K. Cole

Ultrafast Probing of Core Hole Localization in N2

Although valence electrons are clearly delocalized in molecular bonding frameworks, chemists and physicists have long debated the question of whether the core vacancy created in a homonuclear diatomic molecule by absorption of a single x-ray photon is localized on one atom or delocalized over both. We have been able to clarify this question with an experiment that uses Auger electron angular emission patterns from molecular nitrogen after inner-shell ionization as an ultrafast probe of hole localization. The experiment, along with the accompanying theory, shows that observation of symmetry breaking (localization) or preservation (delocalization) depends on how the quantum entangled Bell state created by Auger decay is detected by the measurement.

Resonant Auger decay driving intermolecular Coulombic decay in molecular dimers

In 1997, it was predicted that an electronically excited atom or molecule placed in a loosely bound chemical system (such as a hydrogen-bonded or van-der-Waals-bonded cluster) could efficiently decay by transferring its excess energy to a neighbouring species that would then emit a low-energy electron. This intermolecular Coulombic decay (ICD) process has since been shown to be a common phenomenon, raising questions about its role in DNA damage induced by ionizing radiation, in which low-energy electrons are known to play an important part. It was recently suggested that ICD can be triggered efficiently and site-selectively by resonantly core-exciting a target atom, which then transforms through Auger decay into an ionic species with sufficiently high excitation energy to permit ICD to occur. Here we show experimentally that resonant Auger decay can indeed trigger ICD in dimers of both molecular nitrogen and carbon monoxide. By using ion and electron momentum spectroscopy to measure simultaneously the charged species created in the resonant-Auger-driven ICD cascade, we find that ICD occurs in less time than the 20 femtoseconds it would take for individual molecules to undergo dissociation. Our experimental confirmation of this process and its efficiency may trigger renewed efforts to develop resonant X-ray excitation schemes for more localized and targeted cancer radiation therapy.

Imaging of the Structure of the Argon and Neon Dimer, Trimer, and Tetramer

We Coulomb explode argon and neon dimers, trimers, and tetramers by multiple ionization in an ultrashort 800 nm laser pulse. By measuring all momentum vectors of the singly charged ions in coincidence, we determine the ground state nuclear wave function of the dimer, trimer, and tetramer. Furthermore we retrieve the bond angles of the trimer in position space by applying a classical numerical simulation. For the argon and neon trimer, we find a structure close to the equilateral triangle. The width of the distribution around the equilateral triangle is considerably wider for neon than for argon.

Matter wave optics perspective at molecular photoionization: K-shell photoionization and Auger decay of N2

In this paper, we shed new light on the molecular photoionization of a diatomic molecule. We will elaborate the differences and analogy between a quantum optical and light?matter interaction approach in a study of K-shell photoionization of N2 in which the photoelectron and the subsequently emitted Auger electron are both measured in coincidence in the body fixed frame of the molecule. The two electrons form an entangled state inside a double slit. By changing the photon energy we create different types of interference in the photoelectron and the Auger electron wave.

Imaging the Temporal Evolution of Molecular Orbitals during Ultrafast Dissociation

We investigate the temporal evolution of molecular frame angular distributions of Auger electrons emitted during ultrafast dissociation of HCl following a resonant single-photon excitation. The electron emission pattern changes its shape from that of a molecular σ orbital to that of an atomic p state as the system evolves from a molecule into two separated atoms.

Auger decay of 1 σ g and 1 σ u hole states of the N 2 molecule: Disentangling decay routes from coincidence measurements

Results of the most sophisticated measurements in coincidence with the angular-resolved K-shell photoelectrons and Auger electrons and with two atomic ions produced by dissociation of N{sub 2} molecule are analyzed. Detection of photoelectrons at certain angles makes it possible to separate the Auger decay processes of the 1{sigma}{sub g} and 1{sigma}{sub u} core-hole states. The Auger electron angular distributions for each of these hole states are measured as a function of the kinetic-energy release of two atomic ions and are compared with the corresponding theoretical angular distributions. From that comparison one can disentangle the contributions of different repulsive doubly charged molecular ion states to the Auger decay. Different kinetic-energy-release values are directly related to the different internuclear distances. In this way one can trace experimentally the behavior of the potential energy curves of dicationic final states inside the Frank-Condon region. Presentation of the Auger-electron angular distributions as a function of kinetic-energy release of two atomic ions opens a new dimension in the study of Auger decay.

Electron emission from H 2 + in strong laser fields

M. Odenweller,1 J. Lower,1 K. Pahl,1 M. Schütt,1 J. Wu,2 K. Cole,1 A. Vredenborg,1 L. Ph. Schmidt,1 N. Neumann,1 J. Titze,1 T. Jahnke,1 M. Meckel,1 M. Kunitski,1 T. Havermeier,1 S. Voss,1 M. Schöffler,1 H. Sann,1 J. Voigtsberger,1 H. Schmidt-Böcking,1 and R. Dörner1,* 1Institut für Kernphysik, J. W. Goethe Universität, Max-von-Laue-Strasse1, 60438 Frankfurt, Germany 2State Key Laboratory of Precision Spectroscopy, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, China (Received 29 November 2013; published 31 January 2014)

Experimental Proof of Resonant Auger Decay Driven Intermolecular Coulombic Decay

Resonant Auger decay driven Intermolecular Coulombic Decay through synchrotron radiation in gas phase carbon monoxided dimers and nitrogen dimers has been studied. We report the first experiment where the low-energy ICD-electron has been measured in coincidence with the ionic fragments and Resonant Auger ICD has been proved experimentally.

Double Auger Emission of fixed-in-space Carbon Monoxide following Core-Excitation and Ionization

Double Auger decay after core-level photo excitation and after ionization through synchrotron radiation in gas phase carbon monoxide has been studied. We report the first experiment where both Auger electrons in double Auger decay have been measured in coincidence with the ionic fragments.

Strong Field Electron Emission from Fixed in Space H{sub 2}{sup +} Ions

We have studied electron emission from the H(2)(+) ion by a circularly polarized laser pulse (800 nm, 6×10(14)  W/cm(2)). The electron momentum distribution in the body fixed frame of the molecule is experimentally obtained by a coincident detection of electrons and protons. The data are compared to a solution of the time-dependent Schrödinger equation in two dimensions. We find radial and angular distributions which are at odds with the quasistatic enhanced ionization model. The unexpected momentum distribution is traced back to a complex laser-driven electron dynamics inside the molecule influencing the instant of ionization and the initial momentum of the electron.