2,195 research outputs found
Analytical approximations for the higher energy structure in strong field ionization with inhomogeneous electric fields
Recently, the emergence of a higher energy structure (HES) due to a spatial
inhomogeneity in the laser electric field, as is typically found close to a
nano tip, was reported in Phys.~Rev. Letter {\bf 119}, 053204 (2017). For
practical applications, such as the characterization of near-fields or the
creation of localized sources of monoenergetic electron beams with tunable
energies, further insight into the nature of this higher energy structure is
needed. Here, we give a closed form analytical approximation to describe the
movement of the electron in the inhomogeneous electric field. In particular, we
derive a simple scaling law for the location of the HES peak and give a scheme
to analytically tune the width of the peak, both of which will prove useful in
optimizing the nanostructure size or geometry for creating the HES in
experimental settings
Intensity dependence of Rydberg states
We investigate numerically and analytically the intensity dependence of the
fraction of electrons that end up in a Rydberg state after strong-field
ionization with linearly polarized light. We find that including the intensity
dependent distribution of ionization times and non-adiabatic effects leads to a
better understanding of experimental results. Furthermore, we observe using
Classical Trajectory Monte Carlo simulations that the intensity dependence of
the Rydberg yield changes with wavelength and that the previously observed
power-law dependence breaks down at longer wavelengths. Our work suggests that
Rydberg yield measurements can be used as an independent test for
non-adiabaticity in strong field ionization
Helium-rich EHB Stars in Globular Clusters
Recent UV observations of the most massive Galactic globular clusters show a
significant population of hot stars below the zero-age HB (``blue hook''
stars), which cannot be explained by canonical stellar evolution. Stars which
suffer unusually large mass loss on the red giant branch and thus experience
the helium-core flash while descending the white dwarf cooling curve could
populate this region. They should show higher temperatures than the hottest
canonical HB stars and their atmospheres should be helium-rich and probably
C/N-rich. We have obtained spectra of blue hook stars in omega Cen and NGC 2808
to test this possibility. Our analysis shows that the blue hook stars in these
clusters reach effective temperatures well beyond the hot end of the canonical
EHB and have higher helium abundances than canonical EHB stars. These results
support the hypothesis that the blue hook stars arise from stars which ignite
helium on the white dwarf cooling curve.Comment: LaTeX, 8 pages, 3 figures, uses Kluwer style files (included), to
appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics
and Space Science, Kluwer Academic Publishers, proceedings of the meeting
held in Keele, UK, June 16-20, 200
The effect of electron-electron correlation on the attoclock experiment
We investigate multi-electron effects in strong-field ionization of Helium
using a semi-classical model that, unlike other commonly used theoretical
approaches, takes into account electron-electron correlation. Our approach has
an additional advantage of allowing to selectively switch off different
contributions from the parent ion (such as the remaining electron or the
nuclear charge) and thereby investigate in detail how the final electron angle
in the attoclock experiment is influenced by these contributions. We find that
the bound electron exerts a significant effect on the final electron momenta
distribution that can, however, be accounted for by an appropriately selected
mean field. Our results show excellent agreement with other widely used
theoretical models done within a single active electron approximation
Predictions of ultra-harmonic oscillations in coupled arrays of limit cycle oscillators
Coupled distinct arrays of nonlinear oscillators have been shown to have a
regime of high frequency, or ultra-harmonic, oscillations that are at multiples
of the natural frequency of individual oscillators. The coupled array
architectures generate an in-phase high-frequency state by coupling with an
array in an anti-phase state. The underlying mechanism for the creation and
stability of the ultra-harmonic oscillations is analyzed. A class of
inter-array coupling is shown to create a stable, in-phase oscillation having
frequency that increases linearly with the number of oscillators, but with an
amplitude that stays fairly constant. The analysis of the theory is illustrated
by numerical simulation of coupled arrays of Stuart-Landau limit cycle
oscillators.Comment: 24 pages, 9 figures, accepted to Phys. Rev. E, in pres
Instantaneous ionization rate as a functional derivative
We describe an approach defining instantaneous ionization rate (IIR) as a
functional derivative of the total ionization probability. The definition is
based on physical quantities which are directly measurable, such as the total
ionization probability and the waveform of the pulse. The definition is,
therefore, unambiguous and does not suffer from gauge non-invariance. We
compute IIR by solving numerically the time-dependent Schrodinger equation for
the hydrogen atom in a strong laser field. We find that the IIR lags behind the
electric field, but this lag is entirely due to the long tail effect of the
Coulomb field. In agreement with the previous results using attoclock
methodology, therefore, the IIR we define does not show measurable delay in
strong field tunnel ionization
Complete chaotic synchronization in mutually coupled time-delay systems
Complete chaotic synchronization of end lasers has been observed in a line of
mutually coupled, time-delayed system of three lasers, with no direct
communication between the end lasers. The present paper uses ideas from
generalized synchronization to explain the complete synchronization in the
presence of long coupling delays, applied to a model of mutually coupled
semiconductor lasers in a line. These ideas significantly simplify the analysis
by casting the stability in terms of the local dynamics of each laser. The
variational equations near the synchronization manifold are analyzed, and used
to derive the synchronization condition that is a function of the parameters.
The results explain and predict the dependence of synchronization on various
parameters, such as time-delays, strength of coupling and dissipation. The
ideas can be applied to understand complete synchronization in other chaotic
systems with coupling delays and no direct communication between synchronized
sub-systems.Comment: 22 pages, 6 figure
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