title: Novel collective autoionization process observed in electron spectra of He clusters.
abstract: The ionization dynamics of He nanodroplets irradiated with intense femtosecond extreme ultraviolet pulses of up to 1013  W/cm2 power density have been investigated by photoelectron spectroscopy. Helium droplets were resonantly excited to atomiclike 2p states with a photon energy of 21.4 eV, below the ionization potential (Ip), and directly into the ionization continuum with 42.8 eV photons. While electron emission following direct ionization above Ip is well explained within a model based on a sequence of direct electron emission events, the resonant excitation provides evidence of a new, collective ionization mechanism involving many excited atomiclike 2p states. With increasing power density the direct photoline due to an interatomic Coulombic decay disappears. It indicates that ionization occurs due to energy exchange between at least three excited atoms proceeding on a femtosecond time scale. In agreement with recent theoretical work the novel ionization process is very efficient and it is expected to be important for many other systems.
authors.affiliation: Institut für Optik und Atomare Physik, TU Berlin, 10623 Berlin, Germany.
authors.forename: Y
authors.initials: Y
authors.lastname: Ovcharenko
authors.affiliation: Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany.
authors.forename: V
authors.initials: V
authors.lastname: Lyamayev
authors.affiliation: Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Katzy
authors.affiliation: CIMAINA and Dipartimento di Fisica, Università degli Studi di Milano, 20133 Milano, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Devetta
authors.affiliation: Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany.
authors.forename: A
authors.initials: A
authors.lastname: LaForge
authors.affiliation: CNR Istituto di Metodologie Inorganiche e dei Plasmi, 00016 Monterotondo Scalo, Italy.
authors.forename: P
authors.initials: P
authors.lastname: O'Keeffe
authors.affiliation: Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy.
authors.forename: O
authors.initials: O
authors.lastname: Plekan
authors.affiliation: Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Finetti
authors.affiliation: Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy and Department of Physics, University of Trieste, 34128 Trieste, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Di Fraia
authors.affiliation: Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Mudrich
authors.affiliation: Institut für Optik und Atomare Physik, TU Berlin, 10623 Berlin, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Krikunova
authors.affiliation: CIMAINA and Dipartimento di Fisica, Università degli Studi di Milano, 20133 Milano, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Piseri
authors.affiliation: CNR Istituto di Metodologie Inorganiche e dei Plasmi, 00016 Monterotondo Scalo, Italy and Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Coreno
authors.affiliation: Laboratoire de Chimie Physique Moléculaire, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: N B
authors.initials: NB
authors.lastname: Brauer
authors.affiliation: European XFEL GmbH, 22607 Hamburg, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Mazza
authors.affiliation: Dipartimento di Chimica e Tecnologie del Farmaco, Università La Sapienza, 00185 Rome, Italy and IOM-CNR TASC Laboratory, Basovizza, 34149 Trieste, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Stranges
authors.affiliation: Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy and IOM-CNR TASC Laboratory, Basovizza, 34149 Trieste, Italy and Department of Chemical and Pharmaceutical Sciences, University of Trieste, 34128 Trieste, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Grazioli
authors.affiliation: Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Richter
authors.affiliation: Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy and IOM-CNR TASC Laboratory, Basovizza, 34149 Trieste, Italy.
authors.forename: K C
authors.initials: KC
authors.lastname: Prince
authors.affiliation: Laboratoire de Chimie Physique Moléculaire, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Drabbels
authors.affiliation: Elettra-Sincrotrone Trieste, Basovizza, 34149 Trieste, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Callegari
authors.affiliation: Physikalisches Institut, Universität Freiburg, 79104 Freiburg, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Stienkemeier
authors.affiliation: Institut für Optik und Atomare Physik, TU Berlin, 10623 Berlin, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Möller
mesh_terms: D004583:Electrons; D006371:Helium / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D008956:Models, Chemical; D053758:Nanoparticles / Q000737:chemistry*; D055668:Photochemical Processes; D056951:Photoelectron Spectroscopy / Q000379:methods; D014466:Ultraviolet Rays
chemical_list: D007477:Ions; D006371:Helium