Karl-Heinz Meiwes-Broer

Laser-driven nonlinear cluster dynamics

Laser excitation of nanometer-sized atomic and molecular clusters offers various opportunities to explore and control ultrafast many-particle dynamics. Whereas weak laser fields allow the analysis of photoionization, excited-state relaxation, and structural modifications on these finite quantum systems, large-amplitude collective electron motion and Coulomb explosion can be induced with intense laser pulses. This review provides an overview of key phenomena arising from laser-cluster interactions with focus on nonlinear optical excitations and discusses the underlying processes according to the current understanding. A general survey covers basic cluster properties and excitation mechanisms relevant for laser-driven cluster dynamics. Then, after an excursion in theoretical and experimental methods, results for single-photon and multiphoton excitations are reviewed with emphasis on signatures from time- and angular-resolved photoemission. A key issue of this review is the broad spectrum of phenomena arising from clusters exposed to strong fields, where the interaction with the laser pulse creates short-lived and dense nanoplasmas. The implications for technical developments such as the controlled generation of ion, electron, and radiation pulses will be addressed along with corresponding examples. Finally, future prospects of laser-cluster research as well as experimental and theoretical challenges are discussed.

Photoelectron spectroscopy of silver and palladium cluster anions : electron delocalization versus localization

Photoelectrons (PE) from jet-cooled mass-identified silver and palladium cluster anions (number of atoms, n⩽ 21) were detached by UV laser light and energy-analysed in a time-of-flight (TOF) electron spectrometer of the magnetic-bottle type. For palladium PE threshold energies smoothly increase with n; for Ag, they show clear evidence of shell effects as well as an ‘even–odd oscillation’. The PE energy spectra are strongly structured, the structures being attributed to transitions involving the neutral ground states as well as contributions of low-lying excited neutral states. For silver the results can, in part, be qualitatively understood in terms of a delocalized electron Fermi-gas within the ellipsoidal deformed cluster. This picture fails for the more localized d electrons of palladium. For a thorough interpretation more elaborate calculations are necessary. The first results are available for alkali-metal clusters and will be compared to the silver and copper data.

Ultraviolet photodetachment spectroscopy on jet-cooled metal-cluster anions

Metal-cluster anions have been produced by laser vaporization and cooled in a supersonic expansion, without converting neutral clusters into anions via a secondary ionizing agent. After mass-selection, photoelectrons were detached (hν= 3.68 eV) from defined anion bunches and analysed in a magnetic bottle-type time-of-flight electron spectrometer. The resulting photoelectron distributions from Al–n, Ni–n, Ag–n and Sn–n, n < 22, are compared; all spectra display a dramatic dependence on the cluster size and differ substantially from the respective bulk spectra. The measurements provide information about low-lying excited electronic structures which are missing in the solid metal. Photoelectron thresholds as estimates of electron affinities in part follow a simple electrostatic model.

Spectroscopy on rare gas–doped silver clusters in helium droplets

The optical spectrum of Ag8 in a helium droplet, first measured by Federmann et al. [Eur. Phys. J. D 9, 11 (1999)], is studied over a broad wavelength range (237–450 nm) by resonant two photon ionization. A strong resonance is found in accordance to recent ab initio calculations. Doping the droplet additionally with rare gas atoms leads to a shift of the mean resonance position, which depends on the type and the number of attached atoms. In contrast to the red-shift obtained for argon, krypton, and xenon, for neon a net shift of the resonance to shorter wavelengths is observed. The dosage-dependence of the displacements will qualitatively be explained.

Structure and magnetic moments of mass-filtered deposited nanoparticles

Mass-filtered 3d transition metal nanoparticles have been produced by means of an ultrahigh vacuum compatible arc cluster ion source. High resolution transmission electron microscopy images of individual Fe, Co, and FeCo alloy particles with diameters of about 12 nm reveal the crystalline structure of the nanoparticles. X-ray absorption spectroscopy confirms the purity of the particles after in situ deposition. Analysis of the x-ray magnetic circular dichroism reveals bulklike total magnetic moments in all cases and strongly enhanced orbital moments for the iron nanoparticles. Furthermore, the data hint at a chemically ordered alloy in the case of FeCo particles.

Scanning tunneling spectroscopy on deposited platinum clusters

Abstract We report on investigations of platinum clusters with about five to 400 atoms that are deposited from a cluster beam on to crystalline graphite surfaces. Scanning tunneling spectroscopy in an UHV environment at liquid helium temperatures reveals the existence of distinct peaks in the conductivity of the cluster-on-surface system. For negative voltages, the peak separations scale with the inverse particle height, which hints at a quantum size effect in these metallic “quantum dots”.

Combined temperature-programmed reaction and in situ x-ray scattering studies of size-selected silver clusters under realistic reaction conditions in the epoxidation of propene

The catalytic activity and dynamical shape changes in size-selected nanoclusters at work are studied under realistic reaction conditions by using a combination of simultaneous temperature-programmed reaction with in situ grazing-incidence small angle x-ray scattering. This approach allows drawing a direct correlation between nanocatalyst size, composition, shape, and its function under realistic reaction conditions for the first time. The approach is illustrated in a chemical industry highly relevant selective partial oxidation of propene on a monodisperse silver nanocatalyst. The shape of the catalyst undergoes rapid change already at room temperature upon the exposure to the reactants, followed by a complex evolution of shape with increasing temperature. Acrolein formation is observed around 50 degrees C while the formation of the propylene oxide exhibits a sharp onset at 80 degrees C and is leveling off at 150 degrees C. At lower temperatures acrolein is produced preferentially to propylene oxide; at temperatures above 100 degrees C propylene oxide is favored.

The 3D-architecture of individual free silver nanoparticles captured by X-ray scattering

The diversity of nanoparticle shapes generated by condensation from gaseous matter reflects the fundamental competition between thermodynamic equilibration and the persistence of metastable configurations during growth. In the kinetically limited regime, intermediate geometries that are favoured only in early formation stages can be imprinted in the finally observed ensemble of differently structured specimens. Here we demonstrate that single-shot wide-angle scattering of femtosecond soft X-ray free-electron laser pulses allows three-dimensional characterization of the resulting metastable nanoparticle structures. For individual free silver particles, which can be considered frozen in space for the duration of photon exposure, both shape and orientation are uncovered from measured scattering images. We identify regular shapes, including species with fivefold symmetry and surprisingly large aspect ratio up to particle radii of the order of 100 nm. Our approach includes scattering effects beyond Born’s approximation and is remarkably efficient—opening up new routes in ultrafast nanophysics and free-electron laser science.

Ion induced snowballs as a diagnostic tool to investigate the caging of metal clusters in large helium droplets

Metal clusters embedded in ultracold helium nanodroplets are exposed to femtosecond laser pulses with intensities of 10(13)-10(14) W/cm2. The influence of the matrix on the ionization and fragmentation dynamics is studied by pump-probe time-of-flight mass spectrometry. Special attention is paid to the generation of helium snowballs around positive metal ions (Me(z+)He(N), z=1,2). Closings of the first and second helium shells are found for silver at N(1)=10,12 and N(2)=32,44, as well as for magnesium at N1=19-20. The distinct abundance enhancement of helium snowballs in the presence of isolated atoms and small clusters in the droplets is used as a diagnostics to explore the cage effect. For silver, a reaggregation of the clusters is observed at 30 ps after femtosecond laser excitation.

Thomson scattering from near-solid density plasmas using soft x-ray free electron lasers

We propose a collective Thomson scattering experiment at the VUV free electron laser facility at DESY (FLASH) which aims to diagnose warm dense matter at near-solid density. The plasma region of interest marks the transition from an ideal plasma to a correlated and degenerate many-particle system and is of current interest, e.g. in ICF experiments or laboratory astrophysics. Plasma diagnostic of such plasmas is a longstanding issue. The collective electron plasma mode (plasmon) is revealed in a pump-probe scattering experiment using the high-brilliant radiation to probe the plasma. The distinctive scattering features allow to infer basic plasma properties. For plasmas in thermal equilibrium the electron density and temperature is determined from scattering off the plasmon mode.