title: Imaging the He2 quantum halo state using a free electron laser.
abstract: 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.
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany; zeller@atom.uni-frankfurt.de doerner@atom.uni-frankfurt.de.
authors.forename: Stefan
authors.initials: S
authors.lastname: Zeller
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Maksim
authors.initials: M
authors.lastname: Kunitski
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Voigtsberger
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Anton
authors.initials: A
authors.lastname: Kalinin
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Schottelius
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Carl
authors.initials: C
authors.lastname: Schober
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Waitz
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Hendrik
authors.initials: H
authors.lastname: Sann
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Hartung
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Bauer
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Pitzer
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Trinter
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Goihl
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Janke
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Richter
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Gregor
authors.initials: G
authors.lastname: Kastirke
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Miriam
authors.initials: M
authors.lastname: Weller
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Achim
authors.initials: A
authors.lastname: Czasch
authors.affiliation: Photonics Institute, Vienna University of Technology, 1040 Vienna, Austria.
authors.forename: Markus
authors.initials: M
authors.lastname: Kitzler
authors.affiliation: Deutsches Elektronen-Synchrotron, 22607 Hamburg, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Braune
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.|GSI Helmholtz Centre for Heavy Ion Research, 64291 Darmstadt, Germany.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Grisenti
authors.affiliation: Department of Molecular Physics, Fritz-Haber-Institut, 14195 Berlin, Germany.
authors.forename: Wieland
authors.identifier: 0000-0003-0564-203X
authors.initials: W
authors.lastname: Schöllkopf
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Lothar Ph H
authors.initials: LP
authors.lastname: Schmidt
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Markus S
authors.initials: MS
authors.lastname: Schöffler
authors.affiliation: Department of Physics, University of Nevada, Reno, NV 89557.
authors.forename: Joshua B
authors.initials: JB
authors.lastname: Williams
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany.
authors.forename: Till
authors.initials: T
authors.lastname: Jahnke
authors.affiliation: Institut für Kernphysik, Goethe-Universität Frankfurt, 60438 Frankfurt, Germany; zeller@atom.uni-frankfurt.de doerner@atom.uni-frankfurt.de.
authors.forename: Reinhard
authors.initials: R
authors.lastname: Dörner
keywords: clusters; helium dimer; tunneling; wavefunction