Strong field multiple ionization as a route to electron dynamics in a van der Waals cluster

Alignment Thresholds of Molecules

Molecules have long been known to align in moderately intense, far off-resonance laser fields with a large variety of applications in physics and optics. We illustrate and describe the physical origin of a previously unexplored phenomenon in the adiabatic alignment dynamics of molecules, which is fundamentally interesting and also has an important practical implication. Specifically, the intensity dependence of the degree of adiabatic alignment exhibits a threshold behavior, below which molecules are isotropically distributed rotationally and above which the alignment rapidly reaches a plateau. Furthermore, we show that both the intensity and the temperature dependencies of the alignment of all linear molecules exhibit universal curves and derive analytical forms to describe these dependencies. Finally, we illustrate that the alignment threshold occurs very generally at a lower intensity than the off-resonance ionization threshold, a numerical observation that is readily illustrated analytically. The threshold behavior is attributed to a tunneling mechanism that rapidly switches off at the threshold intensity, where tunneling between the potential wells corresponding to the two orientations of the aligned molecules becomes impossible. The universal threshold behavior of molecular alignment is a simple phenomenon, but one that was not realized before and can be readily tested experimentally.

Heavy N+ ion transfer in doubly charged N2Ar van der Waals cluster

Van der Waals clusters are weakly bound atomic/molecular systems and are an important medium for understanding micro-environmental chemical phenomena in bio-systems. The presence of neighboring atoms may open channels otherwise forbidden in isolated atoms/molecules. In hydrogen-bond clusters, proton transfer plays a crucial role, which involves mass and charge migration over large distances within the cluster and results in its fragmentation. Here we report an exotic transfer channel involving a heavy N⁺ ion observed in a doubly charged cluster produced by 1 MeV Ne⁸⁺ ions: (N₂Ar)²⁺→N⁺+NAr⁺. The neighboring Ar atom decreases the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{N}}_2^{2 + }$$\end{document}N22+ barrier height and width, resulting in significant shorter lifetimes of the metastable molecular ion state \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{N}}_2^{2 + }$$\end{document}N22+(\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\mathrm{X}}^{1}}{\Sigma _{{\mathrm{g}}}^{+}}$$\end{document}X1Σg+). Consequently, the breakup of the covalent N⁺−N⁺ bond, the tunneling out of the N⁺ ion from the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{N}}_2^{2 + }$$\end{document}N22+ potential well, as well as the formation of an N−Ar⁺ bound system take place almost simultaneously, resulting in a Coulomb explosion of N⁺ and NAr⁺ ion pairs.

There are multiple ways by which energy and charge transfer occur in weakly bound systems. Here the authors reveal a heavy ion N⁺ transfer in a doubly charged Van der Waals cluster produced in collisions of the highly charged Ne⁸⁺ ion with N₂Ar, leading to fragmentation of N⁺ and NAr⁺ via Coulomb explosion.

Imaging the He2 quantum halo state using a free electron laser

Quantum tunneling is a ubiquitous phenomenon in nature and crucial for many technological applications. It allows quantum particles to reach regions in space which are energetically not accessible according to classical mechanics. In this "tunneling region," the particle density is known to decay exponentially. This behavior is universal across all energy scales from nuclear physics to chemistry and solid state systems. Although typically only a small fraction of a particle wavefunction extends into the tunneling region, we present here an extreme quantum system: a gigantic molecule consisting of two helium atoms, with an 80% probability that its two nuclei will be found in this classical forbidden region. This circumstance allows us to directly image the exponentially decaying density of a tunneling particle, which we achieved for over two orders of magnitude. Imaging a tunneling particle shows one of the few features of our world that is truly universal: the probability to find one of the constituents of bound matter far away is never zero but decreases exponentially. The results were obtained by Coulomb explosion imaging using a free electron laser and furthermore yielded He2's binding energy of [Formula: see text] neV, which is in agreement with most recent calculations.

Strong-field dissociative double ionization of acetylene

We investigate dissociative double ionization of acetylene, one of the smallest organic molecules yet with a rich electronic structure, in strong laser fields by measuring two fragment ions and two electrons in coincidence. The two-body fragmentation channels are dominated by the removal of electrons from the lower-lying molecular orbitals rather than from the highest occupied one. The electron localization-assisted enhanced ionization mechanism plays a central role for the strong-field deprotonation ionization of acetylene by releasing the second electron from the up-field potential well of the hydrogen site at the internuclear distance near twice the equilibrium value of the C-H bond.