933 research outputs found
Dynamics of Tunneling Ionization using Bohmian Mechanics
Recent attoclock experiments and theoretical studies regarding the
strong-field ionization of atoms by few-cycle infrared pulses revealed new
features that have attracted much attention. Here we investigate tunneling
ionization and the dynamics of the electron probability using Bohmian
Mechanics. We consider a one-dimensional problem to illustrate the underlying
mechanisms of the ionization process. It is revealed that in the major part of
the below-the-barrier ionization regime, in an intense and short infrared
pulse, the electron does not tunnel \through" the entire barrier, but rather
already starts from the classically forbidden region. Moreover, we highlight
the correspondence between the probability of locating the electron at a
particular initial position and its asymptotic momentum. Bohmian Mechanics also
provides a natural definition of mean tunneling time and exit position, taking
account of the time dependence of the barrier. Finally, we find that the
electron can exit the barrier with significant kinetic energy, thereby
corroborating the results of a recent study [Camus et al., Phys. Rev. Lett. 119
(2017) 023201]
Two-photon Double Ionization of H in Intense Femtosecond Laser Pulses
Triple-differential cross sections for two-photon double ionization of
molecular hydrogen are presented for a central photon energy of 30 eV. The
calculations are based on a fully {\it ab initio}, nonperturbative, approach to
the time-dependent Schroedinger equation in prolate spheroidal coordinates,
discretized by a finite-element discrete-variable-representation. The wave
function is propagated in time for a few femtoseconds using the short,
iterative Lanczos method to study the correlated response of the two
photoelectrons to short, intense laser radiation. The current results often lie
in between those of Colgan {\it et al} [J. Phys. B {\bf 41} (2008) 121002] and
Morales {\it et al} [J. Phys. B {\bf 41} (2009) 134013]. However, we argue that
these individual predictions should not be compared directly to each other, but
preferably to experimental data generated under well-defined conditions.Comment: 4 pages, 4 figure
Numerical Green's functions in optical potential calculations for positron scattering from argon and neon
An optical potential method was applied to the calculation of positron scattering from the noble gases in order to determine the effect of open excitation channels on the shape of differential scattering cross sections
U12 type introns were lost at multiple occasions during evolution
<p>Abstract</p> <p>Background</p> <p>Two categories of introns are known, a common U2 type and a rare U12 type. These two types of introns are removed by distinct spliceosomes. The phylogenetic distribution of spliceosomal RNAs that are characteristic of the U12 spliceosome, i.e. the U11, U12, U4atac and U6atac RNAs, suggest that U12 spliceosomes were lost in many phylogenetic groups. We have now examined the distribution of U2 and U12 introns in many of these groups.</p> <p>Results</p> <p>U2 and U12 introns were predicted by making use of available EST and genomic sequences. The results show that in species or branches where U12 spliceosomal components are missing, also U12 type of introns are lacking. Examples are the choanoflagellate <it>Monosiga brevicollis</it>, <it>Entamoeba histolytica</it>, green algae, diatoms, and the fungal lineage Basidiomycota. Furthermore, whereas U12 splicing does not occur in <it>Caenorhabditis elegans</it>, U12 introns as well as U12 snRNAs are present in <it>Trichinella spiralis</it>, which is deeply branching in the nematode tree. A comparison of homologous genes in <it>T. spiralis </it>and <it>C. elegans </it>revealed different mechanisms whereby U12 introns were lost.</p> <p>Conclusions</p> <p>The phylogenetic distribution of U12 introns and spliceosomal RNAs give further support to an early origin of U12 dependent splicing. In addition, this distribution identifies a large number of instances during eukaryotic evolution where such splicing was lost.</p
Quantum coherent control of the photo\-electron angular distribution in bichromatic ionization of atomic neon
We investigate the coherent control of the photo\-electron angular
distribution in bichromatic atomic ionization. Neon is selected as target since
it is one of the most popular systems in current gas-phase experiments with
free-electron lasers (FELSs). In particular, we tackle practical questions,
such as the role of the fine-structure splitting, the pulse length, and the
intensity. Time-dependent and stationary perturbation theory are employed, and
we also solve the time-dependent Schr\"odinger equation in a single-active
electron model. We consider neon ionized by a FEL pulse whose fundamental
frequency is in resonance with either or excitation. The
contribution of the non\-resonant two-photon process and its potential
constructive or destructive role for quantum coherent control is investigated.Comment: 10 pages, 6 figure
Photoelectron angular distribution in two-pathway ionization of neon with femtosecond XUV pulses
We analyze the photoelectron angular distribution in two-pathway interference
between non\-resonant one-photon and resonant two-photon ionization of neon. We
consider a bichromatic femtosecond XUV pulse whose fundamental frequency is
tuned near the atomic states of neon. The time-dependent
Schr\"odinger equation is solved and the results are employed to compute the
angular distribution and the associated anisotropy parameters at the main
photoelectron line. We also employ a time-dependent perturbative approach,
which allows obtaining information on the process for a large range of pulse
parameters, including the steady-state case of continuous radiation, i.e., an
infinitely long pulse. The results from the two methods are in relatively good
agreement over the domain of applicability of perturbation theory
Connection between Superelastic and Inelastic Electron-Atom Collisions Involving Polarized Collision Partners
It is shown how the results of a recent experiment by Jiang, Zuo, Vuković, and Bederson [Phys. Rev. Lett. 68, 915 (1992)], who investigated low-energy electron scattering from laser-excited polarized sodium atoms in the initial (3p) 2P°3/2 (F=3, MF=3) state, can be related to the inelastic 3S→3P transition involving initially unpolarized electron and atom beams. Hence, this method can provide an independent check of the traditional electron-scattering experiment with unpolarized beams
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