Large Energy Dispersion Research Articles

Conformational Control of Donor-Acceptor Molecules Using Non-covalent Interactions.

Controlling the architecture of organic molecules is an important aspect in tuning the functional properties of components in organic electronics. For purely organic thermally activated delayed fluorescence (TADF) molecules, design is focused upon orthogonality orientated donor and acceptor units. In these systems, the rotational dynamics around the donor and acceptor bond has been shown to be critical for activating TADF; however, too much conformational freedom can increase the non-radiative rate, leading to a large energy dispersion of the emitting states and conformers, which do not exhibit TADF. To date, control of the motion around the D-A bond has focused upon steric hindrance. In this work, we computationally investigate eight proposed donor-acceptor molecules, exhibiting a B-N bond between the donor and acceptor. We compare the effect of steric hindrance and noncovalent interactions, achieved using oxygen (sulfur) boron heteroatom interactions, in exerting fine conformational control of the excited state dynamics. This work reveals the potential for judiciously chosen noncovalent interactions to strongly influence the functional properties of TADF emitters, including the accessible conformers and the energy dispersion associated with the charge transfer states.

A new analysis procedure to extract fusion excitation function with large beam energy dispersions: application to the 6Li+120Sn and 7Li+119Sn

In the present paper it is described an analysis procedure suited for experiments where cross-sections strongly varying with energy are measured using beams having large energy dispersion. These cross-sections are typically the sub-barrier fusion excitation function of reactions induced by radioactive beams. The large beam energy dispersion, typical of these experiments, can lead to ambiguities in the association of the effective beam energy to the reaction product yields and consequently to an error in the determination of the excitation function. As a test case, the approach is applied to the experiments 6 Li+120 Sn and 7 Li+119 Sn measured in the energy range 14 MeV ≤ Ec.m. ≤28 MeV. The complete fusion cross sections are deduced from activation measurements using the stacked target technique. The results of these experiments, that employ the two weakly-bound stable Li isotopes, show that the complete fusion cross sections above the barrier are suppressed of about 70% and 85% with respect to the Universal Fusion Function, used as a standard reference, in the 6 Li and 7 Li induced reactions respectively. Moreover, the excitation functions of the two systems at energies below the barrier, do not show significant differences, despite the two systems have different n -transfer Qvalue .

Design of monochromatic X-ray imager with a pinhole and a logarithmic spiral Bragg mirror for Z-pinch diagnose

Monochromatic self-emission X-ray imaging is a powerful way to diagnose the imploding Z-pinch plasmas. Several approaches have been successfully developed to image the 0.1–1keV continuous emission of a time evolving Z-pinch plasma, including the using of a filtered pinhole camera and a filtered pinhole camera combined with a grazing incidence reflection mirror or a planar multilayer mirror, but these techniques might be unfeasible when the instrument is keyed to image the 1–10keV K-shell emission due to the inherent constraints such as limited mirror-filter match and/or large energy dispersion. In this paper, we proposed a novel monochromatic X-ray imaging approach that employs a pinhole camera and a logarithmic spiral Bragg mirror. The logarithmic spiral Bragg mirror is a non-dispersive monochromator that break the constraints meted in the planar mirror and thus can be used to image the K-shell emission. The geometrical imaging theory of this new instrument and the instrument design and evaluation are discussed.

Spectroscopy of laser-plasma accelerated electrons: A novel concept based on Thomson scattering

The spectrum of relativistic electron bunches with large energy dispersion, like the ones usually generated with laser-plasma acceleration processes, is difficult to obtain with conventional methods. A novel spectroscopic concept, based on the analysis of the photons generated by Thomson scattering of a probe laser pulse by the electron bunch, is presented. The feasibility of a single-pulse spectrometer, using an energy-calibrated charge coupled device as detector, is investigated. Numerical simulations performed in conditions typical of a real experiment show the effectiveness and accuracy of the new method.

Momentum-Dependent Charge Transfer Excitations in Sr2CuO 2Cl2 Angle-Resolved Electron Energy Loss Spectroscopy.

Electron-hole pair excitations in the insulating cuprates $Sr_2CuO_2Cl_2$ were investigated by angle-resolved electron energy loss spectroscopy. The optically allowed and optically forbidden transitions were observed to be strongly anisotropic in $Cu-O_2$ plane. The former show a large energy dispersion $\sim 1.5$eV along [110], and the latter appear at a higher energy position ($\sim 4.5$eV) only along [100], but not along [110]. We interpret these results as transitions involving excitons. A small exciton model is examined to explain both the observed features.

Signatures of the substorm recovery phase at high‐altitude spacecraft

The substorm recovery phase typically commences ∼30 min after the substorm expansion phase onset and covers a period of roughly 1 hour. Several signatures have previously been associated with the recovery phase such as plasma sheet expansion in the midtail and magnetic field return toward the quiet time configuration. However, the detailed temporal sequence during the recovery phase is still not very well established. A total of 66 events in February‐April 1979 have been investigated where ISEE 2 observed a plasma sheet expansion associated with fast earthward flows. Of these, 50 events were clearly associated with the substorm recovery phase as identified in ground magnetic records. For this data set, energetic electron (>30 keV) and proton (>145 keV) observations from two geostationary spacecraft were available in 41 cases. Of the 41 cases, 32 of the midtail plasma sheet recoveries were associated with distinctive ion or electron flux increases at geostationary orbit. These flux increases, often in both protons and electrons, were generally observed from the predusk to the postdawn sector. However, very few enhancements were found near local noon. The lack of large energy dispersion in the flux increases and the simultaneous occurrence of both electron and ion enhancements suggests that the particles do not drift from a more distant location (as in substorm expansion phase onsets) but are accelerated or redistributed locally. These events are suggested to be associated with a large‐scale reconfiguration of the near‐Earth tail as the neutral line retreats to large distances.

Angle-resolved resonant photoemission of Ni(100) and Ni(110) single crystals

We have studied the valence band photoemission spectra of Ni(100) and Ni(110) single crystals near the excitation threshold for 3p core electrons. The resonant behavior of the 6 eV satellite does not depend on both the surface orientation and the polarization of the electric vector of an incident light for excitation. These results indicate that the 6 eV satellite should be under little influence of spatial symmetry of the valence band. In the angle-resolved photoemission spectra of Ni(100), we have observed another broad feature near the 6 eV satellite. It shows the large energy dispersion and is interpreted as due to the interband transition. In Ni(110), we have observed the weak valence band satellites at binding energies of about 9.3 eV and 13.4 eV. They do not show well-defined resonance around the 3p threshold.

Valence band structure of TlSbS2 crystals by angle-resolved photoemission

Photoemission is performed on TlSbS2, a layered compound with a large unit cell (16 atoms) and a calculated structure of 28 valence bands. The angle-integrated emission data confirm the existence of three groups of bands as obtained in the tight-binding calculation. In the angle-resolved modem, the observed emission presents a large energy dispersion with an apparent period larger than the reciprocal lattice. This effect, also present in other materials having a large number of atoms by unit cell, is attributed to strong variations versus k/sub /// of the transition matrix elements. In this case, photoemission data are not directly comparable with the band structure and a complete calculation of emission probability is needed to assign the observed bands.

Beam-beam effects with large dispersion function at the interaction point

Beam-beam effects are considered for the case of circular electron-positron colliders with very low beam emittances and a large energy dispersion at the interaction point.