111 research outputs found
Influence of asymmetry and nodal planes on high-harmonic generation in heteronuclear molecules
The relation between high-harmonic spectra and the geometry of the molecular
orbitals in position and momentum space is investigated. In particular we
choose two isoelectronic pairs of homonuclear and heteronuclear molecules, such
that the highest occupied molecular orbital of the former exhibit at least one
nodal plane. The imprint of such planes is a strong suppression in the harmonic
spectra, for particular alignment angles. We are able to identify two distinct
types of nodal planes. If the nodal planes are determined by the atomic
wavefunctions only, the angle for which the yield is suppressed will remain the
same for both types of molecules. In contrast, if they are determined by the
linear combination of atomic orbitals at different centers in the molecule,
there will be a shift in the angle at which the suppression occurs for the
heteronuclear molecules, with regard to their homonuclear counterpart. This
shows that, in principle, molecular imaging, which uses the homonuclear
molecule as a reference and enables one to observe the wavefunction distortions
in its heteronuclear counterpart, is possible.Comment: 14 pages, 7 figures. Figs. 3, 5 and 6 have been simplified in order
to comply with the arXiv size requirement
Excitation, two-center interference and the orbital geometry in laser-induced nonsequential double ionization of diatomic molecules
We address the influence of the molecular orbital geometry and of the
molecular alignment with respect to the laser-field polarization on
laser-induced nonsequential double ionization of diatomic molecules for
different molecular species, namely and . We
focus on the recollision excitation with subsequent tunneling ionization (RESI)
mechanism, in which the first electron, upon return, promotes the second
electron to an excited state, from where it subsequently tunnels. We show that
the electron-momentum distributions exhibit interference maxima and minima due
to the electron emission at spatially separated centers. We provide generalized
analytical expressions for such maxima or minima, which take into account
mixing and the orbital geometry. The patterns caused by the two-center
interference are sharpest for vanishing alignment angle and get washed out as
this parameter increases. Apart from that, there exist features due to the
geometry of the lowest occupied molecular orbital (LUMO), which may be observed
for a wide range of alignment angles. Such features manifest themselves as the
suppression of probability density in specific momentum regions due to the
shape of the LUMO wavefunction, or as an overall decrease in the RESI yield due
to the presence of nodal planes.Comment: 11 pages revtex, 2 figure
Local dynamics in high-order harmonic generation using Bohmian trajectories
We investigate high-order harmonic generation from a Bohmian-mechanical
perspective, and find that the innermost part of the core, represented by a
single Bohmian trajectory, leads to the main contributions to the high-harmonic
spectra. Using time-frequency analysis, we associate this central Bohmian
trajectory to an ensemble of unbound classical trajectories leaving and
returning to the core, in agreement with the three step model. In the Bohmian
scenario, this physical picture builds up non-locally near the core via the
quantum mechanical phase of the wavefunction. This implies that the flow of the
wavefunction far from the core alters the central Bohmian trajectory. We also
show how this phase degrades in time for the peripheral Bohmian trajectories as
they leave the core region.Comment: 7 pages, 3 figures; the manuscript has been considerably extended and
modified with regard to the previous version
The significance of the reflective practitioner in blended learning
This is a case study paper concerned with the introduction of blended learning on a part-time higher education programme for mature students. The interpretive work draws on four action research cycles conducted over two years with two student cohorts. Discussion is based on observations, staff and student focus groups and interviews examining the students’ expectations and experiences.
The initial focus of the action research was on the introduction of technology into the teaching and learning experience. The great advantage of an interpretive approach, however, is to allow the findings to determine the course of the research. During the first action research cycles, the focus of the research soon changed from the use of technology in blended learning to the role of the practitioners involved. We advocate the key role of reflective practitioners in facilitating blended learning and suggest that action research is a useful framework to develop this.
Keywords: Educational Technology Implementation, Electronic Learning (E-Learning), Teacher Improvement, Teacher Preparation, Action Research, Asynchronous Communications, blended learning, part-time student, reflective practice
Forward and hybrid path-integral methods in photoelectron holography: sub-barrier corrections, initial sampling and momentum mapping
We construct two strong-field path integral methods with full Coulomb
distortion, in which the quantum pathways are mimicked by interfering electron
orbits: the rate-based CQSFA (R-CQSFA) and the hybrid forward-boundary CQSFA
(H-CQSFA). The methods have the same starting point as the standard Coulomb
quantum-orbit strong-field approximation (CQSFA), but their implementation does
not require pre-knowledge of the orbits' dynamics. These methods are applied to
ultrafast photoelectron holography. In the rate-based method, electron orbits
are forward propagated and we derive a non-adiabatic ionization rate from the
CQSFA, which includes sub-barrier Coulomb corrections and is used to weight the
initial orbit ensemble. In the H-CQSFA, the initial ensemble provides initial
guesses for a subsequent boundary problem and serves to include or exclude
specific momentum regions, but the ionization probabilities associated with
individual trajectories are computed from sub-barrier complex integrals. We
perform comparisons with the standard CQSFA and \textit{ab-initio} methods,
which show that the standard, purely boundary-type implementation of the CQSFA
leaves out whole sets of trajectories. We show that the sub-barrier Coulomb
corrections broaden the resulting photoelectron momentum distributions (PMDs)
and improve the agreement of the R-CQSFA with the H-CQSFA and other approaches.
We probe different initial sampling distributions, uniform and otherwise, and
their influence on the PMDs. We find that initial biased sampling emphasizes
rescattering ridges and interference patterns in high-energy ranges, while an
initial uniform sampling guarantees accurate modeling of the holographic
patterns near the ionization threshold or polarization axis. Our results are
explained using the initial to final momentum mapping for different types of
interfering trajectories.Comment: 25 pages revtex, 14 figures; in the revised version, some
explanations have been extended and some figures have been modifie
Role of the Mitochondria in Immune-Mediated Apoptotic Death of the Human Pancreatic β Cell Line βLox5
Mitochondria are indispensable in the life and death of many types of eukaryotic cells. In pancreatic beta cells, mitochondria play an essential role in the secretion of insulin, a hormone that regulates blood glucose levels. Unregulated blood glucose is a hallmark symptom of diabetes. The onset of Type 1 diabetes is preceded by autoimmune-mediated destruction of beta cells. However, the exact role of mitochondria has not been assessed in beta cell death. In this study, we examine the role of mitochondria in both Fas- and proinflammatory cytokine-mediated destruction of the human beta cell line, βLox5. IFNγ primed βLox5 cells for apoptosis by elevating cell surface Fas. Consequently, βLox5 cells were killed by caspase-dependent apoptosis by agonistic activation of Fas, but only after priming with IFNγ. This beta cell line undergoes both apoptotic and necrotic cell death after incubation with the combination of the proinflammatory cytokines IFNγ and TNFα. Additionally, both caspase-dependent and -independent mechanisms that require proper mitochondrial function are involved. Mitochondrial contributions to βLox5 cell death were analyzed using mitochondrial DNA (mtDNA) depleted βLox5 cells, or βLox5 ρ0 cells. βLox5 ρ0 cells are not sensitive to IFNγ and TNFα killing, indicating a direct role for the mitochondria in cytokine-induced cell death of the parental cell line. However, βLox5 ρ0 cells are susceptible to Fas killing, implicating caspase-dependent extrinsic apoptotic death is the mechanism by which these human beta cells die after Fas ligation. These data support the hypothesis that immune mediators kill βLox5 cells by both mitochondrial-dependent intrinsic and caspase-dependent extrinsic pathways
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