6,171 research outputs found
On the problem of mass-dependence of the two-point function of the real scalar free massive field on the light cone
We investigate the generally assumed inconsistency in light cone quantum
field theory that the restriction of a massive, real, scalar, free field to the
nullplane is independent of mass \cite{LKS}, but the
restriction of the two-point function depends on it (see, e.g., \cite{NakYam77,
Yam97}). We resolve this inconsistency by showing that the two-point function
has no canonical restriction to in the sense of distribution theory.
Only the so-called tame restriction of the two-point function exists which we
have introduced in \cite{Ull04sub}. Furthermore, we show that this tame
restriction is indeed independent of mass. Hence the inconsistency appears only
by the erroneous assumption that the two-point function would have a
(canonical) restriction to .Comment: 10 pages, 2 figure
A novel method for unambiguous ion identification in mixed ion beams extracted from an EBIT
A novel technique to identify small fluxes of mixed highly charged ion beams
extracted from an Electron Beam Ion Trap (EBIT) is presented and practically
demonstrated. The method exploits projectile charge state dependent potential
emission of electrons as induced by ion impact on a metal surface to separate
ions with identical or very similar mass-to-charge ratio.Comment: 8 pages, 5 figure
Imaging Molecules from Within: Ultra-fast, {\AA}ngstr\"om Scale Structure Determination of Molecules via Photoelectron Holography using Free Electron Lasers
A new scheme based on (i) upcoming brilliant X-ray Free Electron Laser (FEL)
sources, (ii) novel energy and angular dispersive, large-area electron imagers
and (iii) the well-known photoelectron holography is elaborated that provides
time-dependent three-dimensional structure determination of small to medium
sized molecules with {\AA}ngstr\"om spatial and femtosecond time resolution.
Inducing molecular dynamics, wave-packet motion, dissociation, passage through
conical intersections or isomerization by a pump pulse this motion is
visualized by the X-ray FEL probe pulse launching keV photoelectrons within few
femtoseconds from specific and well-defined sites, deep core levels of
individual atoms, inside the molecule. On their way out the photoelectrons are
diffracted generating a hologram on the detector that encodes the molecular
structure at the instant of photoionization, thus providing 'femtosecond
snapshot images of the molecule from within'. Detailed calculations in various
approximations of increasing sophistication are presented and three-dimensional
retrieval of the spatial structure of the molecule with {\AA}ngstr\"om spatial
resolution is demonstrated. Due to the large photo-absorption cross sections
the method extends X-ray diffraction based, time-dependent structure
investigations envisioned at FELs to new classes of samples that are not
accessible by any other method. Among them are dilute samples in the gas phase
such as aligned, oriented or conformer selected molecules, ultra-cold ensembles
and/or molecular or cluster objects containing mainly light atoms that do not
scatter X-rays efficiently.Comment: 18 pages, 11 figure
Three-body Interactions In Proton-helium Angular Scattering
H++He scattering at 0.5 MeV has been investigated using a coincidence technique that completely determines the three-body transverse momentum exchange in single ionization collisions. Three scattering regions could be distinctly recognized that are dominated by proton helium-nucleus, proton-electron, or electron helium-nucleus interactions. Calculations and the experimental data show that the coupling between the electronic and nuclear degrees of freedom is required to understand the dynamics for more than 97% of the ionizing collisions. © 1989 The American Physical Society
XUV Frequency Combs via Femtosecond Enhancement Cavities
We review the current state of tabletop extreme ultraviolet (XUV) sources
based on high harmonic generation (HHG) in femtosecond enhancement cavities
(fsEC). Recent developments have enabled generation of high photon flux (1014
photons/sec) in the XUV, at high repetition rates (>50 MHz) and spanning the
spectral region from 40 nm - 120 nm. This level of performance has enabled
precision spectroscopy with XUV frequency combs and promises further
applications in XUV spectroscopic and photoemission studies. We discuss the
theory of operation and experimental details of the fsEC and XUV generation
based on HHG, including current technical challenges to increasing the photon
flux and maximum photon energy produced by this type of system. Current and
future applications for these sources are also discussed.Comment: invited review article, 38 page
Time-dependent electron transport through a strongly correlated quantum dot: multiple-probe open boundary conditions approach
We present a time-dependent study of electron transport through a strongly
correlated quantum dot. The time-dependent current is obtained with the
multiple-probe battery method, while adiabatic lattice density functional
theory in the Bethe ansatz local-density approximation to the Hubbard model
describes the dot electronic structure. We show that for a certain range of
voltages the quantum dot can be driven into a dynamical state characterized by
regular current oscillations. This is a manifestation of a recently proposed
dynamical picture of Coulomb blockade. Furthermore, we investigate how the
various approximations to the electron-electron interaction affect the
line-shapes of the Coulomb peaks and the I-V characteristics. We show that the
presence of the derivative discontinuity in the approximate
exchange-correlation potential leads to significantly different results
compared to those obtained at the simpler Hartree level of description. In
particular, a negative differential conductance (NDC) in the I-V
characteristics is observed at large bias voltages and large Coulomb
interaction strengths. We demonstrate that such NDC originates from the
combined effect of electron-electron interaction in the dot and the finite
bandwidth of the electrodes.Comment: 10 pages, 7 figure
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