256 research outputs found
Opportunities for future supernova studies of cosmic acceleration
We investigate the potential of a future supernova dataset, as might be
obtained by the proposed SNAP satellite, to discriminate among different ``dark
energy'' theories that describe an accelerating Universe. We find that many
such models can be distinguished with a fit to the effective
pressure-to-density ratio, , of this energy. More models can be
distinguished when the effective slope, , of a changing is also fit,
but only if our knowledge of the current mass density, , is improved.
We investigate the use of ``fitting functions'' to interpret luminosity
distance data from supernova searches, and argue in favor of a particular
preferred method, which we use in our analysis.Comment: Four pages including figures. Final published version. No significant
changes from v
Interatomic-Coulombic-decay-induced recapture of photoelectrons in helium dimers
We investigate the onset of photoionization shakeup induced interatomic
Coulombic decay (ICD) in He2 at the He+*(n = 2) threshold by detecting two He+
ions in coincidence. We find this threshold to be shifted towards higher
energies compared to the same threshold in the monomer. The shifted onset of
ion pairs created by ICD is attributed to a recapture of the threshold
photoelectron after the emission of the faster ICD electron.Comment: 5 Pages, 2 Figure
Vibrationally Resolved Decay Width of Interatomic Coulombic Decay in HeNe
We investigate the ionization of HeNe from below the He 1s3p excitation to
the He ionization threshold. We observe HeNe ions with an enhancement by
more than a factor of 60 when the He side couples resonantly to the radiation
field. These ions are an experimental proof of a two-center resonant
photoionization mechanism predicted by Najjari et al. [Phys. Rev. Lett. 105,
153002 (2010)]. Furthermore, our data provide electronic and vibrational state
resolved decay widths of interatomic Coulombic decay (ICD) in HeNe dimers. We
find that the ICD lifetime strongly increases with increasing vibrational
state.Comment: 7 pages, 5 figure
A measurement of the evolution of Interatomic Coulombic Decay in the time domain
During the last 15 years a novel decay mechanism of excited atoms has been
discovered and investigated. This so called ''Interatomic Coulombic Decay''
(ICD) involves the chemical environment of the electronically excited atom: the
excitation energy is transferred (in many cases over long distances) to a
neighbor of the initially excited particle usually ionizing that neighbor. It
turned out that ICD is a very common decay route in nature as it occurs across
van-der-Waals and hydrogen bonds. The time evolution of ICD is predicted to be
highly complex, as its efficiency strongly depends on the distance of the atoms
involved and this distance typically changes during the decay. Here we present
the first direct measurement of the temporal evolution of ICD using a novel
experimental approach.Comment: 6 pages, 4 figures, submitted to PR
Constraining dark energy with Sunyaev-Zel'dovich cluster surveys
We discuss the prospects of constraining the properties of a dark energy
component, with particular reference to a time varying equation of state, using
future cluster surveys selected by their Sunyaev-Zel'dovich effect. We compute
the number of clusters expected for a given set of cosmological parameters and
propogate the errors expected from a variety of surveys. In the short term they
will constrain dark energy in conjunction with future observations of type Ia
supernovae, but may in time do so in their own right.Comment: 5 pages, 3 figures, 1 table, version accepted for publication in PR
Elevated arousal at time of decision-making is not the arbiter of risk avoidance in chickens
The somatic marker hypothesis proposes that humans recall previously experienced physiological responses to aid decision-making under uncertainty. However, little is known about the mechanisms used by non-human animals to integrate risk perception with predicted gains and losses. We monitored the behaviour and physiology of chickens when the choice between a high-gain (large food quantity), high-risk (1 in 4 probability of receiving an air-puff) option (HGRAP) or a low-gain (small food quantity), no-risk (of an air-puff) (LGNAP) option. We assessed when arousal increased by considering different stages of the decision-making process (baseline, viewing, anticipation, reward periods) and investigated whether autonomic responses influenced choice outcome both immediately and in the subsequent trial. Chickens were faster to choose and their heart-rate significantly increased between the viewing and anticipation (post-decision, pre-outcome) periods when selecting the HGRAP option. This suggests that they responded physiologically to the impending risk. Additionally, arousal was greater following a HGRAP choice that resulted in an air-puff, but this did not deter chickens from subsequently choosing HGRAP. In contrast to human studies, we did not find evidence that somatic markers were activated during the viewing period, suggesting that arousal is not a good measure of avoidance in non-human animals
Revealing the Two-Electron Cusp in the Ground States of He and H2 via Quasifree Double Photoionization
We report on kinematically complete measurements and ab initio
non-perturbative calculations of double ionization of He and H2 by a single 800
eV circularly polarized photon. We confirm the quasifree mechanism of
photoionization for H2 and show how it originates from the two-electron cusp in
the ground state of a two-electron target. Our approach establishes a new
method for mapping electrons relative to each other and provides valuable
insight into photoionization beyond the electric-dipole approximation.Comment: 7 pages, 4 figure
Enantiosensitive Structure Determination by Photoelectron Scattering on Single Molecules
X-ray as well as electron diffraction are powerful tools for structure
determination of molecules. Electron diffraction methods yield
\r{A}ngstrom-resolution even when applied to large systems or systems involving
weak scatterers such as hydrogen atoms. For cases in which molecular crystals
cannot be obtained or the interaction-free molecular structure is to be
addressed, corresponding electron scattering approaches on gas-phase molecules
exist. Such studies on randomly oriented molecules, however, can only provide
information on interatomic distances, which is challenging to analyse in case
of overlapping distance parameters and they do not reveal the handedness of
chiral systems8. Here, we present a novel scheme to obtain information on the
structure, handedness and even detailed geometrical features of single
molecules in the gas phase. Using a loop-like analysis scheme employing input
from ab initio computations on the photoionization process, we are able to
deduce the three dimensional molecular structure with sensitivity to the
position individual atoms, as e.g. protons. To achieve this, we measure the
molecular frame diffraction pattern of core-shell photoelectrons in combination
with only two ionic fragments from a molecular Coulomb explosion. Our approach
is expected to be suitable for larger molecules, as well, since typical size
limitations regarding the structure determination by pure Coulomb explosion
imaging are overcome by measuring in addition the photoelectron in coincidence
with the ions. As the photoelectron interference pattern captures the molecular
structure at the instant of ionization, we anticipate our approach to allow for
tracking changes in the molecular structure on a femtosecond time scale by
applying a pump-probe scheme in the future
Age-Related Attenuation of Dominant Hand Superiority
The decline of motor performance of the human hand-arm system with age is well-documented. While dominant hand performance is superior to that of the non-dominant hand in young individuals, little is known of possible age-related changes in hand dominance. We investigated age-related alterations of hand dominance in 20 to 90 year old subjects. All subjects were unambiguously right-handed according to the Edinburgh Handedness Inventory. In Experiment 1, motor performance for aiming, postural tremor, precision of arm-hand movement, speed of arm-hand movement, and wrist-finger speed tasks were tested. In Experiment 2, accelerometer-sensors were used to obtain objective records of hand use in everyday activities
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