Two-Color Strong-Field Photoelectron Spectroscopy and the Phase of the Phase.

The presence of a weak second-harmonic field in an intense-laser ionization experiment affects the momentum-resolved electron yield, depending on the relative phase between the ω and the 2ω component. The proposed two-color "phase-of-the-phase spectroscopy" quantifies for each final electron momentum a relative-phase contrast (RPC) and a phase of the phase (PP) describing how much and with which phase lag, respectively, the yield changes as a function of the relative phase. Experimental results for RPC and PP spectra for rare gas atoms and CO_{2} are presented. The spectra demonstrate a rather universal structure that is analyzed with the help of a simple model based on electron trajectories, wave-packet spreading, and (multiple) rescattering. Details in the PP and RPC spectra are target sensitive and, thus, may be used to extract structural (or even dynamical) information with high accuracy.

Tracing electron-ion recombination in nanoplasmas produced by extreme-ultraviolet irradiation of rare-gas clusters.

We investigate electron-ion recombination in nanoplasmas produced by the ionization of rare-gas clusters with intense femtosecond extreme-ultraviolet (XUV) pulses. The relaxation dynamics following XUV irradiation is studied using time-delayed 790-nm pulses, revealing the generation of a large number of excited atoms resulting from electron-ion recombination. In medium-sized Ar-Xe clusters, these atoms are preferentially created in the Xe core within 10 ps after the cluster ionization. The ionization of excited atoms serves as a sensitive probe for monitoring the cluster expansion dynamics up to the ns time scale.

Rare-gas clusters in intense extreme-ultraviolet pulses from a high-order harmonic source.

We report evidence for two previously unidentified effects in the ionization of rare-gas clusters by intense extreme-ultraviolet pulses. First, electron spectra indicate multistep photoemission with increasing isotropy for larger clusters due to electron-atom collisions. Second, very slow (meV) electrons are interpreted as the first experimental evidence for Rydberg-like atomic state formation in the nanoplasma expansion. Only small fractions of Xe2+ ions were found, in sharp contrast to previous results recorded under comparable conditions [Murphy et al., Phys. Rev. Lett. 101, 203401 (2008).

Charging of metal clusters in helium droplets exposed to intense femtosecond laser pulses.

We review the strong field (10(13)-10(16) W cm(-2)) laser excitation of metal clusters (Cd(N), Ag(N) and Pb(N)) embedded in He nanodroplets. Plasmon enhanced ionization obtained by stretching the laser pulses to several hundreds of femtoseconds or by using dual pulses with a suitable optical delay leads to a Coulomb explosion of highly charged atomic ions. The charging dynamics can be well described by corresponding semiclassical Vlasov simulations. The influence of the He environment on the ionization process and on the final charge distribution is discussed. Evidence is found that He(2+) is generated in collisions with highly charged metal ions. In contrast, singly and doubly charged ions with low recoil energies induce the formation of He snowballs with a distinct shell structure around the ion. Laser intensity thresholds for snowball formation and for the ionization of clusters are investigated by applying intensity selective scanning.

Plasmon-enhanced electron acceleration in intense laser metal-cluster interactions.

We have measured the energy and angular-resolved electron emission from medium-sized silver clusters (N approximately 500-2000) exposed to dual laser pulses of moderate intensity (I approximately (10(13-14) W/cm2). When the second pulse excites the plasmon resonantly, we observe enhanced emission along the laser polarization axis. The asymmetry of the electron spectrum is strongly increasing with electron energy. Semiclassical simulations reveal the following mechanism: Electrons bound in highly excited states can leave, return to, and traverse the cluster. Those electrons that return at zero plasmon deflection and traverse the cluster during a favorable plasmon half-cycle can experience maximum acceleration by the evolving polarization field. As a result of these constraints energetic electrons are emitted in direction of the laser polarization axis in subcycle bursts.

Controlling the coulomb explosion of silver clusters by femtosecond dual-pulse laser excitation.

Silver clusters grown in helium nanodroplets are excited by intense femtosecond laser pulses resulting in the formation of a hot electron plasma far from equilibrium. The ultrafast dynamics is studied by applying optically delayed dual pulses, which allows us to pursue and control the coupling of the laser field to the clusters on a femtosecond time scale. A distinct influence of the optical delay on the ionization efficiency gives strong evidence that a significant contribution of collective dipolar electron motion is present, which is verified by corresponding Vlasov dynamics simulations on a model system. The microscopic approach demonstrates the outstanding role of giant resonances in clusters also in intense laser fields.