X. H. Liu

Electron–electron correlation in strong laser fields

In the context of strong field–matter interaction, an increasing number of phenomena has been found to be not understood within the single-active-electron approximation. For such phenomena, electron–electron correlation plays an important role in the underlying dynamics. In this review, we will provide a broad overview of electron–electron correlation and examine two distinct cases of its manifestation in strong field physics. In the first example, we examine nonsequential double and multiple ionization of atoms, discussing the experimental manifestation of the electron correlation and the theoretical models that have been developed to describe the effect. The second case examines the interaction of larger systems with intense laser fields, for which multielectron effects have to be invoked for an accurate description of the dynamics.

Comment on “Observation of the Wigner-Huntington transition to metallic hydrogen”

Dias and Silvera (Research Article, 17 February 2017, p. 715) claim the observation of the Wigner-Huntington transition to metallic hydrogen at 495 gigapascals. We show that neither the claims of the record pressure nor the phase transition to a metallic state are supported by data and that the data contradict the authors’ own unconfirmed previous results.

Nonsequential double ionization with polarization-gated pulses

We investigate laser-induced nonsequential double ionization by a polarization-gated laser pulse, constructed by employing two counter-rotating circularly polarized few-cycle pulses with a time delay T(d). We address the problem within a classical framework and mimic the behaviour of the quantum-mechanical electronic wave packet by means of an ensemble of classical electron trajectories. These trajectories are initially weighted with the quasi-static tunnelling rate and with suitably chosen distributions for the momentum components parallel and perpendicular to the laser-field polarization in the temporal region for which it is nearly linearly polarized. We show that, if the time delay Td is of the order of the pulse length, the electron-momentum distributions, as functions of the parallel momentum components, are highly asymmetric and dependent on the carrier-envelope (CE) phase. As this delay is decreased, this asymmetry gradually vanishes. We explain this behaviour in terms of the available phase space, the quasi-static tunnelling rate and the recollision rate for the first electron for different sets of trajectories. Our results show that the polarization-gating technique may provide an efficient way to study the NSDI dynamics in the single-cycle limit without employing few-cycle pulses.

High-Pressure Behavior of Hydrogen and Deuterium at Low Temperatures

In situ high-pressure low-temperature high-quality Raman data for hydrogen and deuterium demonstrate the presence of a novel phase, phase II^{}, unique to deuterium and distinct from the known phase II. Phase II^{} of D_{2} is not observed in hydrogen, making it the only phase that does not exist in both isotopes and occupies a significant part of P-T space from ∼25 to 110xa0GPa and below 125xa0K. For H_{2}, the data show that below 30xa0K the transition to phase II happens at as low as 73xa0GPa. The transformation from phase II to III commences at around ∼155u2009u2009GPa and is completed by 170xa0GPa with the average pressure of ∼160u2009u2009GPa being slightly higher than previously thought. The updated phase diagrams of H_{2} and D_{2} demonstrate the difference between the isotopes at low temperatures and moderate pressures, providing new information on the phase diagrams of both elements.

Speed-up and slow-down collisions in laser-induced non-sequential multiple ionization

We investigate laser-induced non-sequential multiple ionization using a simple statistical model in which an electron recollides inelastically with its parent ion. In this collision, it thermalizes with the remaining Nu2009−u20091 bound electrons within a time interval Δt. Subsequently, all the N electrons leave. We address the question of how the above time delays influence the individual contributions from the orbits in which the first electron, upon return, is accelerated or decelerated by the field, respectively, to the ion momentum distributions. In both cases, the time delays modify the drift momenta obtained by the N electrons when they reach the continuum at t+Δt, by moving such times towards or away from a crossing of the electric field. The contributions from both types of collisions are influenced in distinct ways, and the interplay between such trends determines the widths and the peak momenta of the distributions. Specifically in the few-cycle pulse case, we also show that such time delays do not affect the shapes of the momentum distributions in a radical fashion. Hence, even with a thermalization time Δt, non-sequential multiple ionization could in principle be used for absolute-phase diagnosis.