1,330 research outputs found
Broadband infrared and Raman probes of excited-state vibrational molecular dynamics; Simulation protocols based on loop diagram
Vibrational motions in electronically excited states can be observed by
either time and frequency resolved infrared absorption or by off resonant
stimulated Raman techniques. Multipoint correlation function expressions are
derived for both signals. Three representations for the signal which suggest
different simulation protocols are developed. These are based on the forward
and the backward propagation of the wavefunction, sum over state expansion
using an effective vibration Hamiltonian and a semiclassical treatment of a
bath. We show that the effective temporal () and spectral
() resolution of the techniques is not controlled solely by
experimental knobs but also depends on the system dynamics being probed. The
Fourier uncertainty is never violated
Time-resolved broadband Raman spectroscopies; A unified six-wave-mixing representation
Excited-state vibrational dynamics in molecules can be studied by an
electronically off-resonant Raman process induced by a probe pulse with
variable delay with respect to an actinic pulse. We establish the connection
between several variants of the technique that involve either spontaneous or
stimulated Raman detection and different pulse configurations. By using loop
diagrams in the frequency domain we show that all signals can be described as
six wave mixing which depend on the same four point molecular correlation
functions involving two transition dipoles and two polarizabilities and
accompanied by a different gating. Simulations for the stochastic
two-state-jump model illustrate the origin of the absorptive and dispersive
features observed experimentally
Revisiting two-step Forbush decreases
Interplanetary coronal mass ejections (ICMEs) and their shocks can sweep out galactic cosmic rays (GCRs), thus creating Forbush decreases (FDs). The traditional model of FDs predicts that an ICME and its shock decrease the GCR intensity in a two-step profile. This model, however, has been the focus of little testing. Thus, our goal is to discover whether a passing ICME and its shock inevitably lead to a two-step FD, as predicted by the model. We use cosmic ray data from 14 neutron monitors and, when possible, high time resolution GCR data from the spacecraft International Gamma Ray Astrophysical Laboratory (INTEGRAL). We analyze 233 ICMEs that should have created two-step FDs. Of these, only 80 created FDs, and only 13 created two-step FDs. FDs are thus less common than predicted by the model. The majority of events indicates that profiles of FDs are more complicated, particularly within the ICME sheath, than predicted by the model. We conclude that the traditional model of FDs as having one or two steps should be discarded. We also conclude that generally ignored small-scale interplanetary magnetic field structure can contribute to the observed variety of FD profiles
Cascading and Local-Field Effects in Non-Linear Optics Revisited; A Quantum-Field Picture Based on Exchange of Photons
The semi-classical theory of radiation-matter coupling misses local-field
effects that may alter the pulse time-ordering and cascading that leads to the
generation of new signals. These are then introduced macroscopically by solving
Maxwell's equations. This procedure is convenient and intuitive but ad hoc. We
show that both effects emerge naturally by including coupling to quantum modes
of the radiation field in the vacuum state to second order. This approach is
systematic and suggests a more general class of corrections that only arise in
a QED framework. In the semi-classical theory, which only includes classical
field modes, the susceptibility of a collection of non-interacting
molecules is additive and scales as . Second-order coupling to a vacuum mode
generates an effective retarded interaction that leads to cascading and local
field effects both of which scale as
Multipoint, high time resolution galactic cosmic ray observations associated with two interplanetary coronal mass ejections
[1] Galactic cosmic rays (GCRs) play an important role in our understanding of the interplanetary medium (IPM). The causes of their short timescale variations, however, remain largely unexplored. In this paper, we compare high time resolution, multipoint space-based GCR data to explore structures in the IPM that cause these variations. To ensure that features we see in these data actually relate to conditions in the IPM, we look for correlations between the GCR time series from two instruments onboard the Polar and INTEGRAL (International Gamma Ray Astrophysical Laboratory) satellites, respectively inside and outside Earth\u27s magnetosphere. We analyze the period of 18–24 August 2006 during which two interplanetary coronal mass ejections (ICMEs) passed Earth and produced a Forbush decrease (Fd) in the GCR flux. We find two periods, for a total of 10 h, of clear correlation between small-scale variations in the two GCR time series during these 7 days, thus demonstrating that such variations are observable using space-based instruments. The first period of correlation lasted 6 h and began 2 h before the shock of the first ICME passed the two spacecraft. The second period occurred during the initial decrease of the Fd, an event that did not conform to the typical one- or two-step classification of Fds. We propose that two planar magnetic structures preceding the first ICME played a role in both periods: one structure in driving the first correlation and the other in initiating the Fd
A Continued Fraction Resummation Form of Bath Relaxation Effect in the Spin-Boson Model
In the spin-boson model, a continued fraction form is proposed to
systematically resum high-order quantum kinetic expansion (QKE) rate kernels,
accounting for the bath relaxation effect beyond the second-order perturbation.
In particular, the analytical expression of the sixth-order QKE rate kernel is
derived for resummation. With higher-order correction terms systematically
extracted from higher-order rate kernels, the resummed quantum kinetic
expansion (RQKE) approach in the continued fraction form extends the Pade
approximation and can fully recover the exact quantum dynamics as the expansion
order increases.Comment: accepted by J. Chem. Phy
Detailed Calculation of Test-Mass Charging in the LISA Mission
The electrostatic charging of the LISA test masses due to exposure of the
spacecraft to energetic particles in the space environment has implications in
the design and operation of the gravitational inertial sensors and can affect
the quality of the science data. Robust predictions of charging rates and
associated stochastic fluctuations are therefore required for the exposure
scenarios expected throughout the mission. We report on detailed charging
simulations with the Geant4 toolkit, using comprehensive geometry and physics
models, for Galactic cosmic-ray protons and helium nuclei. These predict
positive charging rates of 50 +e/s (elementary charges per second) for solar
minimum conditions, decreasing by half at solar maximum, and current
fluctuations of up to 30 +e/s/Hz^{1/2}. Charging from sporadic solar events
involving energetic protons was also investigated. Using an event-size
distribution model, we conclude that their impact on the LISA science data is
manageable. Several physical processes hitherto unexplored as potential
charging mechanisms have also been assessed. Significantly, the kinetic
emission of very low-energy secondary electrons due to bombardment of the
inertial sensors by primary cosmic rays and their secondaries can produce
charging currents comparable with the Monte Carlo rates.Comment: 31 pages, 18 figures, 4 tables. to be published in Astroparticle
Physics. Changed due to error found in normalisation of the simulation
result
Fluctuation theorem for counting-statistics in electron transport through quantum junctions
We demonstrate that the probability distribution of the net number of
electrons passing through a quantum system in a junction obeys a steady-state
fluctuation theorem (FT) which can be tested experimentally by the full
counting statistics (FCS) of electrons crossing the lead-system interface. The
FCS is calculated using a many-body quantum master equation (QME) combined with
a Liouville space generating function (GF) formalism. For a model of two
coupled quantum dots, we show that the FT becomes valid for long binning times
and provide an estimate for the finite-time deviations. We also demonstrate
that the Mandel (or Fano) parameter associated with the incoming or outgoing
electron transfers show subpoissonian (antibunching) statistics.Comment: 20 pages, 12 figures, accepted in Phy.Rev.
Relationship of Transmural Variations in Myofiber Contractility to Left Ventricular Ejection Fraction: Implications for Modeling Heart Failure Phenotype With Preserved Ejection Fraction
The pathophysiological mechanisms underlying preserved left ventricular (LV) ejection fraction (EF) in patients with heart failure and preserved ejection fraction (HFpEF) remain incompletely understood. We hypothesized that transmural variations in myofiber contractility with existence of subendocardial dysfunction and compensatory increased subepicardial contractility may underlie preservation of LVEF in patients with HFpEF. We quantified alterations in myocardial function in a mathematical model of the human LV that is based on the finite element method. The fiber-reinforced material formulation of the myocardium included passive and active properties. The passive material properties were determined such that the diastolic pressure-volume behavior of the LV was similar to that shown in published clinical studies of pressure-volume curves. To examine changes in active properties, we considered six scenarios: (1) normal properties throughout the LV wall; (2) decreased myocardial contractility in the subendocardium; (3) increased myocardial contractility in the subepicardium; (4) myocardial contractility decreased equally in all layers, (5) myocardial contractility decreased in the midmyocardium and subepicardium, (6) myocardial contractility decreased in the subepicardium. Our results indicate that decreased subendocardial contractility reduced LVEF from 53.2 to 40.5%. Increased contractility in the subepicardium recovered LVEF from 40.5 to 53.2%. Decreased contractility transmurally reduced LVEF and could not be recovered if subepicardial and midmyocardial contractility remained depressed. The computational results simulating the effects of transmural alterations in the ventricular tissue replicate the phenotypic patterns of LV dysfunction observed in clinical practice. In particular, data for LVEF, strain and displacement are consistent with previous clinical observations in patients with HFpEF, and substantiate the hypothesis that increased subepicardial contractility may compensate for subendocardial dysfunction and play a vital role in maintaining LVEF
- …