6,686 research outputs found
High-order harmonic generation from polyatomic molecules including nuclear motion and a nuclear modes analysis
We present a generic approach for treating the effect of nuclear motion in
the high-order harmonic generation from polyatomic molecules. Our procedure
relies on a separation of nuclear and electron dynamics where we account for
the electronic part using the Lewenstein model and nuclear motion enters as a
nuclear correlation function. We express the nuclear correlation function in
terms of Franck-Condon factors which allows us to decompose nuclear motion into
modes and identify the modes that are dominant in the high-order harmonic
generation process. We show results for the isotopes CH and CD and
thereby provide direct theoretical support for a recent experiment [Baker {\it
et al.}, Science {\bf 312}, 424 (2006)] that uses high-order harmonic
generation to probe the ultra-fast structural nuclear rearrangement of ionized
methane.Comment: 6 pages, 6 figure
Antihydrogen studies in ALPHA
he ALPHA experiment studies antihydrogen as a means to investigate the symmetry of matter and antimatter. Spectroscopic studies of the anti-atom hold the promise of the most precise direct comparisons of matter and antimatter possible. ALPHA was the first to trap antihydrogen in a magnetic trap, allowing the first ever detection of atomic transitions in an anti-atom. More recently, through stochastic heating, we have also been able to put a new limit on the charge neutrality of antihydrogen. ALPHA is currently preparing to perform the first laser-spectroscopy of antihydrogen, hoping to excite the 2s state using a two-photon transition from the 1s state. We discuss the recent results as well as the key developments that led to these successes and discuss how we are preparing to perform the first laser-spectroscopy. We will also discuss plans to use our novel technique for gravitational tests on antihydrogen for a direct measurement of the sign of the gravitational force on antihydrogen
Simulation of transition dynamics to high confinement in fusion plasmas
The transition dynamics from the low (L) to the high (H) confinement mode in
magnetically confined plasmas is investigated using a first-principles
four-field fluid model. Numerical results are in close agreement with
measurements from the Experimental Advanced Superconducting Tokamak - EAST.
Particularly, the slow transition with an intermediate dithering phase is well
reproduced by the numerical solutions. Additionally, the model reproduces the
experimentally determined L-H transition power threshold scaling that the ion
power threshold increases with increasing particle density. The results hold
promise for developing predictive models of the transition, essential for
understanding and optimizing future fusion power reactors
Probing the longitudinal momentum spread of the electron wave packet at the tunnel exit
We present an ellipticity resolved study of momentum distributions arising
from strong-field ionization of Helium at constant intensity. The influence of
the ion potential on the departing electron is considered within a
semi-classical model consisting of an initial tunneling step and subsequent
classical propagation. We find that the momentum distribution can be explained
by the presence of a longitudinal momentum spread of the electron at the exit
from the tunnel. Our combined experimental and theoretical study provides an
estimate of this momentum spread
Semiclassical two-step model for strong-field ionization
We present a semiclassical two-step model for strong-field ionization that
accounts for path interferences of tunnel-ionized electrons in the ionic
potential beyond perturbation theory. Within the framework of a classical
trajectory Monte-Carlo representation of the phase-space dynamics, the model
employs the semiclassical approximation to the phase of the full quantum
propagator in the exit channel. By comparison with the exact numerical solution
of the time-dependent Schr\"odinger equation for strong-field ionization of
hydrogen, we show that for suitable choices of the momentum distribution after
the first tunneling step, the model yields good quantitative agreement with the
full quantum simulation. The two-dimensional photoelectron momentum
distributions, the energy spectra, and the angular distributions are found to
be in good agreement with the corresponding quantum results. Specifically, the
model quantitatively reproduces the fan-like interference patterns in the
low-energy part of the two-dimensional momentum distributions as well as the
modulations in the photoelectron angular distributions.Comment: 31 pages, 7 figure
Nucleation of quark matter bubbles in neutron stars
The thermal nucleation of quark matter bubbles inside neutron stars is
examined for various temperatures which the star may realistically encounter
during its lifetime. It is found that for a bag constant less than a critical
value, a very large part of the star will be converted into the quark phase
within a fraction of a second. Depending on the equation of state for neutron
star matter and strange quark matter, all or some of the outer parts of the
star may subsequently be converted by a slower burning or a detonation.Comment: 13 pages, REVTeX, Phys.Rev.D (in press), IFA 93-32. 5 figures (not
included) available upon request from [email protected]
A Cosmological Three Level Neutrino Laser
We present a calculation of a neutrino decay scenario in the early Universe.
The specific decay is \nu_{2} \to \nu_{1} + \phi, where \phi is a boson. If
there is a neutrino mass hierarchy, m_{\nu_{e}} < m_{\nu_{\mu}} <
m_{\nu_{\tau}}, we show that it is possible to generate stimulated decay and
effects similar to atomic lasing without invoking new neutrinos, even starting
from identical neutrino distributions. Under the right circumstances the decay
can be to very low momentum boson states thereby producing something similar to
a Bose condensate, with possible consequences for structure formation. Finally,
we argue that this type of decay may also be important other places in early
Universe physics.Comment: 7 pages, RevTex, due for publication in Phys. Rev. D, April 15 issu
Mass formulas and thermodynamic treatment in the mass-density-dependent model of strange quark matter
The previous treatments for strange quark matter in the quark
mass-density-dependent model have unreasonable vacuum limits. We provide a
method to obtain the quark mass parametrizations and give a self-consistent
thermodynamic treatment which includes the MIT bag model as an extreme. In this
treatment, strange quark matter in bulk still has the possibility of absolute
stability. However, the lower density behavior of the sound velocity is
opposite to previous findings.Comment: Formatted in REVTeX 3.1, 5 pages, 3 figures, to appear in PRC6
Neural-Network Force Field Backed Nested Sampling: Study of the Silicon p-T Phase Diagram
Nested sampling is a promising method for calculating phase diagrams of
materials, however, the computational cost limits its applicability if
ab-initio accuracy is required. In the present work, we report on the efficient
use of a neural-network force field in conjunction with the nested-sampling
algorithm. We train our force fields on a recently reported database of silicon
structures and demonstrate our approach on the low-pressure region of the
silicon pressure-temperature phase diagram between 0 and \SI{16}{GPa}. The
simulated phase diagram shows a good agreement with experimental results,
closely reproducing the melting line. Furthermore, all of the experimentally
stable structures within the investigated pressure range are also observed in
our simulations. We point out the importance of the choice of
exchange-correlation functional for the training data and show how the meta-GGA
r2SCAN plays a pivotal role in achieving accurate thermodynamic behaviour using
nested-sampling. We furthermore perform a detailed analysis of the exploration
of the potential energy surface and highlight the critical role of a diverse
training data set
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