classical molecular dynamics. The model conditions vary by particle (electron or carbon ion), empirical potential (two parametrizations of Tersoff [J. Tersoff, Phys. Rev. B 39, 5566 (1989); L. Lindsay and D. A. Broido, Phys. Rev. B 81, 205441 (2010)] and one of Brenner et al. [D. W. Brenner, O. A. Shenderova, J. A. Harrison, S. J. Stuart, B. Ni, and S. B. Sinnott, J. Phys.: Condens. Matter 14, 783 (2002)]), and momentum transfer direction (towards or away from the nanotube axis). For electron irradiation simulations, ${E}_{d}$ exhibits a smoothly varying chirality dependence and a characteristic curvature influenced by the momentum transfer direction. Changing the empirical potential shifts the magnitude of ${E}_{d}$, but the trend is preserved for electron simulations. However, the perturbation in the knock-on dynamics introduced by the carbon ion leads to ${E}_{d}$ trends that diverge from the equivalent electron simulation. Thus, the ion interaction has a non-negligible effect on the dynamics of the collision and leads to ${E}_{d}$ values that can distinctly vary depending on the selected carbon nanostructure.

Charge density waves and phonon-electron coupling inZrTe3

Charge-density-wave (CDW) order has long been interpreted as arising from a Fermi-surface instability in the parent metallic phase. While phonon-electron coupling has been suggested to influence the formation of CDW order in quasi-two-dimensional (quasi-2D) systems, the presumed dominant importance of Fermi-surface nesting remains largely unquestioned in quasi-1D systems. Here we show that phonon-electron coupling is also important for the CDW formation in a model quasi-1D system ZrTe$_3$. Our joint experimental and computational study reveals that particular lattice vibrational patterns possess exceedingly strong coupling to the conduction electrons, and are directly linked to the lattice distortions associated with the CDW order. The dependence of the coupling matrix elements on electron momentum further dictates the opening of (partial) electronic gaps in the CDW phase. Since lattice distortions and electronic gaps are the defining signatures of CDW order, our result demonstrates that the conventional wisdom based on Fermi-surface geometry needs to be substantially supplemented by phonon-electron coupling even in the simplest quasi-1D case. As prerequisites for the CDW formation, the highly anisotropic electronic structure and strong phonon-electron coupling in ZrTe$_3$ give rise to a distinct Raman scattering effect, namely, measured phonon linewidths depend on the direction of momentum transfer in the scattering process.

Double ionization of helium by ion impact: second Born order treatment at the fully differential level

In this work, a theoretical study of the double ionization of He by ion impact at the fully differential level is presented. Emphasis is made in the role played by the projectile in the double emission process depending on its charge and the amount of momentum transferred to the target. A Born-CDW model including a second-order term in the projectile charge is introduced and evaluated within an on-shell treatment. We find that emission geometries for which the second-order term dominates lead to asymmetric structures around the momentum transfer direction, a typical characteristic of higher order transitions.

On the relativistic electron impact K-shell (e, (3-1)e) differential cross sections for atoms

In this paper, we present the results of our calculation of four-fold differential cross section (4DCS) for the K-shell electron impact double ionization (DI) on various atoms in the relativistic energy regime. The 4DCS is computed from the five-fold differential cross sections (5DCS) by integrating 5DCS over either the angles of one of the ejected electrons or the scattered electron (this process is referred as an (e, (3-1)e) process). Hence, it requires, possibly feasible double coincidence techniques to supplement our findings experimentally instead of the difficult triple coincidence technique, which is required for the complete description of the DI process. The angular profile of 5DCS is scanned with the angles of one of the ejected electrons for both the (e, (3-1)e) cases. A dominant peak in the momentum transfer direction is found when the scattered and one of the ejected electrons are detected in coincidence. The effects of nucleus (Z), energies of the ejected electrons, and the scattering angle on the 4DCS have been discussed. The angular correlation of the ejected electrons has also been discussed when only the ejected electrons are detected. We consider the spin aspects by scanning the singlet and triplet contributions in the angular profile of 4DCS. We also find that the interference term of the longitudinal and transverse interaction is responsible for the strengthening of the secondary peak. We observe a clear manifestation of the effect of nucleus on the spin asymmetry for behavior targets.

Electron-helium scattering in a 1.17 eV laser field: The effect of polarization direction

We report measurements of one-photon emission during the elastic scattering of electrons by He atoms through ${90}^{\ensuremath{\circ}}$ in the presence of 1.17 eV photons from a Nd:YAG laser. The incident energy of the electrons was in the range 30--200 eV and the linear polarization direction of the laser was varied over ${180}^{\ensuremath{\circ}}$ in the plane, perpendicular to the scattering plane, that contains the momentum transfer direction. Our results are perfectly consistent with the Kroll-Watson approximation. In particular, we see no evidence of free-free transitions when the polarization is perpendicular to the momentum transfer direction.

Electron impact ionization of neon and neonic ions under distorted-wave Born approximation

The (e, 2e) triple differential cross sections of 2s orbitals of neon and neonic ions (Z = 11–14) are calculated using a distorted-wave Born approximation under coplanar asymmetric geometry. The calculated results show that, with the increase in the nuclear charge number Z, the amplitude of triple differential cross sections decreases. The angle difference between the binary peak position and the direction of momentum transfer gradually increases with the increase in the nuclear charge Z, and a new structure appears at an ejected angle 90° < θ2 < 120°. Three kinds of collision processes are proposed to illustrate the formation mechanism of such collision peaks.

Anisotropic Form Factors of Neutron Scattering by Magnetic Octupole in CeB6

The form factor of elastic neutron scattering is calculated for the quadrupole order phase of CeB6 in magnetic fields. It is shown that the dependence of the form factor on the direction of neutron momentum transfer is very small for the Bragg reflections, whereas the scattering due to field-induced octupoles gives rise to a significant anisotropy of the form factor for the super-lattice reflections. These results are discussed in quantitative comparison with recent exprimental results. The similarity and dissimilarity with the results for phase IV in Ce1−xLaxB6 is also discussed.