143 research outputs found
Chain length dependence of the polymer-solvent critical point parameters
We report grand canonical Monte Carlo simulations of the critical point
properties of homopolymers within the Bond Fluctuation model. By employing
Configurational Bias Monte Carlo methods, chain lengths of up to N=60 monomers
could be studied. For each chain length investigated, the critical point
parameters were determined by matching the ordering operator distribution
function to its universal fixed-point Ising form. Histogram reweighting methods
were employed to increase the efficiency of this procedure. The results
indicate that the scaling of the critical temperature with chain length is
relatively well described by Flory theory, i.e. \Theta-T_c\sim N^{-0.5}. The
critical volume fraction, on the other hand, was found to scale like \phi_c\sim
N^{-0.37}, in clear disagreement with the Flory theory prediction \phi_c\sim
N^{-0.5}, but in good agreement with experiment. Measurements of the chain
length dependence of the end-to-end distance indicate that the chains are not
collapsed at the critical point.Comment: 13 Pages Revtex, 9 epsf embedded figs. gzipped tar file. To appear in
J. Chem. Phy
Generation of ultra-short light pulses by a rapidly ionizing thin foil
A thin and dense plasma layer is created when a sufficiently strong laser
pulse impinges on a solid target. The nonlinearity introduced by the
time-dependent electron density leads to the generation of harmonics. The pulse
duration of the harmonic radiation is related to the risetime of the electron
density and thus can be affected by the shape of the incident pulse and its
peak field strength. Results are presented from numerical
particle-in-cell-simulations of an intense laser pulse interacting with a thin
foil target. An analytical model which shows how the harmonics are created is
introduced. The proposed scheme might be a promising way towards the generation
of attosecond pulses.
PACS number(s): 52.40.Nk, 52.50.Jm, 52.65.RrComment: Second Revised Version, 13 pages (REVTeX), 3 figures in ps-format,
submitted for publication to Physical Review E, WWW:
http://www.physik.tu-darmstadt.de/tqe
Plasma-Induced Frequency Chirp of Intense Femtosecond Lasers and Its Role in Shaping High-Order Harmonic Spectral Lines
We investigate the self-phase modulation of intense femtosecond laser pulses
propagating in an ionizing gas and its effects on collective properties of
high-order harmonics generated in the medium. Plasmas produced in the medium
are shown to induce a positive frequency chirp on the leading edge of the
propagating laser pulse, which subsequently drives high harmonics to become
positively chirped. In certain parameter regimes, the plasma-induced positive
chirp can help to generate sharply peaked high harmonics, by compensating for
the dynamically-induced negative chirp that is caused by the steep intensity
profile of intense short laser pulses.Comment: 5 pages, 5 figure
XUV digital in-line holography using high-order harmonics
A step towards a successful implementation of timeresolved digital in-line
holography with extreme ultraviolet radiation is presented. Ultrashort XUV
pulses are produced as high-order harmonics of a femtosecond laser and a
Schwarzschild objective is used to focus harmonic radiation at 38 nm and to
produce a strongly divergent reference beam for holographic recording.
Experimental holograms of thin wires are recorded and the objects
reconstructed. Descriptions of the simulation and reconstruction theory and
algorithms are also given. Spatial resolution of few hundreds of nm is
potentially achievable, and micrometer resolution range is demonstrated.Comment: 8 pages, 8 figure
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Timeâfrequency representation of autoionization dynamics in helium
Autoionization, which results from the interference between direct photoionization and photoexcitation to a discrete state decaying to the continuum by configuration interaction, is a well known example of the important role of electron correlation in lightâmatter interaction. Information on this process can be obtained by studying the spectral, or equivalently, temporal complex amplitude of the ionized electron wave packet. Using an energy-resolved interferometric technique, we measure the spectral amplitude and phase of autoionized wave packets emitted via the sp2+ and sp3+ resonances in helium. These measurements allow us to reconstruct the corresponding temporal profiles by Fourier transform. In addition, applying various timeâfrequency representations, we observe the build-up of the wave packets in the continuum, monitor the instantaneous frequencies emitted at any time and disentangle the dynamics of the direct and resonant ionization channels
Neutrino - nucleon reaction rates in the supernova core in the relativistic random phase approximation
In view of the application to supernova simulations, we calculate neutrino
reaction rates with nucleons via the neutral and charged currents in the
supernova core in the relativistic random phase approximation (RPA) and study
their effects on the opacity of the supernova core. The formulation is based on
the Lagrangian employed in the calculation of nuclear equation of state (EOS)
in the relativistic mean field theory (RMF). The nonlinear meson terms are
treated appropriately so that the consistency of the density correlation
derived in RPA with the thermodynamic derivative obtained from EOS by RMF is
satisfied in the static and long wave length limit. We employ pion and rho
meson exchange interactions together with the phenomenological Landau-Migdal
parameters for the isospin-dependent nuclear interactions. We find that both
the charged and neutral current reaction rates are suppressed from the standard
Bruenn's approximate formula considerably in the high density regime. In the
low density regime, on the other hand, the vector current contribution to the
neutrino-nucleon scattering rate is enhanced in the vicinity of the boundary of
the liquid-gas phase transition, while the other contributions are moderately
suppressed there also. In the high temperature regime or in the regime where
electrons have a large chemical potential, the latter of which is important
only for the electron capture process and its inverse process, the recoil of
nucleons cannot be neglected and further reduces the reaction rates with
respect to the standard approximate formula which discards any energy transfer
in the processes. These issues could have a great impact on the neutrino
heating mechanism of collapse-driven supernovae.Comment: 16pages, 19figures, submitted to PR
High harmonic generation and periodic level crossings
Published versio
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