Transitions between quintet states of doubly excited four-electron ions

Following an earlier observation in F VI we identified the line pair 1s2s2p^2 {^5P}-1s2s2p3d {^5P^0} , {^5D^0} for the elements N, O, Mg, and tentatively for A1 and Si in beam-foil spectra. Assignment was established by comparison with Multi-Configuration Dirac-Fock calculations along the isoelectronic sequence. Using this method we also identified some quartet lines of lithium-like ions with Z > 10.

Atomistic-continuum modeling of ultrafast laser-induced melting of silicon targets

In this work, we present an atomistic-continuum model for simulations of ultrafast laser-induced melting processes in semiconductors on the example of silicon. The kinetics of transient non-equilibrium phase transition mechanisms is addressed with MD method on the atomic level, whereas the laser light absorption, strong generated electron-phonon nonequilibrium, fast heat conduction, and photo-excited free carrier diffusion are accounted for with a continuum TTM-like model (called nTTM). First, we independently consider the applications of nTTM and MD for the description of silicon, and then construct the combined MD-nTTM model. Its development and thorough testing is followed by a comprehensive computational study of fast nonequilibrium processes induced in silicon by an ultrashort laser irradiation. The new model allowed to investigate the effect of laser-induced pressure and temperature of the lattice on the melting kinetics. Two competing melting mechanisms, heterogeneous and homogeneous, were identified in our big-scale simulations. Apart from the classical heterogeneous melting mechanism, the nucleation of the liquid phase homogeneously inside the material significantly contributes to the melting process. The simulations showed, that due to the open diamond structure of the crystal, the laser-generated internal compressive stresses reduce the crystal stability against the homogeneous melting. Consequently, the latter can take a massive character within several picoseconds upon the laser heating. Due to the large negative volume of melting of silicon, the material contracts upon the phase transition, relaxes the compressive stresses, and the subsequent melting proceeds heterogeneously until the excess of thermal energy is consumed. A series of simulations for a range of absorbed fluences allowed us to find the threshold fluence value at which homogeneous liquid nucleation starts contributing to the classical heterogeneous propagation of the solid-liquid interface. A series of simulations for a range of the material thicknesses showed that the sample width we chosen in our simulations (800 nm) corresponds to a thick sample. Additionally, in order to support the main conclusions, the results were verified for a different interatomic potential. Possible improvements of the model to account for nonthermal effects are discussed and certain restrictions on the suitable interatomic potentials are found. As a first step towards the inclusion of these effects into MD-nTTM, we performed nanometer-scale MD simulations with a new interatomic potential, designed to reproduce ab initio calculations at the laser-induced electronic temperature of 18946 K. The simulations demonstrated that, similarly to thermal melting, nonthermal phase transition occurs through nucleation. A series of simulations showed that higher (lower) initial pressure reinforces (hinders) the creation and the growth of nonthermal liquid nuclei. For the example of Si, the laser melting kinetics of semiconductors was found to be noticeably different from that of metals with a face-centered cubic crystal structure. The results of this study, therefore, have important implications for interpretation of experimental data on the kinetics of melting process of semiconductors.

Equilibrium K-shell excitation of highly ionized neon

Augerelectron emission from foil-excited Ne-ions (6 to 10 MeV beam energy) has been measured. The beam-foil time-of-flight technique has been applied to study electronic transitions of metastable states (delayed spectra) and to determine their lifetimes. To achieve a line identification for the complex structure observed in the prompt spectrum, the spectrum is separated into its isoelectronic parts by an Augerelectron-ion coincidence correlating the emitted electrons and the emitting projectiles of well defined final charge states q_f. Well resolved spectra were obtained and the lines could be identified using intermediate coupling Dirac-Fock multiconfiguration calculations. From the total KLL-Augerelectron transition probabilities observed in the electronion coincidence experiment for Ne (10 MeV) the amount of projectiles with one K-hole just behind a C-target can be estimated. For foil-excited Ne-projectiles in contrast to single collision results the comparison of transition intensities for individual lines with calculated transition probabilities yields a statistical population of Li- and Be-like configurations.

Ab initio Molecular Dynamics Simulations of the Structural Response of Solids to Ultrashort Laser and XUV Pulses

The theoretical model and underlying physics described in this thesis are about the interaction of femtosecond-laser and XUV pulses with solids. The key to understand the basics of such interaction is to study the structural response of the materials after laser interaction. Depending on the laser characteristics, laser-solid interaction can result in a wide range of structural responses such as solid-solid phase transitions, vacuum phonon squeezing, ultrafast melting, generation of coherent phonons, etc. During my research work, I have modeled the systems irradiated by low-, medium- and high-laser intensities, and studied different types of structural dynamics of solids at various laser fluences.

Double photoionization of argon into the {3s3p^5} {^1P}- and {^3P}-states

Double photoionization of argon was studied by photon induced fluorescence spectroscopy (PIFS). Cross sections for the double photoionization into the {3s3p^5} {^1P}, {^3P} states of Ar^{+ +} are presented for exciting photon energies between threshold and 120 eV. In the threshold range the energy dependencies of these cross sections were determined for the first time. Singlet and triplet states are populated with comparable probabilities at equal excess energies, in contrast to predictions of the extended Wannier theory. At hv = 100eV the spin-or-bit splitting of the 3s3p^5 ^3P state was resolved, and a cross section for the production of Ar^{+ +} {3s^0}{3p^6 } {^1S_0} was determined for the first time.

On the transition from localized to delocalized electronic states in divalent-metal clusters

The transition from van der Waals to covalent bonding, which is expected to occur in divalent-metal clusters with increasing cluster size, is discussed. We propose a model which takes into account, within the same electronic theory, the three main competing contributions, namely the kinetic energy of the electrons, the Coulomb interactions between electrons, and the s \gdw p intraatomic transitions responsible for van der Waals like bonding. The model is solved by taking into account electron correlations using a generalized Gutzwiller approximation (slave boson method). The occurrence of electron localization is studied as a function of the interaction parameters and cluster size.

Photoionization of Kr near the 4s threshold: I. Experiment and LS coupling theory

Absolute cross sections for the transitions of the Kr atom into the 4s^1 and 4p^4nl states of the Kr^+ ion were measured in the 4s-electron threshold region by photon-induced fluorescence spectroscopy (PIFS). The cross sections for the transitions of the Kr atom into the 4s^1 and 4p^4nl states were also calculated, as well as the 4p^4nln'l' doubly excited states, in the frame of LS-coupling many-body technique. The cross sections of the doubly-excited atomic states were used to illustrate the pronounced contributions of the latter to the photoionization process, evident from the measurements. The comparison of theory and experiment led to conclusions about the origin of the main features observed in the experiment.

Adsorption of element 112 on a Au surface

Während der letzten 20 Jahre hat sich das Periodensystem bis zu den Elementen 114 und 116 erweitert. Diese sind kernphysikalisch nachgewiesen, so dass jetzt die chemische Untersuchung an erster Selle steht. Nachdem sich das Periodensystem bis zum Element 108 so verhält, wie man es dem Periodensystem nach annimmt, wird in dieser Arbeit die Chemie des Elements 112 untersucht. Dabei geht es um die Adsorptionsenergie auf einer Gold-Ober fläche, weil dies der physikalisch/chemische Prozess ist, der bei der Analyse angewandt wird. Die Methode, die in dieser Arbeit angwandt wird, ist die relativistische Dichtefunktionalmethode. Im ersten Teil wird das Vielkörperproblem in allgemeiner Form behandelt, und im zweiten die grundlegenden Eigenschaften und Formulierungen der Dichtefunktionaltheorie. Die Arbeit beschreibt zwei prinzipiell unterschiedliche Ansätze, wie die Adsorptionsenergie berechnet werden kann. Zum einen ist es die sogenannte Clustermethode, bei der ein Atom auf ein relativ kleines Cluster aufgebracht und dessen Adsorptionsenergie berechnet wird. Wenn es gelingt, die Konvergenz mit der Größe des Clusters zu erreichen, sollte dies zu einem Wert für die Adsorptionsenergie führen. Leider zeigt sich in den Rechnungen, dass aufgrund des zeitlichen Aufwandes die Konvergenz für die Clusterrechnungen nicht erreicht wird. Es werden sehr ausführlich die drei verschiedenen Adsorptionsplätze, die Top-, die Brücken- und die Muldenposition, berechnet. Sehr viel mehr Erfolg erzielt man mit der Einbettungsmethode, bei der ein kleiner Cluster von vielen weiteren Atomen an den Positionen, die sie im Festkörpers auf die Adsorptionsenergie soweit sichergestellt ist, dass physikalisch-chemisch gute Ergebnisse erzielt werden. Alle hier gennanten Rechnungen sowohl mit der Cluster- wie mit der Einbettungsmethode verlangen sehr, sehr lange Rechenzeiten, die, wie oben bereits erwähnt, nicht zu einer Konvergenz für die Clusterrechnungen ausreichten. In der Arbeit wird bei allen Rechnungen sehr detailliert auf die Abhängigkeit von den möglichen Basissätzen eingegangen, die ebenfalls in entscheidender Weise zur Länge und Qualität der Rechnungen beitragen. Die auskonvergierten Rechnungen werden in der Form von Potentialkurven, Density of States (DOS), Overlap Populations sowie Partial Crystal Overlap Populations analysiert. Im Ergebnis zeigt sich, dass die Adsoptionsenergie für das Element 112 auf einer Goldoberfläche ca. 0.2 eV niedriger ist als die Adsorption von Quecksilber auf der gleichen Ober fläche. Mit diesem Ergebnis haben die experimentellen Kernchemiker einen Wert an der Hand, mit dem sie eine Anhaltspunkt haben, wo sie bei den Messungen die wenigen zu erwartenden Ereignisse finden können.

Theory for the optical properties of small Hg_n clusters

The static and dynamical polarizabilities of the Hg-dimer are calculated by using a Hubbard Hamiltonian to describe the electronic structure. The Hamiltonian is diagonalized exactly within a subspace of second-quantized electronic states from which only multiply ionized atomic configurations have been excluded. With this approximation we can describe the most important electronic transitions including the effect of charge fluctuations. We analyze the polarizability as a function of the intraatomic Coulomb interaction which represents the repulsion between electrons. We obtain that this interaction results in strong electronic correlations in the excited states and increases the first excitation energy of the dimer by 0.8 eV in comparison to a calculation which neglects correlations, resulting in a better agreement with the experiment.

Study of 3d - 4f and 3d - 5f quartet and doublet transitions of doubly excited three-electron ions

We present the first observation of optical transitions between doubly excited doublet states in the term systems N V, 0 VI and F VII. The spectra were produced by foil excitation of fast ion beams. The assignment of the spectral lines was made by comparison with the results of MCDP calculations along the isoelectronic sequence. The same method also led to the identification of two 3d - 4f quartet transitions in Mg X.

State-dependent volume isotope shifts of low-lying states of group-IIa and -IIb elements

Results of relativistic multiconfiguration Dirac-Fock calculations with an extended nucleus are used to analyze the volume isotope shifts of the resonance transitions in the group-IIa and -IIb elements as well as in Yb. This is done together with a review of the isotope shift theory, including a critical evaluation and comparison of the semiempirical calculation of volume isotope shifts commonly used today. Electronic factors F_i, proportional to differences of electronic densities over the nuclear volume, are discussed within various approximations and compared with experimental results.

First-principles electronic theory of non-collinear magnetic order in transition-metal nanowires

The structural, electronic and magnetic properties of one-dimensional 3d transition-metal (TM) monoatomic chains having linear, zigzag and ladder geometries are investigated in the frame-work of first-principles density-functional theory. The stability of long-range magnetic order along the nanowires is determined by computing the corresponding frozen-magnon dispersion relations as a function of the 'spin-wave' vector q. First, we show that the ground-state magnetic orders of V, Mn and Fe linear chains at the equilibrium interatomic distances are non-collinear (NC) spin-density waves (SDWs) with characteristic equilibrium wave vectors q that depend on the composition and interatomic distance. The electronic and magnetic properties of these novel spin-spiral structures are discussed from a local perspective by analyzing the spin-polarized electronic densities of states, the local magnetic moments and the spin-density distributions for representative values q. Second, we investigate the stability of NC spin arrangements in Fe zigzag chains and ladders. We find that the non-collinear SDWs are remarkably stable in the biatomic chains (square ladder), whereas ferromagnetic order (q =0) dominates in zigzag chains (triangular ladders). The different magnetic structures are interpreted in terms of the corresponding effective exchange interactions J(ij) between the local magnetic moments μ(i) and μ(j) at atoms i and j. The effective couplings are derived by fitting a classical Heisenberg model to the ab initio magnon dispersion relations. In addition they are analyzed in the framework of general magnetic phase diagrams having arbitrary first, second, and third nearest-neighbor (NN) interactions J(ij). The effect of external electric fields (EFs) on the stability of NC magnetic order has been quantified for representative monoatomic free-standing and deposited chains. We find that an external EF, which is applied perpendicular to the chains, favors non-collinear order in V chains, whereas it stabilizes the ferromagnetic (FM) order in Fe chains. Moreover, our calculations reveal a change in the magnetic order of V chains deposited on the Cu(110) surface in the presence of external EFs. In this case the NC spiral order, which was unstable in the absence of EF, becomes the most favorable one when perpendicular fields of the order of 0.1 V/Å are applied. As a final application of the theory we study the magnetic interactions within monoatomic TM chains deposited on graphene sheets. One observes that even weak chain substrate hybridizations can modify the magnetic order. Mn and Fe chains show incommensurable NC spin configurations. Remarkably, V chains show a transition from a spiral magnetic order in the freestanding geometry to FM order when they are deposited on a graphene sheet. Some TM-terminated zigzag graphene-nanoribbons, for example V and Fe terminated nanoribbons, also show NC spin configurations. Finally, the magnetic anisotropy energies (MAEs) of TM chains on graphene are investigated. It is shown that Co and Fe chains exhibit significant MAEs and orbital magnetic moments with in-plane easy magnetization axis. The remarkable changes in the magnetic properties of chains on graphene are correlated to charge transfers from the TMs to NN carbon atoms. Goals and limitations of this study and the resulting perspectives of future investigations are discussed.