K. Hoffmann

Observation of shells in Coulomb explosions of rare-gas clusters

The explosions of noble gas clusters from argon and xenon irradiated by intense 35-fs infrared laser pulses have been studied. The kinetic energy spectra of ions produced in small clusters (<700 atoms) show a two-mode shell structure that is attributed to originating from a radial charge distribution. With a simple classical particle simulation of Coulomb explosions, the energy structure was reproduced using information on the arrangement of charge in the cluster. It was found that, during the explosion, the inner atoms of the clusters were less ionized than the outer atoms.

Fragmentation of clusters and recombination induced by intense and ultrashort x-ray laser pulses

Understanding the ultrafast dynamics of matter under extreme conditions is relevant for structural studies and plasma physics with X-ray lasers. We used the pulses from free-electron lasers (FLASH in Hamburg and LCLS in Stanford) to trigger X-ray induced explosions in atomic atoms (Xe) and molecular clusters (CH4 and CD4). The explosion dynamics depends on cluster size and the intensity of the X-ray pulse, and a transition from Coulomb explosion to hydrodynamic expansion is expected with increasing size and increasing pulse intensity. In methane clusters experiments at FLASH, the time-of-flight spectrometry shows the appearance of molecular adducts which are the result of molecular recombination between ions and molecules. The recombination depends on the cluster size and the expansion mechanism and becomes significant in larger clusters. In Xenon cluster experiments at the LCLS, measurements of the ion charge states in clusters suggest a formation of Xe nanoplasma which expands hydrodynamically. The dominance of low charge states of Xe is due to three-body recombination processes involving electron and Xe ions, and it depends on the X-ray intensity and nanoplasma formation.

Explosions of clusters in intense X-Ray pulses

Cluster explosion in ultrashort intense X-ray laser fields have been studied in first experiments at the LCLS with time-of flight techniques. Ion charge states and kinetic energy spectra indicate hydrodynamic and Coulombic plasma expansion contributions.

High Intensity Femtosecond XUV Pulse Interactions with Atomic Clusters

The interactions of large xenon clusters irradiated by intense, femtosecond extreme‐ultraviolet pulses at a wavelength of 38 nm have been studied. Using high harmonic generation from a 35 fs near‐infrared terawatt laser, clusters have been irradiated by XUV pulses of 1011 W/cm2 intensity. Charge states up to Xe8+ are observed, states well above that produced by single atom illumination, indicating that plasma continuum lowering is important. Furthermore the kinetic energy distribution of the exploding ions is consistent with a quasineutral hydrodynamic expansion, rather than a Coulomb explosion.

Tuning the absorption and emission of CdSe and ZnS core-shell nanoparticles by laser radiation

CdSe and ZnS core-shell nanoparticles made by LAM (Laser Ablation of Microparticles) show photoluminesence (PL) peaks in a region of wavelengths around 400 nm. Control over the size and PL peak position is obtained by irradiating the nanoparticles multiple times. In LAM, micropaticle powder passes through an aerosol generator and then into a laser ablation glass cell, where a laser pulse (high energy excimer laser) ablates the microparticle aerosol. Nanoparticles are formed after condensation. At this stage the nanoparticles can be covered with a second material or irradiated multiple times to change their size. The size distribution of these particles is successfully investigated with TEM (Transmission Electron Microscopy). PL blue shifts are seen as the mean size decreases.