Controlling Below-Threshold Nonsequential Double Ionization via Quantum Interference
May 23rd, 2018 ANDREW MAXWELL University College London

Laser-induced nonsequential double ionization (NSDI) is the archetypal example of electron-electron correlation occurring in the context of matter in intense laser field. The underlying physical mechanism is laser-induced recollision, in which an electron returns to its parent ion and, by sharing part of its kinetic energy with the core, releases a second electron. Due to the success of classical-trajectory models in reproducing key features in NSDI electron-momentum distributions, this correlation has been viewed as classical for over two decades. This holds especially in the direct-ionization regime, for which the second electron gains enough energy to overcome the second ionization potential .

In the present work, we address the question of whether features related to quantum interference may be visible in below-threshold NSDI, for which the the first electron, upon return, does not have enough energy to directly ionize the second electron. We focus on the recollision-excitation with subsequent ionization (RESI) mechanism, in which the first electron excites the second electron upon recollision with its parent ion. Using semi-analytic methods based on the strong-field approximation, we identify two types of quantum interference, related to (i) events displaced in time and electron indistinguishability; (ii) the electron being excited to different intermediate states, and provide analytical conditions for the interference fringes encountered. These conditions agree well with our computations, and give hyperbolic fringes, and go well beyond previous investigations in which this interference been found.

We show that both interference types are of paramount importance, and may survive the integration over the momentum components parallel to the laser-field polarization and focal averaging. This has several consequences. First, this implies that both types of interference can be observed experimentally, so that classical RESI models must be viewed with care. Second, by manipulating the interference effects one may obtain a myriad of shapes for the electron momentum distributions, including correlated and anti-correlated. This means that correlated NSDI distributions in the below-threshold intensity regime, which have been related to direct ionization, may be in fact RESI. Third, since excitations to s, p or d states lead to very distinct shapes in the RESI distributions, different coherent superpositions of channels and events could be used to reconstruct or even control the intermediate state of the second electron. As a testing ground, we model experimental data for RESI on Argon from , where an increasing laser pulse results in FIG. 1: RESI distributions for argon (E1g = 0:58, E2g = 1:02 a.u.). The specific the excited ionization potential, different coherent superposition. The number in the top left represent the pulse length being modeled. cross-shaped distributions collapsing to a slightly backto- back correlated electron emission distribution. For very short pulses, the prevalent intermediate (excited) state of the second electron resembles an s state, which leads to cross-shaped RESI distributions. As the pulse length increases, this intermediate state consists of a coherent superposition of p and d states. Estimates for frequency and intensity regions for which s, p or d states are accessed, depending on the pulse frequency and intensity widths, are consistent with this picture. This strongly suggests that below-threshold NSDI may be used for quantum-state reconstruction.

Seminar, May 23, 2018, 15:30. ICFO’s Seminar Room

Hosted by Prof. Maciej Lewenstein