The interaction of intense extreme ultraviolet femtosecond laser pulses ( 32:8 nm) from the FLASH free electron laser (FEL) with clusters has been investigated by means of photoelectron spectroscopy and modeled by Monte Carlo simulations. For laser intensities up to 5 10 13 W=cm 2 , we find that the cluster ionization process is a sequence of direct electron emission events in a developing Coulomb field. A nanoplasma is formed only at the highest investigated power densities where ionization is frustrated due to the deep cluster potential. In contrast with earlier studies in the IR and vacuum ultraviolet spectral regime, we find no evidence for electron emission from plasma heating processes.

We report differential measurements of Ar++ ion momentum distributions from nonsequential double ionization in phase-stabilized few-cycle laser pulses. The distributions depend strongly on the carrier-envelope (CE) phase. Via control over the CE phase one is able to direct the nonsequential double-ionization dynamics. Data analysis through a classical model calculation reveals that the influence of the optical phase enters via (i) the cycle dependent electric field ionization rate, (ii) the electron recollision time, and (iii) the accessible phase space for inelastic collisions. Our model indicates that the combination of these effects allows a look into single cycle dynamics already for few-cycle pulses.

Abstract. We use correlated electron-ion momentum measurement to investigate laser induced non-sequential double ionization of Ar and Ne. Light intensities are chosen in a regime at and below the threshold where, within the rescattering model, electron impact ionization of the singly charged ion core is expected to become energetically forbidden. Yet, we find Ar ++ ion momentum distributions and an electron-electron momentum correlation indicative of direct impact ionization. Within the quasistatic model this may be understood by assuming that the electric field of the light wave reduces the ionization potential of the singly charged ion core at the instant of scattering. The width of the projection of the ion momentum distribution onto an axis perpendicular to the light beam polarization vector is found to scale with the square root of the peak electric field strength in the light pulse. A scaling like this is not expected from the phase space available after electron impact ionization. It may indicate that the electric field at the instant of scattering is usually different from zero and determines the transverse momentum distribution. A comparison of our experimental results with several theoretical results is given.

The disintegration mechanisms for xenon clusters in intense femtosecond soft x-ray pulses from the FLASH free electron laser are investigated. The clusters are irradiated at a wavelength of λ = 13.7 nm (hν = 90.5 eV) and power densities of 5 × 1014 W cm−2. During the 10 fs pulse the Xe clusters are transformed into a highly excited, multiply charged nanoplasma. Simulating the ion kinetic energies in an electrostatic model suggests that highly charged ions explode off the surface due to Coulomb repulsion while the inner core expands in a hydrodynamic expansion. The current results yield evidence for efficient ionization of the clusters in addition to direct multistep photoemission.

Femtosecond x-ray laser flashes with power densities of up to 10(14) W/cm(2) at 13.7 nm wavelength were scattered by single xenon clusters in the gas phase. Similar to light scattering from atmospheric microparticles, the x-ray diffraction patterns carry information about the optical constants of the objects. However, the high flux of the x-ray laser induces severe transient changes of the electronic configuration, resulting in a tenfold increase of absorption in the developing nanoplasma. The modification in opaqueness can be correlated to strong atomic charging of the particle leading to excitation of Xe(4+). It is shown that single-shot single-particle scattering on femtosecond time scales yields insight into ultrafast processes in highly excited systems where conventional spectroscopy techniques are inherently blind.