The studies presented in this thesis deal with resonant and non-resonant excitation of free variable size clusters using synchrotron radiation in the soft X-ray regime. The post collision interaction (PCI) effect is investigated in free variable size krypton and argon clusters near the Kr 3d and Ar 2p ionization energies. The core ionization energies of surface and bulk sites in variable size clusters can be clearly distinguished. This is mostly due to the polarization screening. It is found that the asymmetry, which is a consequence of PCI, is characteristically smaller for clusters than for isolated atoms. Moreover, there is less asymmetry for bulk sites than for surface sites in variable size rare gas clusters. We assign the results in terms of mechanisms that are based on quantum mechanical models of post collision interaction. Complementary experiments on the photoionization of free van der Waals clusters are performed by using zero kinetic energy (ZEKE) photoelectron spectroscopy in the Ar 2p-, Kr 3d-, Ne 1s-, and N2-regimes. The experimental approach is also suitable to detect cluster size dependent changes in electronic structure. This also allows us to study post collision interaction in variable size clusters. The parameters of the PCI profiles deduced for ZEKE experiments indicate that there are no significant changes in core ionization dynamics compared to near-threshold experiments. Results from model calculations in Kr 3d ionization energy indicate that different geometric sites can be clearly distinguished from each other by their substantial shift in Kr 3d ionization energy, though the dimer shows almost the same Kr 3d ionization energy as the free atom. A comparison with the experimental results indicates that there is resemblance with the model calculations, even though close-lying ionization energies are blended and require deconvolutions of the experimental spectra. It is evident from the present work that one can observe distinct shifts in core ionization energies in van der Waals clusters that are formed in wide size distributions of a jet expansion. The emission of ultraviolet fluorescence radiation from variable size argon clusters is investigated with high spectral resolution in the Ar 2p-excitation regime. The fluorescence excitation spectra reveal strong fluorescence intensity in the Ar 2p-continuum, but no evidence for the occurrence of discrete low-lying core-exciton states in the near-edge regime. This finding is different from the absorption and photoionization cross sections of argon clusters and the solid. The dispersed fluorescence shows a broad molecular band centered near 280 nm. The present results are consistent with the formation of singly charged, excited moieties within the clusters, which are assigned as sources of the radiative relaxation in the 280 nm regime. A fast energy transfer process (interatomic Coulombic decay, ICD) is assigned to be primarily the origin of these singly charged, excited cations besides intra-cluster electron impact ionization by Auger electrons. Our findings give possibly the first experimental evidence for ICD in the core level regime. Free, variable size nitrogen clusters are investigated in the N 1s excitation regime in comparison with the free molecule and solid nitrogen. The conversion of Rydberg states into core excitons, surface and bulk, was studied. The experimental results are simulated by ab initio calculations using (N2)13 as a reasonable prototype cluster structure that allows us to simulate both surface and bulk properties in comparison with the isolated molecule. The present results clearly show that there are specific properties, such as molecular orientation, in molecular van der Waals clusters, which do not exist in atomic van der Waals clusters. It is shown that inner and outer surface sites give rise to distinct energy shifts of the low lying surface core excitons. / In der vorliegenden Dissertation wurden Experimente zur resonanten und nicht-resonanten Anregung von Clustern variabler Größe durchgeführt. Hierzu kam Synchrotronstrahlung im weichen Röntgenbereich zum Einsatz. Der "Post-Collision Interaction"-Effekt (PCI) wurde im Detail am Beispiel von Krypton und Argon-Clustern im Bereich der Kr 3d- und Ar 2p-Anregung studiert. Es lassen sich die Ionisierungsenergien von Atomen, die an der Oberfläche bzw. im Volumen gebunden sind, klar unterscheiden. Dies ist aufgrund der unterschiedlichen Polarisationsabschirmung möglich, die zu einer Verschiebung der Innerschalen-Ionisierungsenergien führt. Die Linienformen der Photoelektronenbanden werden asymmetrisch, wenn die Anregungsenergie geringfügig über der Ionisierungsenergie liegt. Dies lässt sich auf den PCI-Effekt zurückführen. Es wird beobachtet, dass die Asymmetrie vom isolierten Atom über Oberflächenatome zu den im Volumen gebundenen Atomen abnimmt. Diese Veränderung der Linienformen wird mit Hilfe von Mechanismen, die auf Grundlage von quantenmechanischen Modellen basieren, interpretiert. Komplementäre Experimente wurden an Argon- und Neon-Clustern zur Nullvolt-Photoelektronen-Spektroskopie (ZEKE) durchgeführt (Anregung der Ar 2p-Kante, Kr 3d-Kante, N2 1s und Ne 1s-Kante). Auch mit diesem Ansatz lassen sich größenabhängige Veränderungen der elektronischen Struktur in Clustern sowie die Bedeutung des PCI Effektes bestimmen. Ein Vergleich dieser Resultate mit der Anregung, die nahe der Ionisationsschwelle liegt, zeigt, dass es zu keiner signifikanten Veränderung der Ionisationsdynamik als Funktion der Anregungsenergie kommt. Berechnungen zur Ioniserungsenergien von Krypton-Clustern im Bereich der Kr 3d-Anregung zeigen, dass sich einzelne geometrische Orte klar in ihrer Ionisierungsenergie unterscheiden. Das Krypton-Dimer zeigt allerdings fast dieselbe 3d-Ioniserungsenergie wie das freie Atom. Der Vergleich mit den experimentellen Resultaten zeigt, dass eine gute übereinstimmung zwischen Modell und Experiment besteht. Allerdings müssen die experimentellen Spektren entfaltet werden, da die relativen Verschiebungen der Ionisierungsenergien zu gering sind und die Rumpflochlebensdauer zu einer Verbreiterung der Banden führt. Die Resultate belegen, dass sich ausgezeichnete Werte für Rumpfniveau-Ionisierungsenergien bestimmen lassen, obwohl die Cluster in breiten Größenverteilungen vorliegen. Dies lässt sich durch die ortsspezifische Photoionisation erklären. Die Emission von Fluoreszenzstrahlung im ultravioletten Spektralbereich nach Rumpfniveauanregung wurde im Fall von 2p-angeregten Argon-Clustern untersucht. Die hochaufgel östen Spektren zeigen hohe Intensität im 2p-Kontinuum, jedoch keinen Hinweis auf signifikante Beiträge im Bereich der Rumpfniveau-Excitonen. Dieses Ergebnis unterscheidet sich vom Absoprtions- und Photoionisationsquerschnitt von Argon-Clustern sowie festem Argon. Die dispergierte Fluoreszenz liefert eine intensive Bande bei 280 nm. Dieses Resultat lässt sich mit der Fluoreszenz von einfach geladenen, angeregten Argon-Clustern erklären. Die Bildung von einfach geladenen Ionen nach primärer Doppelionisation im Ar 2p-Kontinuum wird durch einen schnellen Energietransfer-Prozess (Interatomic Coulombic Decay, ICD) erklärt. Er läuft nach der Rumpfniveauanregung ab und liefert, neben der Elektronenstoßionisation durch schnelle Auger-Elektronen, einfach geladene Clusterfragmente, die nachfolgend strahlend relaxieren. Dieses Ergebnis ist als erster Hinweis darauf zu werten, dass der ICD-Prozess auch im Bereich der Innerschalenanregung auftritt. Freie Stickstoff-Cluster variabler Größe wurden im Bereich der N 1s-Anregung untersucht. Hier stand die Umwandlung der Rydberg-Zustände in die entsprechenden Oberflächen- und Volumen-Excitonen in Fokus der Studien. Die Resultate wurden mit denen zu freiem und kondensiertem Stickstoff verglichen. Die experimentellen Resultate lassen auch einen Vergleich mit ab initio Rechnungen zu, wofür (N2)13 als Prototyp-Cluster genutzt wurde, da hier sowohl oberflähen - als auch volumengebundene Moleküle auftreten. Diese Resultate zeigen signifikante Unterschiede im Vergleich zu atomaren Clustern. Es zeigt sich, dass die molekulare Orientierung die Lage der Excitonenbanden beeinflusßt. Ebenso treten signifikante Energieverschiebungen relativ zum isolierten Molekül auf, die sich durch Absorption von Zentren erklären lassen, die entweder auf der inneren bzw. äußeren Oberfläche der Cluster gebunden sind.

Thesis (M.S.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / 2031-01-01

Thesis. 1977. Sc.D.--Massachusetts Institute of Technology. Dept. of Physics. / Microfiche copy available in Archives and Science. / Bibliography : leaves 221-223. / by Riad Ahmad Bitar. / Sc.D.

Made available in DSpace on 2014-06-12T17:39:37Z (GMT). No. of bitstreams: 2 arquivo6925_1.pdf: 2994319 bytes, checksum: fdeaa6794e2982744e5325805b2f482c (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2006 / Estuda-se a dinâmica do sistema cardiorespiratório à luz da Teoria de Shannon. O estudo tem dois suportes metodológicos, um é o estudo de dois osciladores de van der Pol acoplados de diferentes maneiras e o outro é a ferramenta estatística, Informação Mútua, da Teoria da Informação de Shannon. Faz-se, primeiramente, uma revisão da fisiologia dos dois sistemas: o cardiovascular e o respiratório. Explora-se a interação entre a freqüência respiratória e a freqüência cardíaca como um possível marcador de eventuais disfunções fisiológicas. Analisa-se as possíveis formas de acoplamento entre dois osciladores de van der Pol. Os resultados reais, obtidos com o estudo de quatro pacientes do Instituto do Coração da Faculdade de Medicina da Universidade de São Paulo, confirmaram a grande potencialidade da ferramenta Informação Mútua como um indicador fisiológico

We investigate the formation of droplets in a thin liquid film on a solid substrate due to the combined action of surface tension and van der Waals forces. Current models for droplet formation assume that droplets have a shallow profile. By removing that assumption and numerically solving for stable droplet profiles, we have modelled droplets that separate from the substrate on which they sit.

In modelling the energetics of molecules and solids, the need for practical electron density functionals that seamlessly include the van der Waals interaction is growing. Such functionals are still in their infancy, and there is yet much experimentation to be performed in the formulation and numerical testing of the requisite approximations. A ground-state density functional approach that uses the exact relations of the adiabatic connection formula and the fluctuation-dissipation theorem to obtain the xc energy from the density-density response function seems promising, though a direct local density approximation for the interacting susceptibility will fail to yield the vdW interaction. Significant nonlocality can be built into the interacting susceptibility by screening a 'bare' susceptibility, for which a carefully chosen constraint-obeying local approximation is sufficient to yield a non-trivial van der Waals energy [6]. The constraints of charge conservation, and no response to a constant potential, are guaranteed by expressing the bare susceptibility in terms of the double gradients of a nonlocal bare polarisability. for which it should be easier to make an approximation based on physical principles than it would be for the susceptibility. The 'no-flow' condition is also deemed important. In this work, a simple delta-function approximation for the nonlocal polarisability is fully constrained by a new version of a recently-discovered force theorem (sum rule), requiring the additional input of the independent-electron Kohn-Sham potential. This constrained polarisability cannot be used as input for the seamless vdW scheme, which requires a non-delta-function bare polarisability, and is instead applied to systems containing spherical fragments in a perturbative/asymptotic fashion for calculation of the widely-separated van der Waals interaction. The main thrust of this work is an investigation of the efficacy of the force theorem to constrain simple approximations for response quantities. Many recent perturbative vdW density functionals are based on response functions that are electron-hydrodynamical approximations to the response of the uniform electron gas. These schemes require their response functions to be 'cut off' at low density and high density-gradient, where the approximation overestimates the true response. The imposition of the cut-off is crucial to the success of such schemes. Here, we replace the cut-off with an exact theorem (the force theorem) which naturally 'ties down' the response, based on the potential- and density-functions of the system. This is the first time that the force theorem has been directly applied as a constraint upon a model response function (its original use, by Vignale and Kohn (7), was as an exact identity in time-dependent DFT). Also new in this work is the orbital-by-orbital Kohn-Sham version of the force theorem, and its proof (differing significantly from Vignale's original derivation (8) of the interacting theorem) by directly appealing to the Kohn-Sham orbitals makes its first appearance here. For quantum dots, our constrained response-approximation exactly recovers the net linear dipole response, due mainly to the force theorem's ideal applicability to harmonically confined systems. For angularly-averaged atoms, reasonable static dipole polarisabilities are obtained for the independent-electron Kohn-Sham (bare) case. The results are poor for the fully-interacting case, attributable to the local nature of the approximation. This lends weight to the assertion that it is better to approximate a bare quantity, then screen it, than it is to directly approximate a fully-interacting quantity. Dynamic net polarisabilities constrained by the force theorem are guaranteed to have the correct high-frequency asymptotic convergence to the free electron response. It is seen that the calculated dynamic polarisabilities for atoms are too small at intermediate frequencies, since the calculated vdW C6 coefficients (Hamaker constants) of atomic dimers are up to an order of magnitude too small, even without the use of a low-density cutoff. It is seen that our constrained local model response is non-analytic along the imaginary-frequency axis, and this is very detrimental to the C6 calculations, even though the integrated net polarisability is analytic. Improvement of the polarisability ansatz is indicated, perhaps to a non-deltafunction uniform-gas-based approximation. The use of pseudopotentials may improve the force theorem results, by softening the extreme nature of the bare Coulomb potential.

Zugl.: Göttingen, Univ., Diss., 2005

Zugl.: Göttingen, Univ., Diss., 2006

No description available.

No description available.