Orientational Ordering of Nonplanar Phthalocyanines on Cu(111): Strength and Orientation of the Electric Dipole Moment

In order to investigate the orientational ordering of molecular dipoles and the associated electronic properties, we studied the adsorption of chlorogallium phthalocyanine molecules (GaClPc, Pc=C_32N_8H_16) on Cu(111) using the X-ray standing wave technique, photoelectron spectroscopy, and quantum chemical calculations. We find that for sub-monolayer coverages on Cu(111) the majority of GaClPc molecules adsorb in a 'Cl-down' configuration by forming a covalent bond to the substrate. For bilayer coverages the XSW data indicate a co-existence of the 'Cl-down' and 'Cl-up' configuration on the substrate. The structural details established for both cases and supplementary calculations of the adsorbate system allow us to analyze the observed change of the work function.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let

Charge coupling in multi-stage laser wakefield acceleration

The multi-stage technique for laser driven acceleration of electrons become a critical part of full-optical, jitter-free accelerators. Use of several independent laser drivers and shorter length plasma targets allows the stable and reproducible acceleration of electron bunches (or beam) in the GeV energies with lower energy spreads. At the same time the charge coupling, necessary for efficient acceleration in the consecutive acceleration stage(s), depends collectively on the parameters of the injected electron beam, the booster stage, and the non-linear transverse dynamics of the electron beam in the laser pulse wake. An unmatched electron beam injected in the booster stage(s), and its non-linear transverse evolution may result in perturbation and even reduction of the field strength in the acceleration phase of the wakefield. Analysis and characterization of charge coupling in multi-stage laser wakefield acceleration (LWFA) become ultimately important. Here, we investigate two-stage LWFA via fully relativistic multi-dimensional particle-in-cell simulations, and underlying the most critical parameters, which affect the efficient coupling and acceleration of the electron beam in the booster stage.Comment: 10 figure

Radiation Reaction Effects in Cascade Scattering of Intense, Tightly Focused Laser Pulses by Relativistic Electrons

Non-linear cascade scattering of intense, tightly focused laser pulses by relativistic electrons is studied numerically in the classical approximation including the radiation damping for the quantum parameter hwx-ray/E<1 and an arbitrary radiation parameter Kai. The electron energy loss, along with its side scattering by the ponderomotive force, makes the scattering in the vicinity of high laser field nearly impossible at high electron energies. The use of a second, co-propagating laser pulse as a booster is shown to solve this problem

Exploring the Bonding of Large Hydrocarbons on Noble Metals: Diindoperylene on Cu(111), Ag(111), and Au(111)

We present a benchmark study for the adsorption of a large pi-conjugated organic molecule on different noble metal surfaces, which is based on X-ray standing wave (XSW) measurements and density functional theory calculations with van der Waals (vdW) interactions. The bonding distances of diindenoperylene on Cu(111), Ag(111), and Au(111) surfaces (2.51 A, 3.01 A, and 3.10 A, respectively) determined with the normal incidence XSW technique are compared with calculations. Excellent agreement with the experimental data, i.e. deviations less than 0.1 A, is achieved using the Perdew-Burke-Ernzerhof functional with vdW interactions that include the collective response of substrate electrons (PBE+vdW^{surf} method). Noteworthy, the calculations show that the vdW contribution to the adsorption energy increases in the order Au(111) < Ag(111) < Cu(111).Comment: 6 pages, 4 figures, accepted by Phys. Rev.

Space- and time-resolved observation of extreme laser frequency upshifting during ultrafast-ionization

A 65-fs, 800-nm, 2-TW laser pulse propagating through a nitrogen gas jet has been experimentally studied by 90 Thomson scattering. Time-integrated spectra of scattered light show unprecedented broadening towards the blue which exceeds 300 nm. Images of the scattering region provide for the first time a space- and time-resolved description of the process leading quite regularly to such a large upshift. The mean shifting rate was as high as dk/dt3A ̊/fs, never observed before. Interferometry shows that it occurs after partial laser defocusing. Numerical simulations prove that such an upshift is consistent with a laser-gas late interaction, when laser intensity has decreased well below relativistic values (a0 1) and ionization process involves most of the laser pulse. This kind of interaction makes spectral tuning of ultrashort intense laser pulses possible in a large spectral range. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4818602

Attentional Set-Shifting Deficit in Parkinson’s Disease Is Associated with Prefrontal Dysfunction: An FDG-PET Study

The attentional set-shifting deficit that has been observed in Parkinson’s disease (PD) has long been considered neuropsychological evidence of the involvement of meso-prefrontal and prefrontal-striatal circuits in cognitive flexibility. However, recent studies have suggested that non-dopaminergic, posterior cortical pathologies may also contribute to this deficit. Although several neuroimaging studies have addressed this issue, the results of these studies were confounded by the use of tasks that required other cognitive processes in addition to set-shifting, such as rule learning and working memory. In this study, we attempted to identify the neural correlates of the attentional set-shifting deficit in PD using a compound letter task and 18F-fluoro-deoxy-glucose (FDG) positron emission tomography during rest. Shift cost, which is a measure of attentional set-shifting ability, was significantly correlated with hypometabolism in the right dorsolateral prefrontal cortex, including the putative human frontal eye field. Our results provide direct evidence that dysfunction in the dorsolateral prefrontal cortex makes a primary contribution to the attentional set-shifting deficit that has been observed in PD patients

Energy level alignment at strongly coupled organic metal interfaces

Energy-level alignment at organic–metal interfaces plays a crucial role for the performance of organic electronic devices. However, reliable models to predict energetics at strongly coupled interfaces are still lacking. We elucidate contact formation of 1,2,5,6,9,10-coronenehexone (COHON) to the (1 1 1)-surfaces of coinage metals by means of ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, the x-ray standing wave technique, and density functional theory calculations. While for low COHON thicknesses, the work-functions of the systems vary considerably, for thicker organic films Fermi-level pinning leads to identical work functions of 5.2 eV for all COHON-covered metals irrespective of the pristine substrate work function and the interfacial interaction strength.Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659Soochow University-Western University Joint Center for Synchrotron Radiation ResearchCollaborative Innovation Center of Suzhou Nano Science & Technology111 Project of the Chinese State Administration of Foreign Experts AffairsAustrian Science Fundhttps://doi.org/10.13039/501100002428National Key R&D Program of ChinaPeer Reviewe