46 research outputs found
Reaction Dynamics in Double Ionization of Helium by Electron Impact
We present theoretical fully differential cross sections (FDCS) for double ionization of helium by 500 eV and 2 keV electron impact. Contributions from various reaction mechanisms to the FDCS were calculated separately and compared to experimental data. Our theoretical methods are based on the first Born approximation. Higher-order effects are incorporated using the Monte Carlo event generator technique. Earlier, we successfully applied this approach to double ionization by ion impact, and in the work reported here it is extended to electron impact. We demonstrate that at 500 eV impact energy, double ionization is dominated by higher-order mechanisms. Even at 2 keV, double ionization does not predominantly proceed through a pure first-order process
Double Ionization of Helium by Highly-Charged-Ion Impact Analyzed within the Frozen-Correlation Approximation
We apply the frozen-correlation approximation (FCA) to analyze double ionization of helium by energetic highly charged ions. In this model the double ionization amplitude is represented in terms of single ionization amplitudes, which we evaluate within the continuum distorted wave-eikonal initial state (CDW-EIS) approach. Correlation effects are incorporated in the initial and final states, but are neglected during the time the collision process takes place. We implement the FCA using the Monte Carlo event generator technique, which allows us to generate theoretical event files and to compare theory and experiment using the same analysis tools. The comparison with previous theoretical results and with experimental data demonstrates, on the one hand, the validity of our earlier simple models to account for higher-order mechanisms, and, on the other hand, the robustness of the FCA
Torsion in quantum field theory through time-loops on Dirac materials
Assuming dislocations could be meaningfully described by torsion, we propose
here a scenario based on the role of time in the low-energy regime of
two-dimensional Dirac materials, for which coupling of the fully antisymmetric
component of the torsion with the emergent spinor is not necessarily zero.
Appropriate inclusion of time is our proposal to overcome well-known
geometrical obstructions to such a program, that stopped further research of
this kind. In particular, our approach is based on the realization of an exotic
, that could be seen as oscillating particle-hole pairs. Although
this is a theoretical paper, we moved the first steps toward testing the
realization of these scenarios, by envisaging on the
interplay between an external electromagnetic field (to excite the pair
particle-hole and realize the time-loops), and a suitable distribution of
dislocations described as torsion (responsible for the measurable holonomy in
the time-loop, hence a current). Our general analysis here establishes that we
need to move to a nonlinear response regime. We then conclude by pointing to
recent results from the interaction laser-graphene that could be used to look
for manifestations of the torsion-induced holonomy of the time-loop, e.g., as
specific patterns of suppression/generation of higher harmonics.Comment: 24 pages, 5 figure
Controlling polarization of attosecond pulses with plasmonic-enhanced bichromatic counter-rotating circularly polarized fields
The use of bichromatic counter-rotating laser field is known to generate
high-order harmonics with non-zero ellipticity. By combining such laser field
with a plasmonic-enhanced spatially inhomogeneous field, we propose a way to
influence the sub-cycle dynamics of the high-harmonic generation process. Using
the numerical solution of the time-dependent Schr{\"o}dinger equation combined
with classical trajectory Monte Carlo simulations, we show that the change of
the direction and the strength of the plasmonic field selectively enhances or
suppresses certain recombining electron trajectories. This in turn modifies the
ellipticity of the emitted attosecond pulses
Wannier-Bloch approach to localization in high harmonics generation in solids
Emission of high-order harmonics from solids provides a new avenue in
attosecond science. On one hand, it allows to investigate fundamental processes
of the non-linear response of electrons driven by a strong laser pulse in a
periodic crystal lattice. On the other hand, it opens new paths toward
efficient attosecond pulse generation, novel imaging of electronic wave
functions, and enhancement of high-order harmonic generation (HHG) intensity. A
key feature of HHG in a solid (as compared to the well-understood phenomena of
HHG in an atomic gas) is the delocalization of the process, whereby an electron
ionized from one site in the periodic lattice may recombine with any other.
Here, we develop an analytic model, based on the localized Wannier wave
functions in the valence band and delocalized Bloch functions in the conduction
band. This Wannier-Bloch approach assesses the contributions of individual
lattice sites to the HHG process, and hence addresses precisely the question of
localization of harmonic emission in solids. We apply this model to investigate
HHG in a ZnO crystal for two different orientations, corresponding to wider and
narrower valence and conduction bands, respectively. Interestingly, for
narrower bands, the HHG process shows significant localization, similar to
harmonic generation in atoms. For all cases, the delocalized contributions to
HHG emission are highest near the band-gap energy. Our results pave the way to
controlling localized contributions to HHG in a solid crystal, with hard to
overestimate implications for the emerging area of atto-nanoscience