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We demonstrate the photoassociation of ultracold rubidium dimers using coherent femtosecond pulses. Starting from a cloud of ultracold rubidium atoms, electronically excited rubidium molecules are formed with shaped photoassociation pump pulses. The excited state molecules are projected with a time-delayed probe pulse onto molecular ion states which are detected in a mass spectrometer. Coherent transient oscillations of the excited state population are observed in the wings of the pump pulse, in agreement with the time-dependent solution of the Schrödinger equation of the excitation process.

We report on an element-dependent critical size for argon physisorption at 80 K on transition-metal-doped silicon clusters. Argon does not attach to elemental silicon clusters but only to surface-located transition-metal atoms. Thus physisorption provides structural information. Specifically, the minimal cluster size for the formation of endohedral singly metal-doped silicon cages has been determined. The observed critical size for doubly doped silicon clusters indicates that larger caged molecules can be formed, eventually leading to the growth of metal-doped silicon nanorods.

The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted (V(2)O(5))(n-1)(VTiO(5))(-) (n=2-4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral (V(2)O(5))(n) clusters. Upon systematic exchange of V by Ti atoms in V(4-n)Ti(n)O(-)(10) (n=1-4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n-1) for the Ti-rich systems, leading to a quartet ground state for Ti(4)O(-)(10).

The filamentation of ultrashort pulses in air is investigated theoretically and experimentally. From the theoretical point of view, beam propagation is shown to be driven by the interplay between random nucleation of small-scale cells and relaxation to long waveguides. After a transient stage along which they vary in location and in amplitude, filaments triggered by an isotropic noise are confined into distinct clusters, called "optical pillars," whose evolution can be approximated by an averaged-in-time two-dimensional (2D) model derived from the standard propagation equations for ultrashort pulses. Results from this model are compared with space- and time-resolved numerical simulations. From the experimental point of view, similar clusters of filaments emerge from the defects of initial beam profiles delivered by the Teramobile laser facility. Qualitative features in the evolution of the filament patterns are reproduced by the 2D reduced model.

We report on selective optimization of different isotopes in an ionization process by means of spectrally broad shaped fs-laser pulses. This is demonstrated for (39,39)K2 and (39,41)K2 by applying evolution strategies in a feedback loop, whereby a surprisingly high enhancement of one isotope versus the other and vice versa is achieved (total factor approximately 140). Information about the dynamics on the involved vibrational states is extracted from the optimal pulse shapes, which provides a new spectroscopical approach of yielding distinct frequency pattern on fs-time scales. The method should, in principle, be feasible for all molecules.

The filamentation of femtosecond light pulses in air is numerically and experimentally investigated for beam powers reaching several TW. Beam propagation is shown to be driven by the interplay between intense, robust spikes created by the defects of the input beam and random nucleation of light cells. Evolution of the filament patterns can be qualitatively reproduced by an averaged-in-time (2D+1)-dimensional model derived from the propagation equations for ultrashort pulses.

Ultrashort, high-power laser pulses propagating vertically in the atmosphere have been observed over more than 20 km using an imaging 2-m astronomical telescope. This direct observation in several wavelength bands shows indications for filament formation at distances as far as 2 km in the atmosphere. Moreover, the beam divergence at 5 km altitude is smaller than expected, bearing evidence for whole-beam parallelization about the nonlinear focus. We discuss implications for white-light Lidar applications.

We present two-color fs pump-probe spectra of Na2F which were recorded by employing excitation wavelengths around 1208 nm (pump) and ionization wavelengths around 405 nm (probe). The observed oscillatory structure of the signal with a period of 185 fs shows an excellent agreement with our simulated spectra. The employed ab initio Wigner distribution approach provides clear evidence that this observation is caused by photoinduced metal bond breaking followed by a butterfly-type periodic geometric rearrangement.