Allen Lee Landers

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.

Complete photo-fragmentation of the deuterium molecule

All properties of molecules—from binding and excitation energies to their geometry—are determined by the highly correlated initial-state wavefunction of the electrons and nuclei. Details of these correlations can be revealed by studying the break-up of these systems into their constituents. The fragmentation might be initiated by the absorption of a single photon, by collision with a charged particle or by exposure to a strong laser pulse: if the interaction causing the excitation is sufficiently understood, the fragmentation process can then be used as a tool to investigate the bound initial state. The interaction and resulting fragment motions therefore pose formidable challenges to quantum theory. Here we report the coincident measurement of the momenta of both nuclei and both electrons from the single-photon-induced fragmentation of the deuterium molecule. The results reveal that the correlated motion of the electrons is strongly dependent on the inter-nuclear separation in the molecular ground state at the instant of photon absorption.

Momentum-imaging investigations of the dissociation of D2+ and the isomerization of acetylene to vinylidene by intense short laser pulses

We present momentum images of the ionic products from the ionization of D2 and C2H2 by short laser pulses. For D2, we use a pump–probe approach to investigate the dependence of the enhanced ionization on the internuclear distance. Evidence for two (not well separated) regions of enhancement is found near internuclear distances of 6 and 10 au. In the case of acetylene, we report clear evidence for the production of both acetylene and vinylidene dications with kinetic energy releases similar to those reported earlier by core electron removal. We also find very different angular distributions for the fragments in the two channels, consistent with a finite time for the isomerization.

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.

Dissociative electron attachment to carbon dioxide via the 2

Author(s): Moradmand, A; Slaughter, DS; Haxton, DJ; Rescigno, TN; McCurdy, CW; Weber, T; Matsika, S; Landers, AL; Belkacem, A; Fogle, M | Abstract: Momentum imaging measurements from two experiments are presented and interpreted with the aid of ab initio theoretical calculations to describe the dissociative electron attachment (DEA) dynamics of CO2. The dynamics of the transient negative ions of CO2- involve several conical intersections taking part in mechanisms that have only recently been understood. We address the problem of how the 4-eV 2Πu shape resonance in CO2 proceeds to dissociate to CO(1Σ+) + O-(2P) by DEA.

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We present experimental measurements and theoretical calculations for the photoionization of CH4 at the carbon K-edge. Measurements performed using cold target recoil ion momentum spectroscopy (COLTRIMS) combined with complex Kohn variational calculations of the photoelectron in the molecular frame demonstrate the surprising result that the low energy photoelectrons effectively image the molecule by emerging along the bond axes. Furthermore, we observe a dynamic breakdown of axial recoil behaviour in one of the dissociation pathways of the intermediate dication, which we interpret using electronic structure calculations. (Some figures may appear in colour only in the online journal)

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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.

Probing the dynamics of dissociation of methane following core ionization using three-dimensional molecular-frame photoelectron angular distributions

We have measured the production of both Lyα and Hα fluorescence from atomic H and D for the photodissociation of H2 and D2 by linearly polarized photons with energies between 24 and 60 eV. In this energy range, excited photofragments result primarily from the production of doubly excited molecular species which promptly autoionize or dissociate into two neutrals. Our data are compared with ab initio calculations of the dissociation process, in which both doubly excited state production and prompt ionization (nonresonant) channels are considered. Agreement between our experimental data and that of earlier work, and with our theoretical calculations, is qualitative at best. (Some figures in this article are in colour only in the electronic version)

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

We present a joint experimental and theoretical study of the gerade and ungerade amplitudes of the photo double ionization of helium at excess energies of 100 eV and 450 eV above the threshold. We describe a method of extracting the amplitudes from a COLTRIMS data set. The experimental results are well reproduced by convergent close-coupling (CCC) calculations. The fully differential cross section data underlying this study can be found in our companion papers immediately preceding this one.