L. Wöste

Triggering and guiding megavolt discharges by use of laser-induced ionized filaments

We have demonstrated the ability to trigger and guide high-voltage discharges with ionized filaments generated by femtosecond terawatt laser pulses. The plasma filaments extended over the whole gap, providing a direct ohmic connection between the electrodes. Laser-guided straight discharges have been observed for gaps of as much as 3.8 m at a high voltage reduced to 68% of the natural breakdown voltage. The triggering efficiency was found to depend critically on the spatial connection of the laser filaments to the electrode as well as on the temporal coincidence of the laser with the peak of the high voltage.

Infrared extension of the supercontinuum generated by femtosecond terawatt laser pulses propagating in the atmosphere

We investigated the spectral behavior of a white-light continuum generated in air by 2-TW femtosecond laser pulses at 800 nm. The spectrum extends at least from 300 nm to 4.5 mum. From 1 to 1.6 mum the continuums intensity increases strongly with the laser energy and depends on the initial chirp.

Long-distance remote laser-induced breakdown spectroscopy using filamentation in air

We demonstrate remote elemental analysis at distances up to 90m, using a laser-induced breakdown spectroscopy scheme based on filamentation induced by the nonlinear propagation of unfocused ultrashort laser pulses. A detailed signal analysis suggests that this technique, remote filament-induced breakdown spectroscopy, can be extended up to the kilometer range.

Homogeneous nucleation rates of supercooled water measured in single levitated microdroplets

Homogeneous nucleation rates are determined for micrometer sized water droplets levitated inside an electrodynamic Paul-trap. The size of a single droplet is continuously measured by analyzing the angle-resolved light scattering pattern of the droplets with classical Mie theory. The freezing process is detected by a pronounced increase in the depolarization of the scattered light. By statistical analysis of the freezing process of some thousand individual droplets, we obtained the homogeneous nucleation rate of water between 236 and 237 K. The values are in agreement with former expansion cloud chamber measurements but could be determined with considerably higher precision. The measurements are discussed in the light of classical nucleation theory in order to obtain the size and the formation energy of the critical nucleus.

Evolution of the electronic structure of lithium clusters between four and eight atoms

Absorption spectra of lithium clusters containing four to eight atoms have been measured using depletion spectroscopy. Few intense transitions are observed, always located in two predominant spectral regions, ∼480 and 680 nm. The spectra are interpreted using ab initio configuration interaction (CI) calculations, leading to a complete characterization of the excited states and a straightforward determination of the ground state geometrical structure. Intense transitions are explained by interference effects in the transition amplitude and symmetry considerations. Comparisons with semiclassical models, in which an effective mass correction is introduced, are also presented.

Optical spectroscopy of Na3 by two-photon ionization in a supersonic molecular beam

Abstract We report the first measurements of the electronic absorption spectrum of a metal cluster under collision-free gas-phase conditions. Na 3 molecules were produced in a supersonic molecular beam and mass-selective absorption spectroscopy was performed in the visible wavelength region (405–690 nm) by two-photon ionization together with mass-spectrometric detection. Two band systems were found at 625 and 670 nm. The experimental results are compared with recent theoretical calculations.

Remote LIBS with ultrashort pulses: characteristics in picosecond and femtosecond regimes

Using a container-integrated mobile femtosecond terawatt laser system with integrated detection unit (Teramobile), we have demonstrated remote laser-induced breakdown spectroscopy (R-LIBS) on copper and aluminium samples with targets located at 25 m away from the container. The ability of our laser system to generate pulses in the femtosecond, picosecond and nanosecond regimes allowed us to perform direct comparisons between these three pulse durations. The dependence of the fluorescence signal on laser pulse energy showed a nonlinear behavior with a threshold, which is consistent with the previous observations for laser ablation. Such nonlinear behavior leads to a dependence of the LIBS signal on the temporal-spectral shape of the laser pulse. We showed especially that the transform-limited pulse does not optimize the fluorescence. A properly applied chirp allows an increase of the LIBS signal. Understanding and optimization of the chirp effect would improve the detection limit of the LIBS using a femtosecond laser (Femto-LIBS) and lead to a larger detection distance. Furthermore the use of pulse shaping should enhance the detection specificity for the cases of spectral overlapping between several elements to be identified.

Coadsorption of CO and O2 on small free gold cluster anions at cryogenic temperatures: Model complexes for catalytic CO oxidation

The cluster ions Aun(CO)m(O2)u− (n = 2,3; m = 1, u = 1,2) are successfully synthesized via coadsorption of oxygen and carbon monoxide onto mass-selected gold clusters inside a temperature controlled ion trap at 100 K. The investigated gold clusters Au1–3− show a significant size dependence in the reactivity toward oxygen and carbon monoxide. Whereas no reaction products are detected for the monomer ion, simultaneous adsorption of CO and O2 onto the dimer leads to the formation of Au2(CO)(O2)− which is regarded as a model complex to study the mechanism of catalytic CO oxidation on gold clusters. In the case of Au3− the conditioning of the cluster by CO preadsorption is found essential to enable O2 coadsorption.

Feedback optimization of shaped femtosecond laser pulses for controlling the wavepacket dynamics and reactivity of mixed alkaline clusters

Abstract In this paper we describe the branching control of different ionization and fragmentation channels of coherently excited Na 2 K by means of feedback optimization of shaped femtosecond laser pulses. For this purpose the system was first excited with one photon into a pair of intersecting electronic states, from where it was ionized in a two-photonic process, so the resulting ions could size selectively be detected. By employing an evolutionary algorithm for optimizing phase and amplitude of the applied laser field, the relative signal intensities of the resulting mother and fragment ions could significantly be influenced. The peak intensities in the obtained pulse shapes correspond very well to the cross sections of the irradiated transitions; their temporal structure reflects perfectly the wavepacket dynamics of the resulting particles. Hence it was possible to extract with the self-learning algorithm – as postulated by Judson and Rabitz [Phys. Rev. Lett. 68 (1992) 1500] – intrinsic properties from the reactive system in real time.

Gas phase infrared spectroscopy of mono- and divanadium oxide cluster cations

The vibrational spectroscopy of the mono- and divanadium oxide cluster cations VO(1-3)+ and V2O(2-6)+ is studied in the region from 600 to 1600 wave numbers by infrared photodissociation of the corresponding cluster cation-helium atom complexes. The comparison of the experimental depletion spectra with the results of density functional calculations on bare vanadium oxide cluster cations allows for an unambiguous identification of the cluster geometry in most cases and, for VO(1-3)+ and V2O(5,6)+, also of the electronic ground state. A common structural motif of all the studied divanadium cluster cations is a four-membered V-O-V-O ring, with three characteristic absorption bands in the 550-900 wave number region. For the V-O-V and V=O stretch modes the relationship between vibrational frequencies and V-O bond distances follows the Badger rule.