1,721,017 research outputs found
Probing The Vibrational Wave Packet Dynamics On The Electronic Ground State Of Neutral Silver Tetramer: Vibrational Frequencies, Anharmonicities And Anisotropy
Full text linkSmall silver clusters possess remarkable luminescence and photoelectric properties, making them subject of current research.\footnote{Grandjean, D. et al. Science 2018, 361, 686–690.}
However, obtaining vibrations on small, neutral silver clusters remain challenging, due to difficulties in mass-selecting neutral clusters and a lack of easily accessible and widely wavelength-tunable far infrared light sources.
Here, we report our study on experimentally probing the vibrational wave packet dynamics on the ground state potential energy surface of the neutral silver tetramer Ag, a benchmark system for small neutral metal clusters, and unambiguously assign its structure. We combine femtosecond pump-probe spectroscopy employing the NeNePo (negative-neutral-positive) excitation scheme\footnote{Wolf, S. et al., Phys. Rev.Lett. 74(21), 4177; Hess, H. et al., Eur. Phys. J. D, 16(1), 145-149.} with a cryogenic ion-trap tandem mass spectrometer. A linear polarized ultrafast pump pulse (40 fs, tunable center wavelength from 700 nm - 820 nm) is used to selectively prepare a coherent wave packet by photodetachment from thermalized (20 - 300 K) Ag anions. The wave packet dynamics on the electronic ground state are then probed using a second polarized ultrafast pulse (50 fs, centered at 400 nm), which ionizes Ag in a two-photon process. The mass-selected cation yield as a function of the delay time (0 - 60 ps) between the two laser pulses yields the fs-NeNePo spectrum. Frequency analysis with a resolution down to about 0.5 cm by using Fourier transform of transient traces reveal one prime frequency band (109.5 0.4 cm) in all conditions and four bands at 32 cm, 78 cm, 186 cm and 295 cm dependent on pump wavelengths and temperatures. These frequencies are consists with predicted fundamental vibration frequencies (\nub{1}, \nub{2}, \nub{5} and \nub{6}) and one combination (\nub{1} + \nub{2}) for rhombic D geometry of Ag. The rephrasing period of the wave packet allows determining vibrational anharmonicities. A strong dependence of the NeNePo cation signal on the polarization of ultrafast pulses is observed, revealing information on the anisotropy of the partial waves involved in the photodetachment process
Theoretical Investigation and Structural Assignment of Small Metal Oxide Clusters
Full text linkAnhand von theoretischen Untersuchungen wird eine umfassende Beschreibung von kleinen Metalloxidclustern gegeben. Bei den untersuchten Systemen handelt es sich um Aluminium- und Eisenoxid-Ionen sowie entsprechende Oxid-Cluster, die beide Metalle enthalten. Neben der Bestimmung der geometrischen Struktur der Cluster werden auch die allgemeinen elektronischen Eigenschaften der eisenhaltigen Verbindungen untersucht. Alle Vorhersagen werden durch Vergleich mit verfügbaren experimentellen Ergebnissen -- hauptsächlich aus der Infrarot-Photodissoziations- und der Photoelektronenspektroskopie -- überprüft und bewertet. Soweit möglich werden die Bewegungen von Atomen oder kleinen Gruppen innerhalb der Cluster einzelnen experimentellen Signalen zugeordnet. Besondere Aufmerksamkeit wird dem Eisendioxidmolekül und seinem Anion gewidmet. Es wird mit spezialisierten Wellenfunktionsmethoden untersucht, mit denen ab initio Franck-Condon-Simulationen einschließlich nicht-adiabatischer und Spin-Orbit-Kopplungen für die Photoionisation des Anions erstellt werden. Sie liefern Erklärungen für die komplizierte Schwingungsstruktur des experimentellen hochauflösenden Photoelektronenspektrums.By means of theoretical investigations, a comprehensive description of small metal oxide clusters is given. The studied systems are aluminum and iron oxide ions as well as respective bi-metallic oxide clusters. Besides the determination of the geometrical structure of the clusters, the general electronic properties of the iron-containing compounds are investigated. All predictions are checked and assessed by comparison with available experimental results, mainly infrared photodissociation and photoelectron spectroscopy measurements. As far as possible, motions of atoms or small groups within the clusters are assigned to distinct experimental vibrational features. Particular attention is paid to the iron dioxide molecule and its anion. It is studied with sophisticated wave function methods based on which ab initio Franck-Condon simulations for the photodetachment from the anion, including non-adiabatic and spin-orbit couplings, are generated. They provide explanations for the complicated vibrational structure of the experimental high-resolution photoelectron spectrum
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Quantum chemical calculations on small Aluminium, Iron and Cobalt containing oxide clusters
No full textDas Verständnis von Katalysatormaterialien ist nach wie vor von größter Bedeutung für die Verbesserung umweltfreundlicher Technologien und die chemische Industrie. Die Untersuchung von Metalloxidclustern in der Gasphase ergänzt das Verständnis dieser Klasse von Katalysatormaterialien. Ein aktives Zentrum kann in der Gasphase isoliert untersucht werden, was bedeutet, dass der Cluster ein katalytisch aktives Zentrum in einem industriell relevanten Katalysatormaterial darstellen kann, das unter streng kontrollierten Bedingungen verfügbar ist. Dichtefunktionaltheorie (DFT) und Wellenfunktionsmethoden werden verwendet, um die geometrischen und elektronischen Strukturen sowie die Reaktivität von zwei verschiedenen Aluminiumoxidsystemen zu untersuchen: [Al3(μ2-OH)2(O)(PhSi(OSiPh2O)3)2]ꟷ, mit Ph = Phenyl ist, und MAl7O12+, mit M = Fe oder Co ist.
Die Ergebnisse für [Al3(μ2-OH)2(O)(PhSi(OSiPh2O)3)2]ꟷ deuten darauf hin, dass die konjugierte Base nach der Deprotonierung eine zusätzliche Stabilisierung erfährt. Dadurch wird die Azidität der AlIV-O(H)-AlIV Einheiten vergleichbar mit derjenigen von verbrückenden SiIV-O(H)-AlIV-Einheiten in Zeolithen. Dies ist im Zusammenhang mit Brønstedsauren Zeolithen von Bedeutung, die Al-Spezies außerhalb des Gerüsts aufweisen. In Bezug auf MAl7O12+, mit M = Fe oder Co, sind die vorgeschlagenen Strukturen isomorph mit der ursprünglichen Al8O12+ Struktur, enthalten aber eine mehrfach gebundene [M(IV)=Ot]2+ Einheit anstelle der terminalen [Al(III)ꟷOt●]2+ Radikalstelle. Dies erklärt die stark reduzierte Fähigkeit zur Abstraktion eines H-Atoms aus CH4 für M = Fe, Co im Vergleich zu M = Al, da die Stabilität der Metall-Oxo-Mehrfachbindung die Abstraktion von den H-Atoms erschwert. Diese kontrollierte Änderung der Reaktivität ist nur möglich, weil die Übergangsmetalle Fe und Co in der Oxidationsstufe +IV stabil sind, die für Al nicht zugänglich ist.The understanding of materials for catalysis remains of utmost importance to improve green technologies and chemical industries. The study of metal oxide gas-phase clusters complements the understanding of this class of catalytic materials. An active site in isolation can be studied in the gas-phase, meaning the cluster can represent a catalytic active centre in an industrially relevant catalyst material, available under highly controlled conditions. Density functional theory (DFT) and wavefunction calculations are used to study the geometric and electronic structures as well as the reactivity of two distinct aluminium oxide systems; [Al3(μ2-OH)2(O)(PhSi(OSiPh2O)3)2]ꟷ, where Ph = Phenyl; and MAl7O12+, where M = Fe or Co. Relating to [Al3(μ2-OH)2(O)(PhSi(OSiPh2O)3)2]ꟷ, the results suggest that upon deprotonation, the conjugate base gains additional stabilization. This renders the acidity of the AlIV-O(H)-AlIV units comparable to that of bridging SiIV-O(H)-AlIV entities in zeolites. Which is relevant in the context of Brønsted acidic zeolite materials that exhibit extra-framework Al species. Relating to MAl7O12+, with M = Fe or Co, the proposed structures are isomorphous with the parent Al8O12+ structure, but contain a multiply bonded [M(IV)=Ot]2+ unit instead of the terminal [Al(III)ꟷOt●]2+ radical site. This explains the largely reduced ability to abstract an H-atom from CH4 for M = Fe, Co compared to M = Al, as the stability of the multiple metal-oxo bond results in a barrier for H-atom abstraction. This controlled change in reactivity is only possible because the transition metals Fe and Co are stable in the +IV oxidation state, which is not accessible for Al
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Struktur, Anharmonie und Fluxionalität
Full text linkGas-phase clusters are aggregates of a countable number of particles, which
exhibit size-dependent physical and chemical properties that typically lie in
the non-scalable size regime. These properties can be systematically
characterized at a molecular level with respect to composition, size and
charge state. This allows studying how macroscopic properties of condensed
matter, e.g. phase transitions or metallic behavior, emerge from the atomic or
molecular properties as a function of cluster size. Furthermore, smaller
clusters are also amenable to high-level quantum chemical calculations, making
them ideal model systems for understanding phenomena in more complex
heterogeneous matter. The main advantage here is that clusters can be studied
with a very high degree of selectivity and sensitivity, under well-defined
conditions and in the absence of perturbing interaction with an environment.
The studies presented in this theses focus on the structure characterization
of ionic clusters using cryogenic ion vibrational spectroscopy. This technique
combines cryogenic ion trapping with mass spectrometric schemes and infrared
photodissociation (IRPD) spectroscopy. It makes use of an ion-trap triple mass
spectrometer in combination with various light sources that grant access to a
wide range of the infrared spectrum (210-4000 cm-1). Structures are typically
assigned by comparing experimental IRPD spectra with computed vibrational
spectra. The structures of aluminum oxide clusters and their interaction with
water are studied in the framework of the collaborative research center
CRC1109 "Understanding of Metal Oxide / Water Systems at the Molecular Scale:
Structural Evolution, Interfaces, and Dissolution". This project aims at
gaining a molecular level understanding of the mechanisms involved in oxide
formation and dissolution. Section 4.1 and 4.2 present results of IRPD
spectroscopy experiments on small mono and dialuminum oxide anions and on the
anionic cluster series (Al2O3)nAlO2 with n = 0 to 6. These studies discuss the
effects of the distribution of the excess charge on the cluster structure,
analyze how structural properties evolve with size and how these relate to
those of nanoparticles and crystal surfaces. The dissociative adsorption of
water by Al-oxide clusters is investigated in Section 4.3.2. Boron exhibits a
rich variety of polymorphs with the B12 icosahedron as a common building
block. This three dimensional (3D) structure is retained in the halogenated
closo-dodecaborate dianions (B12X122-). On the other hand, small pure boron
clusters are essentially planar. The study presented in Section 5.2
investigates the 3D to 2D structural transition by probing the vibrational
spectra of partially deiodinated B12In2- clusters as a function of decreasing
n. The results presented in Section 5.1 show that B13+ has a planar structure
consisting of two concentric rings. As a result of delocalized aromatic
bonding, this structure is particularly stable without being rigid as it
permits an almost free rotation of the inner ring. Protonated water clusters
are model systems for understanding protons in aqueous solutions. The
interpretation of their vibrational spectra is a challenge for state-of-the-
art electronic structure calculations and therefore often prone to
controversies. The results presented in Chapter 6 clear existing doubts over
the assignment of the protonated water pentamer structure and the vibrational
fingerprints of the embedded distorted H3O+. This study laid the foundation
for a subsequent series of measurements which provided crucial new insights
into the proton transfer mechanism in water.Gasphasencluster sind Aggregate mit einer zählbaren Anzahl von Teilchen, deren
größenabhängige Eigenschaften typischerweise im nicht-skalierbaren Regime
liegen. Deren Eigenschaften lassen sich auf molekularer Ebene hinsichtlich
Zusammensetzung, Größe und Ladungszustand systematisch charakterisieren. Dies
ermöglicht zu erforschen, wie sich Eigenschaften kondensierter Materie, wie
beispielsweise Phasenübergänge oder metallisches Verhalten, aus den
Eigenschaften der Bestandteile mit zunehmender Größe entwickeln. Ferner können
kleinere Cluster mit hochwertigen quantenchemischen Methoden modelliert
werden, was sie zu idealen Modellsystemen für heterogene Materialien macht.
Cluster können nicht nur größenselektiv und mit hoher Empfindlichkeit, sondern
auch unter wohldefinierten Bedingungen und in Abwesenheit des störenden
Einflusses einer Umgebung erforscht werden. Das Ziel der vorliegenden
Dissertation ist die Strukturcharakterisierung ionischer Cluster mittels der
Schwingungsspektroskopie kryogener Ionen. Hierzu wurden moderne
massenspektrometrische Methoden mit der Infrarot-
Photodissoziation(IRPD)-Spektroskopie (IRPD) kombiniert. Mit Hilfe
unterschiedlicher Strahlungsquellen konnte ein möglichst breiter
Spektralbereich (210 bis 4000 cm–1 ) abgedeckt werden. Die Strukturbestimmung
erfolgte typischerweise durch den Vergleich gemessener IRPD-Spektren mit
simulierten IR-Spektren. Die Untersuchungen zur Struktur von
Aluminiumoxidclustern sowie deren Wechselwirkung mit Wasser wurden im Rahmen
des Sonderforschungsbereichs SFB-1109 “Understanding of Metal Oxide / Water
Systems at the Molecular Scale” durchgeführt. Ziel des SFBs ist, die
Mechanismen der Oxidbildung und -auflösung auf molekularer Ebene aufzuklären.
In den Abschnitten 4.1 und 4.2 werden die Resultate der IRPD-Spektroskopie an
kleinen Mono- und Dialuminiumoxidanionen sowie an anionischen Clustern der
Serie (Al2O3)nAlO2– mit n = 0 bis 6 beschrieben. In diesen Studien wurde
untersucht, welche Auswirkung die Ladungsverteilung auf die Clusterstruktur
hat, wie sich Eigenschaften mit der Clustergröße entwickeln und inwieweit
diese sich mit denen entsprechender Nanopartikel und Kristalloberflächen
vergleichen lassen. Die dissoziative Wasseradsorption an Aluminiumoxidclustern
wurde in Abschnitt 4.3.2) erforscht. Bor weist einen umfangreichen
Polymorphismus auf, wobei als Grundbaustein häufig ein B12 - Ikosaeder
auftritt. Diese dreidimensionale Struktur liegt auch im halogenierten Closo-
Dodekaborat B12X122– Dianion vor. Andererseits weisen kleine Borcluster
planare Strukturen auf. Die in Abschnitt 5.2 beschriebenen Messungen dienen
der Untersuchung des Übergangs von einer drei- zu einer zwei- dimensionalen
Struktur am Beispiel des deiodierten B12In–. In Abschnitt 5.1 wird gezeigt,
dass das kationische B13+ eine, aus zwei konzentrischen Ringen bestehende,
planare Struktur aufweist, die sich durch eine fast freie Rotation des inneren
Rings auszeichnet. Aufgrund der aromatischen Bindungsverhältnisse ist dieses
System somit besonders stabil, ohne starr zu sein. Protonierte Wassercluster
dienen als Modellsystem für Protonen in wässriger Lösung. Die Modellierung der
IR-Spektren dieser Cluster bringt selbst die besten quantenchemischen Methoden
an ihre Grenzen, was zu erheblichen Kontroversen geführt hat. Die in Kapitel 6
dargelegten Ergebnisse erlauben eine vollständige Zuordnung der IR-Banden der
verzerrten H3O+ -Gruppe, beseitigen Zweifel an der Struktur des protonierten
Wasserpentamers und ebneten einer Reihe von weiteren Messungen den Weg, die
neue Einsichten in den Mechanismus des Protonentransfers in Wasser
ermöglichten
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