Coherent strong-field control of multiple states by a single chirped femtosecond laser pulse

We present a joint experimental and theoretical study on strong-field photo-ionization of sodium atoms using chirped femtosecond laser pulses. By tuning the chirp parameter, selectivity among the population in the highly excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways enabling control are identified by simultaneous ionization and measurement of photoelectron angular distributions employing the velocity map imaging technique. Free electron wave packets at an energy of around 1 eV are observed. These photoelectrons originate from two channels. The predominant 2+1+1 Resonance Enhanced Multi-Photon Ionization (REMPI) proceeds via the strongly driven two-photon transition $4s\leftarrow\leftarrow3s$, and subsequent ionization from the states 5p, 6p and 7p whereas the second pathway involves 3+1 REMPI via the states 5f and 6f. In addition, electron wave packets from two-photon ionization of the non-resonant transiently populated state 3p are observed close to the ionization threshold. A mainly qualitative five-state model for the predominant excitation channel is studied theoretically to provide insights into the physical mechanisms at play. Our analysis shows that by tuning the chirp parameter the dynamics is effectively controlled by dynamic Stark-shifts and level crossings. In particular, we show that under the experimental conditions the passage through an uncommon three-state "bow-tie" level crossing allows the preparation of coherent superposition states

Embedded cluster approach to simulate single atom adsorption on surfaces: Cu on Cu surface

Within full relativistic four-component ab initio density functional calculations, we examined the adsorption of a Cu-adatom on a Cu(1 0 0)-surface. As a first step we simulated the surface by a cluster of atoms and increased the size successively up to nearly 100 atoms. We found that using more than 60 atoms causes no significant changes in adsorption energy and bond distance. In a second step we used an embedding approach where a relatively small cluster was embedded in different types of environments. With only 26 embedded Cu-atoms we were able to reproduce the converged values we had calculated before and which are in good agreement with other solid-state calculations

Embedding method to simulate single atom adsorption: Cu on Cu(100)

Within full relativistic four-component ab initio density functional calculations we examined the adsorption of a Cu adatom on a Cu(100)-surface. The surface was simulated by a cluster of Cu atoms in which the number of atoms was successively increased to 99 atoms. Through extensive studies we were able to get convergence in adsorption energy and bond distance with about 60 atoms. Using converged cluster sizes, the results of the binding characteristics are in good agreement with other solid-state calculations. The same adsorption process was then studied with much smaller clusters that were embedded into different types of environments. By this scheme we were able to reproduce the same converged results with a decreased cluster size of only about 25 embedded atoms

Embedding method to simulate single atom adsorption: Cu on Cu(100)

Within full relativistic four-component ab initio density functional calculations we examined the adsorption of a Cu adatom on a Cu(100)-surface. The surface was simulated by a cluster of Cu atoms in which the number of atoms was successively increased to 99 atoms. Through extensive studies we were able to get convergence in adsorption energy and bond distance with about 60 atoms. Using converged cluster sizes, the results of the binding characteristics are in good agreement with other solid-state calculations. The same adsorption process was then studied with much smaller clusters that were embedded into different types of environments. By this scheme we were able to reproduce the same converged results with a decreased cluster size of only about 25 embedded atoms

Adsorption of super-heavy elements on metal surfaces

The theoretical description of the adsorption of atoms on surfaces is still a big problem especially when the atoms involved are very heavy such that relativistic effects play an important role. During the last years we have developed a relativistic molecular program [1] which solves the relativistic Kohn-Sham equations with the use of various density functionals. We discuss here the adsorption of heavy elements on a Au(100) surface. The surface is simulated by different clusters in order to check at which position the ad-atom is adsorbed. Our main task here is to calculate the difference in the adsorption energies between the super-heavy element 112 and its homologue Hg