Jan-Michael Rost

Ionization of Clusters in Intense Laser Pulses through Collective Electron Dynamics

The motion of electrons and ions in medium-sized rare gas clusters ( approximately 1000 atoms) exposed to intense laser pulses is studied microscopically by means of classical molecular dynamics using a hierarchical tree code. Pulse parameters for optimum ionization are found to be wavelength dependent. This resonant behavior is traced back to a collective electron oscillation inside the charged cluster. It is shown that this dynamics can be well described by a driven and damped harmonic oscillator allowing for a clear discrimination against other energy absorption mechanisms.

Small Rare-Gas Clusters in Soft X-Ray Pulses

We develop a microscopic model for the interaction of small rare-gas clusters with soft x-ray radiation from a free electron laser. It is shown that, while the overall charging of the clusters is rather low, unexpectedly high atomic charge states can arise due to charge imbalances inside the cluster. These findings are explained by an increased absorption via inverse bremsstrahlung due to high intermediate charge states and by a nonhomogenous charge distribution inside the cluster.

Separation and identification of dominant mechanisms in double photoionization

Double photoionization by a single photon is often discussed in terms of two contributing mechanisms, knockout (two-step-one) and shakeoff, with the latter being a pure quantum effect. It is shown that a quasiclassical description of knockout and a simple quantum calculation of shakeoff provides a clear separation of the mechanisms and facilitates their calculation considerably. The relevance of each mechanism at different photon energies is quantified for helium. Photoionization ratios, integral, and singly differential cross sections obtained by us are in excellent agreement with benchmark experimental data and recent theoretical results.

Ionization of clusters in strong x-ray laser pulses.

The effect of intense x-ray laser interaction on argon clusters is studied theoretically with a mixed quantum/classical approach. In comparison to a single atom we find that ionization of the cluster is suppressed, which is in striking contrast to the observed behavior of rare-gas clusters in intense optical laser pulses. We have identified two effects responsible for this phenomenon: A high space charge of the cluster in combination with a small quiver amplitude and delocalization of electrons in the cluster. We elucidate their impact for different field strengths and cluster sizes.

Fast electrons from multi-electron dynamics in xenon clusters induced by inner-shell ionization

Fast electrons emitted from xenon clusters in strong femtosecond 90 eV pulses have been measured at the Free-electron Laser in Hamburg (FLASH). Energy absorption occurs mainly through atomic inner-shell photo-ionization. Photo-electrons are trapped in the strong Coulomb potential of the cluster ions and form a non-equilibrium plasma with supra-atomic density. Its equilibration through multiple energy-exchanging collisions within the entire cluster volume produces electrons with energies well beyond the dominant emission line of atomic xenon. Here, in contrast to traditional low-frequency laser plasma heating, the plasma gains energy from electrons delivered through massive single-photon excitation from bound states. Electron emission induced by thermalization of a non-equilibrium plasma is expected to be a general phenomenon occurring for strong atomic x-ray absorption in extended systems.

Diffraction effects in the photoionization of clusters

Abstract In many atomic or molecular clusters an electron cloud exists which is delocalized over a volume of well defined shape. It is shown that the total and partial photoionization cross sections of those clusters oscillate on a scale of energy typically reached by synchrotron radiation. The frequencies of the oscillations are rrlated to geometrical properties of the electron cloud, such as its thickness and the diameter of the cluster. These properties can in principle be extracted from the experimental photo cross section.. As specific examples we discuss an alkali-metal cluster (Na 40 ) and the fullerene C 60 .

Enhanced ionization in small rare-gas clusters

A detailed theoretical investigation of rare-gas atom clusters under intense short laser pulses reveals that the mechanism of energy absorption is akin to enhanced ionization first discovered for diatomic molecules. The phenomenon is robust under changes of the atomic element (neon, argon, krypton, xenon), the number of atoms in the cluster (16-30 atoms have been studied), and the fluence of the laser pulse. In contrast to molecules it does not disappear for circular polarization. We develop an analytical model relating the pulse length for maximum ionization to characteristic parameters of the cluster.

Massively parallel ionization of extended atomic systems.

Massively parallel ionization of many atoms in a cluster or bio-molecule is identified as new phenomenon of light-matter interaction which becomes feasible through short and intense FEL pulses. Almost simultaneously emitted from the illuminated target the photo-electrons can have such a high density that they interact substantially even after photoionization. This interaction results in a characteristic electron spectrum which can be interpreted as convolution of a mean-field electron dynamics and binary electron-electron collisions. We demonstrate that this universal spectrum can be obtained analytically by summing synthetic two-body Coulomb collision events. Moreover, we propose an experiment with hydrogen clusters to observe massively parallel ionization.

Semiclassical S-matrix theory for atomic fragmentation

Abstract A semiclassical scattering approach is developed which can handle long-range (Coulomb) forces without the knowledge of the asymptotic wave function for multiple charged fragments in the continuum. The classical cross section for potential and inelastic scattering including fragmentation (ionization) is derived from first principles in a form which allows for a simple extension to semiclassical scattering amplitudes as a sum over classical orbits and their associated actions. The object of primary importance is the classical deflection function which can show regular and chaotic behavior. Applications to electron impact ionization of hydrogen and electron–atom scattering in general are discussed in a reduced phase space, motivated by partial fixed points of the respective scattering systems. Special emphasis, also in connection with chaotic scattering, is put on threshold ionization. Finally, motivated by the reflection principle for molecules, a semiclassical hybrid approach is introduced for photoabsorption cross sections of atoms where the time-dependent propagator is approximated semiclassically in a short-time limit with the Baker–Hausdorff formula. Applications to one- and two-electron atoms are followed by a presentation of double photoionization of helium, treated in combination with the semiclassical S-matrix for scattering.

Collective energy absorption of ultracold plasmas through electronic edge-modes

We investigate the collective dynamics of electrons in ultracold neutral plasmas driven by an oscillating radio-frequency field. We point out the importance of a sharp density drop at the plasma boundary that arises due to unavoidable charge imbalances, and show that this plasma edge provides the major mechanism for energy absorption from the external field. Using a cold fluid theory, we derive the corresponding absorption frequency and validate our findings by microscopic molecular dynamics simulations. The proposed edge-mode is shown to provide a consistent explanation for the observed absorption spectra measured in different experiments. Understanding the response of the electronic plasma component to weak external driving is essential since it grants experimental access to the time-evolving density and temperature of ultracold plasmas.