arXiv subjects

N. Suárez

Publications and source records attributed to N. Suárez.

4 recordsLinked to original sources

arXiv2018

Determination of the spectral variation origin in high-order harmonic generation in noble gases

One key parameter in the high-order harmonic generation (HHG) phenomenon is the exact frequency of the generated harmonic field. Its deviation from perfect harmonics of the laser frequency can be explained considering (i) the single-atom laser-matter interaction and (ii) the spectral changes of the driving laser. In this work, we perform an experimental and theoretical study of the causes that generate spectral changes in the HHG radiation. We measured the driving laser spectral shift after high harmonic generation in long medium using a correction factor to take into account the multiple possible HHG initiation distances along the laser path. We separate out the contribution of laser spectral shift from the resultant high harmonic spectral shift in order to elucidate the microscopic effect of spectral shift in HHG. Therefore, in some cases we are able to identify the dominant electron trajectory from the experimental data. Our investigations lead to valuable conclusions about atomic dipole phase contribution to a high harmonic spectral shift. We demonstrate that the significant contribution of a long electron path leads to a high harmonic shift, which differs from that expected from the driving laser. Moreover, we assess the origin of the high-order harmonics spectral broadening and provide an explanation for the narrowest high harmonic spectral width in our experiment.

physics.atom-ph↗

arXiv2017

Above-threshold ionization (ATI) in multicenter molecules: the role of the initial state

A possible route to extract electronic and nuclear dynamics from molecular targets with attosecond temporal and nanometer spatial resolution is to employ recolliding electrons as `probes'. The recollision process in molecules is, however, very challenging to treat using {\it ab initio} approaches. Even for the simplest diatomic systems, such as H$_2$, today's computational capabilities are not enough to give a complete description of the electron and nuclear dynamics initiated by a strong laser field. As a consequence, approximate qualitative descriptions are called to play an important role. In this contribution we extend the work presented in N. Suárez {\it et al.}, Phys.~Rev. A {\bf 95}, 033415 (2017), to three-center molecular targets. Additionally, we incorporate a more accurate description of the molecular ground state, employing information extracted from quantum chemistry software packages. This step forward allows us to include, in a detailed way, both the molecular symmetries and nodes present in the high-occupied molecular orbital. We are able to, on the one hand, keep our formulation as analytical as in the case of diatomics, and, on the other hand, to still give a complete description of the underlying physics behind the above-threshold ionization process. The application of our approach to complex multicenter - with more than 3 centers, targets appears to be straightforward.

physics.atom-ph↗

arXiv2016

Double-electron ionization driven by inhomogeneous fields

Electron-electron correlation effects play an instrumental role in our understanding of sequential (SDI) and non-sequential double ionization (NSDI) mechanisms. Here, we present a theoretical study of NSDI driven by plasmonic-enhanced spatial inhomogeneous fields. By numerically solving the time-dependent Schrödinger equation for a linear reduced model of He and a double-electron time-evolution probability analysis, we provide evidence for the enhancement effects in NSDI showing that the double ionization yield at lower laser peak intensities is increased due to the inhomogeneity of the laser field. Furthermore, our quantum mechanical model, as well as classical trajectory Monte Carlo simulations, show that inhomogeneous fields are a useful tool for splitting the binary and recoil processes in the rescattering scenario.

physics.atom-ph↗

arXiv2016

Above-Threshold Ionization and Laser-Induced Electron Diffraction in Diatomic Molecules

Strong field photoemission and electron recollision provide a viable route to extract electronic and nuclear dynamics from molecular targets with attosecond temporal resolution. However, since an {\em ab-initio} treatment of even the simplest diatomic systems is beyond today's capabilities approximate qualitative descriptions are warranted. In this paper, we develop such a theoretical approach to model the photoelectrons resulting from intense laser-molecule interaction. We present a general theory for symmetric diatomic molecules in the single active electron approximation that, amongst other capabilities, allows adjusting both the internuclear separation and molecular potential in a direct and simple way. More importantly we derive an analytic approximate solution of the time dependent Schrödinger equation (TDSE), based on a generalized strong field approximation (SFA) version. Using that approach we obtain expressions for electrons emitted transition amplitudes from two different molecular centres, and accelerated then in the strong laser field. One innovative aspect of our theory is the fact that the dipole matrix elements are free from non-physical gauge and coordinate system dependent terms -- this is achieved by adapting the coordinate system, in which SFA is performed, to the centre from which the corresponding part of the time dependent wave function originates. Our analytic results agree very well with the numerical solution of the full three-dimensional TDSE for the H$_2^+$ molecule. Moreover, the theoretical model was applied to describe laser-induced electron diffraction (LIED) measurements of O$_2^+$ molecules, obtained at ICFO, and reproduces the main features of the experiment very well. Our approach can be extended in a natural way to more complex molecules and multi-electron systems.

physics.atom-ph↗