7,977 research outputs found
Photoassociation and coherent transient dynamics in the interaction of ultracold rubidium atoms with shaped femtosecond pulses - I. Experiment
We experimentally investigate various processes present in the
photoassociative interaction of an ultracold atomic sample with shaped
femtosecond laser pulses. We demonstrate the photoassociation of pairs of
rubidium atoms into electronically excited, bound molecular states using
spectrally cut femtosecond laser pulses tuned below the rubidium D1 or D2
asymptote. Time-resolved pump-probe spectra reveal coherent oscillations of the
molecular formation rate, which are due to coherent transient dynamics in the
electronic excitation. The oscillation frequency corresponds to the detun-ing
of the spectral cut position to the asymptotic transition frequency of the
rubidium D1 or D2 lines, respectively. Measurements of the molecular
photoassociation signal as a function of the pulse energy reveal a non-linear
dependence and indicate a non-perturbative excitation process. Chirping the
association laser pulse allowed us to change the phase of the coherent
transients. Furthermore, a signature for molecules in the electronic ground
state is found, which is attributed to molecule formation by femtosecond
photoassociation followed by spontaneous decay. In a subsequent article [A.
Merli et al., submitted] quantum mechanical calculations are presented, which
compare well with the experimental data and reveal further details about the
observed coherent transient dynamics
Compensation of the skin effect in low-frequency potential drop measurements
Potential drop measurements are routinely used in the non-destructive evaluation of component integrity. Potential drop measurements use either direct current (DC) or alternating current (AC), the latter will have superior noise performance due to the ability to perform phase sensitive detection and the reduction of flicker noise. AC measurements are however subject to the skin effect where the current is electromagnetically constricted to the surface of the component. Unfortunately, the skin effect is a function of magnetic permeability, which in ferromagnetic materials is sensitive to a number of parameters including stress and temperature, and consequently in-situ impedance measurements are likely to be unstable. It has been proposed that quasi-DC measurements, which benefit from superior noise performance, but also tend to the skin-effect independent DC measurement, be adopted for in-situ creep measurements for power station components. Unfortunately, the quasi-DC measurement will only tend to the DC distribution and therefore some remnant sensitivity to the skin effect will remain. This paper will present a correction for situations where the remnant sensitivity to the skin effect is not adequately suppressed by using sufficiently low frequency; the application of particular interest being the in-situ monitoring of the creep strain of power station components. The correction uses the measured phase angle to approximate the influence of the skin effect and allow recovery of the DC-asymptotic value of the resistance. The basis of the correction, that potential drop measurements are minimum phase is presented and illustrated on two cases; the creep strain sensor of practical interest and a conducting rod as another common case to illustrate generality. The correction is demonstrated experimentally on a component where the skin effect is manipulated by application of a range of elastic stresses
The Thermal Memory of Reionization History
The recent measurement by WMAP of a large electron scattering optical depth
tau_e = 0.17 +- 0.04 is consistent with a simple model of reionization in which
the intergalactic medium (IGM) is ionized at redshift z ~ 15, and remains
highly ionized thereafter. Here, we show that existing measurements of the IGM
temperature from the Lyman-alpha forest at z ~ 2 - 4 rule out this ``vanilla''
model. Under reasonable assumptions about the ionizing spectrum, as long as the
universe is reionized before z = 10, and remains highly ionized thereafter, the
IGM reaches an asymptotic thermal state which is too cold compared to
observations. To simultaneously satisfy the CMB and forest constraints, the
reionization history must be complex: reionization begins early at z >~ 15, but
there must have been significant (order unity) changes in fractions of neutral
hydrogen and/or helium at 6 < z < 10, and/or singly ionized helium at 4 < z <
10. We describe a physically motivated reionization model that satisfies all
current observations. We also explore the impact of a stochastic reionization
history and show that a late epoch of (HeII --> HeIII) reionization induces a
significant scatter in the IGM temperature, but the scatter diminishes with
time quickly. Finally, we provide an analytic formula for the thermal
asymptote, and discuss possible additional heating mechanisms that might evade
our constraints.Comment: 10 pages, submitted to ApJ, new references, additional discussion on
earlier work and partial HeII reionizatio
Formation of interstellar SH from vibrationally excited H: Quantum study of S + H SH + H reactions and inelastic collisions
The rate constants for the formation, destruction, and collisional excitation
of SH are calculated from quantum mechanical approaches using two new
SH potential energy surfaces (PESs) of and electronic
symmetry. The PESs were developed to describe all adiabatic states correlating
to the SH () + H() channel. The formation of SH
through the S + H reaction is endothermic by 9860 K, and
requires at least two vibrational quanta on the H molecule to yield
significant reactivity. Quasi-classical calculations of the total formation
rate constant for H() are in very good agreement with the quantum
results above 100K. Further quasi-classical calculations are then performed for
, 4, and 5 to cover all vibrationally excited H levels significantly
populated in dense photodissociation regions (PDR). The new calculated
formation and destruction rate constants are two to six times larger than the
previous ones and have been introduced in the Meudon PDR code to simulate the
physical and illuminating conditions in the Orion bar prototypical PDR. New
astrochemical models based on the new molecular data produce four times larger
SH column densities, in agreement with those inferred from recent ALMA
observations of the Orion bar.Comment: 8 pages, 7 figure
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Models for discriminating image blur from loss of contrast
Observers can discriminate between blurry and low-contrast images (Morgan, 2017). Wang and Simoncelli (2004) demonstrated that a code for blur is inherent to the phase relationships between localized pattern detectors of different scale. To test whether human observers actually use local phase coherence when discriminating between image blur and loss of contrast, we compared phase-scrambled chessboards with unscrambled chessboards. Although both stimuli had identical amplitude spectra, local phase coherence was disrupted by phase-scrambling. Human observers were required to concurrently detect and identify (as contrast or blur) image manipulations in the 2x2 forced-choice paradigm (Nachmias & Weber, 1975; Watson & Robson, 1981) traditionally considered to be a litmus test for "labelled lines" (i.e. detection mechanisms that can be distinguished on the basis of their preferred stimuli). Phase scrambling reduced some observers’ ability to discriminate between blur and a reduction in contrast. However, none of our observers produced data consistent with Watson & Robson’s most stringent test for labelled lines, regardless whether phases were scrambled or not. Models of performance fit significantly better when either a) the blur detector also responded to contrast modulations, b) the contrast detector also responded to blur modulations, or c) noise in the two detectors was anticorrelate
Dynamics of viscoelastic pipe flow in the maximum drag reduction limit
Polymer additives can substantially reduce the drag of turbulent flows and
the upper limit, the so called "maximum drag reduction" (MDR) asymptote is
universal, i.e. independent of the type of polymer and solvent used. Until
recently, the consensus was that, in this limit, flows are in a marginal state
where only a minimal level of turbulence activity persists. Observations in
direct numerical simulations using minimal sized channels appeared to support
this view and reported long "hibernation" periods where turbulence is
marginalized. In simulations of pipe flow we find that, indeed, with increasing
Weissenberg number (Wi), turbulence expresses long periods of hibernation if
the domain size is small. However, with increasing pipe length, the temporal
hibernation continuously alters to spatio-temporal intermittency and here the
flow consists of turbulent puffs surrounded by laminar flow. Moreover, upon an
increase in Wi, the flow fully relaminarises, in agreement with recent
experiments. At even larger Wi, a different instability is encountered causing
a drag increase towards MDR. Our findings hence link earlier minimal flow unit
simulations with recent experiments and confirm that the addition of polymers
initially suppresses Newtonian turbulence and leads to a reverse transition.
The MDR state on the other hand results from a separate instability and the
underlying dynamics corresponds to the recently proposed state of
elasto-inertial-turbulence (EIT).Comment: 18 pages, 5 figure
Many Roads to Synchrony: Natural Time Scales and Their Algorithms
We consider two important time scales---the Markov and cryptic orders---that
monitor how an observer synchronizes to a finitary stochastic process. We show
how to compute these orders exactly and that they are most efficiently
calculated from the epsilon-machine, a process's minimal unifilar model.
Surprisingly, though the Markov order is a basic concept from stochastic
process theory, it is not a probabilistic property of a process. Rather, it is
a topological property and, moreover, it is not computable from any
finite-state model other than the epsilon-machine. Via an exhaustive survey, we
close by demonstrating that infinite Markov and infinite cryptic orders are a
dominant feature in the space of finite-memory processes. We draw out the roles
played in statistical mechanical spin systems by these two complementary length
scales.Comment: 17 pages, 16 figures:
http://cse.ucdavis.edu/~cmg/compmech/pubs/kro.htm. Santa Fe Institute Working
Paper 10-11-02
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