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Identifier 000419511
Title Formation, stability and structure of transition metal oxide clusters
Alternative Title Δημιουργία, σταθερότητα και δομή συσσωματωμάτων οξειδίων μετάλλων μεταπτώσεως
Author Glodić, Pavle B.
Thesis advisor Κιτσόπουλος, Θεοφάνης
Reviewer Βελεγράκης, Μιχάλης
Άγγλος Δημήτριος
Αναστασιάδης, Σπύρος
Ρακιτζής, Πέτρος
Κουτσολέλος, Αθανάσιος
Φαράντος, Σταύρος
Abstract The overall goal and motivation of the research presented in this thesis is to deepen the knowledge of the physical and chemical properties of small molecular and ionic clusters. The research discussed in this thesis has two different aspects. In the first part we investigated the influence of solvation on the photodissociation dynamics of a diatomic molecule. First, we performed a detailed study on iodine-monochloride (ICl) in order to understand photodissociation of the ICl monomer. Following this experiment, we studied changes in photochemical behavior as a result of clustering of ICl molecules with xenon (Xe) atoms. In the second part of thesis we investigated structure, stability and fragmentation mechanisms of transition metal oxide clusters, which is crucial in order to understand the chemical reactivity of these important catalysts. The experiments were performed on small niobium and yttrium oxide clusters. Additionally, theoretical calculations based on density functional theory (DFT) have been performed in the case of YxOy+ clusters in order to compare with the experimental findings. Chapter II serves to describe the experimental apparatus in detail and the conditions under which these studies were conducted. The experiments on the rare gas clusters were conducted using well established slice imaging technique, developed in the Laboratory for Chemical Dynamics. For the transition metal oxide clusters studies we applied two collision induced dissociation (CID) methods, the conventional one employing a collision cell, and a novel method using crossed molecular beams, developed in Laboratory for Clusters. The experiments described in Chapter III concern ICl photolysis in the ultraviolet region of the spectrum (235–265 nm), which was studied using the slice imaging technique. We conclude that photolysis of ICl in this UV region is a relatively “clean” source of spin‐orbit excited chlorine atoms that can be used in crossed molecular beam experiments. In Chapter IV we present slice imaging data demonstrating the influence of clustering on the photodissociation dynamics of a diatomic molecule: iodine monochloride (ICl) was dissociated at 235 nm in He and Xe seed gasses, probing both Cl and I photofragment energy and angular distributions. We observe that the kinetic energy releases of both Cl and I fragments change from He to Xe seeding. For Cl fragments, the seeding in Xe increases the kinetic energy release of some Cl fragments with a narrow kinetic energy distribution, and leads to some fragments with rather broad statistical distribution falling off exponentially from near‐zero energies up to about 2.5 eV. Iodine fragment distribution changes even more dramatically from He to Xe seeding: sharp features essentially disappear and a broad distribution arises reaching to about 2.5 eV. Both these observations are rationalized by a simple qualitative cluster model assuming ICl dissociation inside larger xenon clusters and “on surface” of smaller Xe species. Chapter V presents research on oxygen‐rich niobium oxide clusters, formed by mixing laser‐produced Nb plasma with pure oxygen. Their stability is investigated by mass spectrometry and collision‐induced dissociation (CID). The research described in this chapter is focused on the application of two CID methods – (1) the conventional one, and (2) a novel experimental configuration recently developed by our group, where the cluster ions beam is crossed with a secondary beam of noble gas atoms, and the fragments are rejected by a retarding field energy analyzer. Briefly, the relative collision cross sections of NbxOy+ (x = 1, 2, y = 2–12) clusters have been measured in order to obtain information on their stability and structure. In addition, information about their fragmentation channels has been obtained. Finally, Chapter VI is dedicated to yttrium oxide cluster cations, which have been experimentally and theoretically studied. We produced small, oxygen‐rich yttrium oxide clusters, YxOy+ (x = 1, 2, y = 1–13), by mixing the laser produced yttrium plasma with a molecular oxygen jet. Mass spectrometry measurements showed that the most stable clusters are those consisting of one yttrium and an odd number of oxygen atoms of the form YO+2k+1 (k = 0–6). Additionally, we performed collision induced dissociation experiments, which indicated that the loss of pairs of oxygen atoms down to a YO+ core is the preferred fragmentation channel for all clusters investigated. Furthermore, we conduct DFT calculations and we obtained two types of low energy structures: one containing an yttrium cation core and the other composed of YO+ core and O2 ligands, being in agreement with the observed fragmentation pattern. Finally, from the fragmentation studies, total collision cross sections are obtained and these are compared with geometrical cross sections of the calculated structures.
Language English
Subject Collision induced dissociation
Slice imaging
Φασματομετρία μάζας
Issue date 2018-11-13
Collection   Faculty/Department--Faculty of Sciences and Engineering--Department of Chemistry--Doctoral theses
  Type of Work--Doctoral theses
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