Dissertation:
Relaxationsdynamik und Femtochemie in massenselektierten Metall-Clustern

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2009
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Westhäuser, Wilko
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Relaxation Dynamic and Femtochemistry of mass-selected Metal Clusters
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In this work, the relaxation dynamic of photo-excited pure Cu-clusteranions as well as the photodesorption of oxygen from Ag8O2- are studied. For that purpose, copper was vaporized in a PACIS cluster source and the mass-selected clusters were investigated by time-resolved pump-probe-photoelectron spectroscopy using ultra-short femto-laserpulses. The focus of this research was directed to the possible application of cluster building-blocks in future technologies for catalysis or energy storage and conversion.

1. The investigation of dynamical properties of pure Cu-clusteranions offers the basic research of the dominant relaxation processes in these clusters. The measured dynamics of Cun- were compared to the known behaviour of other sp-metal clusters, like Ag-, Au- and Al-clusteranions, to obtain a systematic understanding. The main interest was on extraordinary long-lasting relaxation dynamics. Pump-probe-signals could be measured from Cun- with n = 5 9, 11 13, 18 and 19. The most relaxation times hit the average of 100 fs. An exception is the electronically magic cluster Cu7-, which shows a remarkable long-living excited state of some picoseconds. Contrary to expectations, the next bigger, electronically magic Cluster, Cu19-, has a relatively fast relaxation dynamic, which does not differ perceptibly from those of the other neighbouring, non-magic clusters. Qualitatively, this special behaviour was explained on the basis of internal conversion via conical intersection and was illustrated by using a Clemenger-Nilsson-diagram. As sp-metal clusters have a low density of states (DOS) in the neighbourhood of the highest occupied molecular orbital (HOMO), fast relaxations via Auger-like processes are unlikely. But the dominating sp-character in the clusters bonds opens an additional relaxation channel: The flexible geometrical structure enables a fast, radiationless relaxation via deformation of the geometry in terms of internal conversion. This ability was already proposed to be responsible for the unexpected short relaxation times in the case of previously studied photo-excited doubly magic Al13-. Qualitatively and quantitatively, small Ag-clusteranions show almost identical dynamics in comparison of measured Cun-. This can be attributed to similarities in their electronic and geometric structures. On the contrary, Au-clusteranions have a clearly more rigid geometric structure due to relativistic effects and stronger influence of d-orbital in the bonds. Therefore, in Au-clusters the fast relaxation channel via geometrical deformation is not likely, resulting in conspicuously longer-lasting relaxation dynamics.

2. Time-resolved photoelectron spectra of Ag8O2- show pump-probe signals shifting to higher binding energies with increasing time delay between pump- and probe-pulse. The signals finally localize at energies which can be attributed to the ones of the pure Ag8-. The temporal evolution of the measuring signals was explained on the basis of a direct photodesorption process: First, the pump-pulse photoexcites the system in an excited binding state, which develops quickly in an excited anti-binding state. The direct desorption of O2 from Ag8O2- occurs out of this anti-binding state. Meanwhile, the potential energy of the state is reduced, which means the binding energy grows. Correspondingly, the measured spectra show a shifting of the time delay depending signal to higher binding energies as the time delay increases. This process results in the breaking of the O2-Ag8-bond, leaving the pure Ag8- as fragment product. This is consistent with the measured data showing a stop of the shifting of the pump-probe signals at energies which agree well with the known ones of Ag8-. The discussed photoelectron spectra are typical for the direct photo-induced desorption. In contrast, the indirect process takes place via an intermediate step of thermalization of the excited state and is well known for metal surfaces.
Summary in another language
Die vorliegende Arbeit behandelt zum einen die Relaxationsdynamik von photoangeregten reinen Cu-Clusteranionen und zum anderen die Photodesorption des Sauerstoffs von Ag8O2-.
Zu diesem Zweck wurden Cluster in einer PACIS-Quelle hergestellt und massenselektiert mittels zeitaufgelöster Pump-Probe-Photoelektronenspektroskopie mit ultra-kurzen Femto-Laserpulsen untersucht. Dabei erfolgte die Forschungsarbeit im Hinblick auf die Anwendbarkeit von möglichen Cluster-Bausteinen in zukünftigen Katalyse-, Energiespeicher- bzw. Energieumwandlungstechnologien.

1. Die Untersuchung der Dynamik in reinen Cu-Clusteranionen ermöglicht die grundlegende Erforschung der in diesen Clustern dominanten Relaxationsprozesse. Um zu einem systematischen Verständnis dieser Vorgänge zu gelangen, wurde das Verhalten der untersuchten Cun- mit dem anderer bekannter sp-Metallcluster, wie Ag-, Au- und Al-Clusteranionen, verglichen. In diesem Zusammenhang lag das besondere Interesse auf Clustern mit einer außergewöhnlich lang andauernden Relaxationsdynamik.
Pump-Probe-Signale konnten bei Cun- mit n = 5 - 9, 11 - 13, 18 und 19 gemessen werden. Die meisten Relaxationszeiten liegen im Größenbereich von einigen 100 fs. Eine Ausnahme bildet der elektronisch magische Cluster Cu7- mit relativ langlebigen angeregten Zuständen von mehreren Pikosekunden. Der nächst größere, elektronisch magische Cluster, Cu19-, zeigt wider Erwarten eine relativ schnelle Relaxationsdynamik, die sich nicht merklich von denen der benachbarten, nicht-magischen Cluster unterscheidet. Dieses besondere Verhalten wurde qualitativ auf der Grundlage der internen Konversion mit Conical Intersection erklärt und anhand von Clemenger-Nilsson-Diagrammen veranschaulicht. Da sp-Metallcluster eine geringe Zustandsdichte (DOS) im Bereich des höchsten besetzten Molekularorbitals (HOMO) besitzen, ist eine schnelle Relaxation über Auger-artige Prozesse sehr unwahrscheinlich. Wegen des dominierenden sp-Charakters in den Bindungen steht Cu-Clustern jedoch ein anderer schneller Relaxationskanal offen: Die flexible geometrische Struktur ermöglicht eine schnelle, strahlungslose Relaxation durch Geometrieänderung in Form von interner Konversion. Mit diesem Mechanismus konnte bereits die unerwartet kurze Relaxationszeit im photoangeregten doppelt magischen Al13- erklären werden.
Kleine Ag-Clusteranionen zeigen im Vergleich zu den untersuchten Cun- nahezu identisches qualitatives wie auch quantitatives Verhalten, was auf große Gemeinsamkeiten in der elektronischen und geometrischen Struktur zurückgeführt werden kann. Im Gegensatz dazu weisen Au-Clusteranionen aufgrund des relativistischen Effekts und des stärkeren Einflusses der d-Orbitale in den Bindungen eine deutlich rigidere geometrische Struktur auf. Somit steht der schnelle Relaxationskanal über eine Geometriedeformation in Au-Clustern nicht zur Verfügung, was sich in deutlich längeren Relaxationsdauern im Vergleich zu Cun- äußert.

2. Die zeitaufgelösten Photoelektronenspektren des Ag8O2- zeigen Pump-Probe-Signale, die mit zunehmender Zeitverzögerung zwischen dem Pump- und Probe-Puls zu höherer Bindungsenergie schieben, bis sie schließlich bei einer Energie lokalisieren, die dem reinen Ag8- zugeordnet werden kann. Die zeitliche Entwicklung der Messsignale konnte auf Basis des direkten Photodesorptionsprozesses qualitativ erklärt werden: Der Pump-Puls regt das System zunächst in einen bindenden Zustand an, welcher relativ schnell in einen anti-bindenden angeregten Zustand übergeht. Die direkte Desorption des O2 von Ag8O2- erfolgt dann aus diesem anti-bindenden Zustand heraus. Dabei wird die potentielle Energie des Zustandes reduziert, was einen Anstieg der Bidnungsenergie bedeutet. In den Spektren tritt entsprechend eine Verschiebung des zeitdelayabhängigen Messsignals zu größeren Bindungsenergien hin mit zunehmender Zeitverzögerung auf. Dieser Prozess resultiert im Bruch der O2-Ag8-Bindung, sodass schließlich nach Desorption des O2 das Fragmentprodukt Ag8- verbleibt. Dies ist in Einklang mit den gemessenen Photoelektronenspektren, da diese zeigen, wie die Bewegung der Pump-Probe-Peaks zu höheren Bindungsenergien bei Energien endet, die mit dem bekannten Photoelektronensignalen des Ag8- übereinstimmen. Die gezeigten Spektren sind typisch für eine direkte photo-induzierte Desorption. Im Gegensatz dazu steht der bei Metalloberflächen bekannte indirekte Prozess, bei dem die Desorption über einen Zwischenschritt der Thermalisierung des angeregten Zustandes als thermisch aktivierter Abdampfprozess und nicht direkt aus einem anti-bindenden Zustand heraus erfolgt.
Subject (DDC)
530 Physics
Keywords
Metallcluster , Ionenstrahl , Ultrakurzzeitspektroskopie , PES , Femtochemie , cluster physics , cluster , photochemistry , femtochemistry , time-resolved photoelectron spectroscopy
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ISO 690WESTHÄUSER, Wilko, 2009. Relaxationsdynamik und Femtochemie in massenselektierten Metall-Clustern [Dissertation]. Konstanz: University of Konstanz
BibTex
@phdthesis{Westhauser2009Relax-5060,
  year={2009},
  title={Relaxationsdynamik und Femtochemie in massenselektierten Metall-Clustern},
  author={Westhäuser, Wilko},
  address={Konstanz},
  school={Universität Konstanz}
}
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