Above-threshold ionization and photoelectron spectra in atomic systems driven by strong laser fields

Above-threshold ionization (ATI) results from strong field laser-matter interaction and it is one of the fundamental processes that may be used to extract electron structural and dynamical information about the atomic or molecular target. Moreover, it can also be used to characterize the laser field itself. Here, we develop an analytical description of ATI, which extends the theoretical Strong Field Approximation (SFA), for both the direct and re-scattering transition amplitudes in atoms. From a non-local, but separable potential, the bound-free dipole and the re-scattering transition matrix elements are analytically computed. In comparison with the standard approaches to the ATI process, our analytical derivation of the re-scattering matrix elements allows us to study directly how the re-scattering process depends on the atomic target and laser pulse features -we can turn on and off contributions having different physical origins or corresponding to different physical mechanisms. We compare SFA results with the full numerical solutions of the time-dependent Schroedinger equation (TDSE) within the few-cycle pulse regime. Good agreement between our SFA and TDSE model is found for the ATI spectrum. Our model captures also the strong dependence of the photoelectron spectra on the carrier envelope phase of the laser field.Comment: 29 pages, 6 figures, submitted to PR

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

Atomic concealment due to loss of coherence of the incident beam of projectiles in collision processes

In the study of collision processes, a series of conditions is usually assumed. One of them is that the beam of projectiles is coherent in lengths greater than those of the targets against which it strikes. However, recent experimental results and theoretical analyzes have shown that this assumption can not only fail, but that it is possible to manipulate the coherence length experimentally to go from a coherent situation to an incoherent one. The most conspicuous and studied manifestation of such loss of coherence is the disappearance of interference effects. However, in the present work we show that a strong decrease can also occur in the magnitude of the cross section, not only differential but also total, due to an atomic concealment effect.Fil: Barrachina, Raúl O.. Universidad Nacional de Cuyo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Navarrete, Francisco. Universität Rostock; AlemaniaFil: Ciappina, Marcelo Fabián. Technion - Israel Institute of Technology; Israel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

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 $time-loop$, 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 $Gedankenexperiments$ 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