**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**