Strong-field molecular ionization beyond the single active electron approximation

Abstract

The present work explores quantitative limits to the Single-Active Electron (SAE) approximation, often used to deal with strong-field ionization and subsequent attosecond dynamics. Using a time-dependent multiconfiguration approach, specifically a Time-Dependent Configuration Interaction (TDCI) method, we solve the time-dependent Schrödinger equation (TDSE) for the two-electron dihydrogen molecule, with the possibility of tuning at will the electron-electron interaction by an adiabatic switch-on/switch-off function. We focus on signals of the single ionization of H2 under a strong near-infrared (NIR) four-cycle, linearly-polarized laser pulse of varying intensity, and within a vibrationally frozen molecule model. The observables we address are post-pulse total ionization probability profiles as a function of the laser peak intensity. Three values of the internuclear distance R taken as a parameter are considered, R = Req = 1.4 a.u, the equilibrium geometry of the molecule, R = 5.0 a.u for an elongated molecule and R = 10.2 a.u for a dissociating molecule. The most striking observation is the non-monotonous behavior of the ionization probability profiles at intermediate elongation distances with an instance of enhanced ionization and one of partial ionization quenching. We give an interpretation of this in terms of a Resonance-Enhanced-Multiphoton Ionization (REMPI) mechanism with interfering overlapping resonances resulting from excited electronic states.

Electro-Nuclear Dynamics of Single and Double Ionization of H2 in Ultrafast Intense Laser Pulses

“…In theoretical studies dealing with the ionization of H 2 in an intense laser field, it is common to invoke the small mass of the electrons relative to the mass of the nuclei to get rid of the motion of the nuclei by modeling the electronic dynamics at a fixed internuclear distance. 33 This approximation is chosen mainly for numerical reasons since a complete quantum calculation including all degrees of freedom of the system, both electronic and nuclear, is computationally extremely expensive. We nevertheless expect that for ultrashort single-cycle pulses, the impact of vibrational dynamics remains limited, even for a molecule as light as H 2 .…”

“…As a result, many theoretical studies have been performed using simplified models that can only be used in certain limited ranges of laser parameters. Examples include ionization dynamics studies performed with the position of the nuclei fixed, , photodissociation dynamics studies that do not consider ionization processes, ,, or studies performed in reduced dimensionality. − These types of approaches, which have proven to be very interesting in order to better analyze, understand, and sometimes control the processes that take place during or after laser excitation, often impose severe constraints on the conditions under which these models can be applied, for example, on the intensity range in which they can realistically be used. While it is true that much more sophisticated models have been developed recently, such as the time-dependent Feshbach close-coupling approach, there are applications where it would be extremely useful to have approximate models that could estimate, with reduced computational times, the single and double ionization probabilities of H 2 and at the same time predict the energy of the protons emitted during the fragmentation of the molecule.…”