1,865 research outputs found
Influence of asymmetry and nodal planes on high-harmonic generation in heteronuclear molecules
The relation between high-harmonic spectra and the geometry of the molecular
orbitals in position and momentum space is investigated. In particular we
choose two isoelectronic pairs of homonuclear and heteronuclear molecules, such
that the highest occupied molecular orbital of the former exhibit at least one
nodal plane. The imprint of such planes is a strong suppression in the harmonic
spectra, for particular alignment angles. We are able to identify two distinct
types of nodal planes. If the nodal planes are determined by the atomic
wavefunctions only, the angle for which the yield is suppressed will remain the
same for both types of molecules. In contrast, if they are determined by the
linear combination of atomic orbitals at different centers in the molecule,
there will be a shift in the angle at which the suppression occurs for the
heteronuclear molecules, with regard to their homonuclear counterpart. This
shows that, in principle, molecular imaging, which uses the homonuclear
molecule as a reference and enables one to observe the wavefunction distortions
in its heteronuclear counterpart, is possible.Comment: 14 pages, 7 figures. Figs. 3, 5 and 6 have been simplified in order
to comply with the arXiv size requirement
Excitation, two-center interference and the orbital geometry in laser-induced nonsequential double ionization of diatomic molecules
We address the influence of the molecular orbital geometry and of the
molecular alignment with respect to the laser-field polarization on
laser-induced nonsequential double ionization of diatomic molecules for
different molecular species, namely and . We
focus on the recollision excitation with subsequent tunneling ionization (RESI)
mechanism, in which the first electron, upon return, promotes the second
electron to an excited state, from where it subsequently tunnels. We show that
the electron-momentum distributions exhibit interference maxima and minima due
to the electron emission at spatially separated centers. We provide generalized
analytical expressions for such maxima or minima, which take into account
mixing and the orbital geometry. The patterns caused by the two-center
interference are sharpest for vanishing alignment angle and get washed out as
this parameter increases. Apart from that, there exist features due to the
geometry of the lowest occupied molecular orbital (LUMO), which may be observed
for a wide range of alignment angles. Such features manifest themselves as the
suppression of probability density in specific momentum regions due to the
shape of the LUMO wavefunction, or as an overall decrease in the RESI yield due
to the presence of nodal planes.Comment: 11 pages revtex, 2 figure
Local dynamics in high-order harmonic generation using Bohmian trajectories
We investigate high-order harmonic generation from a Bohmian-mechanical
perspective, and find that the innermost part of the core, represented by a
single Bohmian trajectory, leads to the main contributions to the high-harmonic
spectra. Using time-frequency analysis, we associate this central Bohmian
trajectory to an ensemble of unbound classical trajectories leaving and
returning to the core, in agreement with the three step model. In the Bohmian
scenario, this physical picture builds up non-locally near the core via the
quantum mechanical phase of the wavefunction. This implies that the flow of the
wavefunction far from the core alters the central Bohmian trajectory. We also
show how this phase degrades in time for the peripheral Bohmian trajectories as
they leave the core region.Comment: 7 pages, 3 figures; the manuscript has been considerably extended and
modified with regard to the previous version
High-harmonic generation from a confined atom
The order of high harmonics emitted by an atom in an intense laser field is
limited by the so-called cutoff frequency. Solving the time-dependent
Schr\"odinger equation, we show that this frequency can be increased
considerably by a parabolic confining potential, if the confinement parameters
are suitably chosen.
Furthermore, due to confinement, the radiation intensity remains high
throughout the extended emission range. All features observed can be explained
with classical arguments.Comment: 4 pages(tex files), 4 figures(eps files); added references and
comment
Nonsequential Double Ionization with Polarization-gated Pulses
We investigate laser-induced nonsequential double ionization by a
polarization-gated laser pulse, constructed employing two counter-rotating
circularly polarized few cycle pulses with a time delay . We address the
problem within a classical framework, and mimic the behavior of the
quantum-mechanical electronic wave packet by means of an ensemble of classical
electron trajectories. These trajectories are initially weighted with the
quasi-static tunneling rate, and with suitably chosen distributions for the
momentum components parallel and perpendicular to the laser-field polarization,
in the temporal region for which it is nearly linearly polarized. We show that,
if the time delay is of the order of the pulse length, the
electron-momentum distributions, as functions of the parallel momentum
components, are highly asymmetric and dependent on the carrier-envelope (CE)
phase. As this delay is decreased, this asymmetry gradually vanishes. We
explain this behavior in terms of the available phase space, the quasi-static
tunneling rate and the recollision rate for the first electron, for different
sets of trajectories. Our results show that polarization-gating technique may
provide an efficient way to study the NSDI dynamics in the single-cycle limit,
without employing few-cycle pulses.Comment: 17 pages, 6 figure
A framework for the selection of the right nuclear power plant
Civil nuclear reactors are used for the production of electrical energy. In the nuclear industry vendors propose several nuclear reactor designs with a size from 35–45 MWe up to 1600–1700 MWe. The choice of the right design is a multidimensional problem since a utility has to include not only financial factors as levelised cost of electricity (LCOE) and internal rate of return (IRR), but also the so called “external factors” like the required spinning reserve, the impact on local industry and the social acceptability. Therefore it is necessary to balance advantages and disadvantages of each design during the entire life cycle of the plant, usually 40–60 years. In the scientific literature there are several techniques for solving this multidimensional problem. Unfortunately it does not seem possible to apply these methodologies as they are, since the problem is too complex and it is difficult to provide consistent and trustworthy expert judgments. This paper fills the gap, proposing a two-step framework to choosing the best nuclear reactor at the pre-feasibility study phase. The paper shows in detail how to use the methodology, comparing the choice of a small-medium reactor (SMR) with a large reactor (LR), characterised, according to the International Atomic Energy Agency (2006), by an electrical output respectively lower and higher than 700 MWe
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