Atoms, molecules and clusters all constitute building blocks of macroscopic matter. Therefore, understanding the electronic and geometrical properties of such systems is the key to understanding the properties of solid state objects. In this thesis, some atomic, molecular and cluster systems (clusters of O2, CH3Br, Ar/O2, Ar/Xe and Ar/Kr; dimers of Na; Na and K atoms) have been investigated using synchrotron radiation, and in the two last instances, laser light. We have performed x-ray photoelectron spectroscopy (XPS) on all of these systems. We have also applied ultraviolet photoelectron spectroscopy (UPS), resonant Auger spectroscopy (RAS) and near-edge x-ray absorption spectroscopy (NEXAFS) to study many of the systems. Calculations using ab initio methods, namely density functional theory (DFT) and Møller-Plesset perturbation theory (MP), were employed for electronic structure calculations. The geometrical structure was studied using a combination of ab initio and molecular dynamics (MD) methods. Results on the dissociation behavior of CH3Br and O2 molecules in clusters are presented. The dissociation of the Na2 molecule has been characterized and the molecular field splitting of the Na 2p level in the dimer has been measured. The molecular field splitting of the CH3Br 3d level has been measured and the structure of CH3Br clusters has been determined to be similar to the structure of the bulk solid. The diffusion behavior of O2, Kr and Xe on large Ar clusters, as a function of doping rate, has been investigated. The shake-down process has been observed from excited states of Na and K. Laser excited Na atoms have been shown to be magnetically aligned. The shake-down process was used to characterize the origin of various final states that can be observed in the spectrum of ground-state K.

In this thesis we have performed core-electron spectroscopy studies of gas phase molecular systems starting with smaller diatomic, continuing with triatomic and extending our research to more complex polyatomic ones. We can subdivide the results presented here into two categories: the first one focusing on electronic fine structure and effect of the chemical bonds on molecular core-levels and the other one dealing with nuclear dynamics induced by creation of a core hole. In our research we have mostly used synchrotron radiation based techniques such as X-ray Photoelectron (XPS), X-ray Absorption (XAS), normal and Resonant Auger (AES and RAS, respectively) and Energy-Selected Auger Electron PhotoIon COincidence (ES-AEPICO) spectroscopies. We have demonstrated that resonant Auger spectroscopy can be used to aid interpretation of the features observed in XAS for Rydberg structures in the case of Cl2 and C1s−1π*1 states of allene molecules. The combined use of high-resolution spectroscopy with ab initio calculations can help the interpretation of strongly overlapped spectral features and disentangle their complex profiles. This approach enabled us to determine the differences in the lifetimes for core-hole 2p sublevels of Cl2 which are caused by the presence of the chemical bond. We have shown that contribution in terms of the Mulliken population of valence molecular orbitals is a determining factor for resonant enhancement of different final states and fragmentation patterns reached after resonant Auger decays in N2O. We have also performed a systematic study of the dependence of the C1s resonant Auger kinetic energies on the presence of different substituents in CH3X compounds. For the first time we have studied possible isomerization reaction induced by core excitation of acetylacetone. We could observe a new spectral feature in the resonant Auger decay spectra which we interpreted as a signature of core-excitation-induced keto-enol tautomerism.

Ideal magnetohydrodynamics (MHD) simulations are known to have problems in satisfying the solenoidal constraint (i.e. the divergence of magnetic field should be equal to zero, $<p>ablacdotvec{B} = 0$). The simulations become unstable unless specific measures have been taken.<p><p>In this thesis, a solenoidal constraint satisfying technique that allows discrete satisfaction of the solenoidal constraint up to the machine accuracy is presented and validated with a variety of test cases. Due to its inspiration from Chorin's artificial compressibility method developed for incompressible CFD applications, the technique was named as \ / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished