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Using the example of metal clusters, an experimental setup and procedure is presented, which allows for the generation of size and charge-state selected polyanions from monoanions in a molecular beam. As a characteristic feature of this modular setup, the further charging process via sequential electron attachment within a 3-state digital trap takes place after mass-selection. In contrast to other approaches, the rf based concept permits to access heavy particles. The procedure is highly flexible with respect to the preparation process and potentially suitable for a wide variety of anionic species. By adjusting the storage conditions, i.e., the radio frequency, to the change in the mass-to-charge ratio, we succeeded to produce clusters in highly negative charge states, i.e., Ag$_{800}^{7-}$. The capabilities of the setup are demonstrated by experiments extracting electronic and optical properties of polyanionic metal clusters by analyzing the corresponding photoelectron spectra.

The laser intensity dependence of the recoil energies from the Coulomb explosion of small argon clusters has been investigated by resolving the contributions of the individual charge states to the ion recoil energy spectra. Between $10^{14}$ and $10^{15}$ W/cm$^2$ the high-energy tail of the ion energy spectra changes its shape and develops into the well-known knee feature, which results from the cluster size distribution, laser focal averaging, and ionization saturation. Resolving the contributions of the different charge states to the recoil energies, the experimental data reveal that the basic assumption of an exploding homogeneously charged sphere cannot be maintained in general. In fact, the energy spectra of the high-$q$ show distinct gaps in the yields at low kinetic energies, which hints at more complex radial ion charge distributions developing during the laser pulse impact., Comment: 15 pages, 5 figures

Magnesium atoms fully embedded in helium nanodroplets are exposed to two-color laser pulses, which trigger multiphoton above-threshold ionization (ATI). This allows to exemplarily study the contribution of a dense, neutral and finite medium on single electron propagation. The angular-resolved photoelectron spectra show striking differences with respect to results obtained on free atoms. Scattering of the individual Mg photoelectrons, when traversing the neutral helium environment, causes the angular distribution to become almost isotropic. Furthermore, the appearance of higher energy electrons is observed, pointing out the impact of the droplet on the concerted emission process. Phase-of-the-phase spectroscopy, however, reveals a marked loss in the \2ww phase dependence of the electron signal. Taking into account sideband formation on a quantitative level, a Monte-Carlo simulation which includes laser-assisted electron scattering can reproduce the experimental spectra and give insights into the strong-field-induced electron emission from disordered systems.

Phase-of-the-phase (PoP) spectroscopy is extended to two-color laser fields having a circularly counter-rotating polarization. In particular, the higher harmonics of the (two-color) phase information are analyzed in order to extract the laser-coherent part of the photoelectron spectra taken under complex target conditions. We illustrate this with a proof-of-principle simulation by considering strong-field electron emission from argon atoms within helium nanodroplets under realistic experimental conditions, i.e., a limited number of photoemission events. Multiple elastic scattering on neutral helium atoms creates a laser-incoherent background, but the higher harmonics of the PoP-signal allow to resolve the coherent contribution to the photoemission., Comment: 8 pages, 6 figures, IOP style

The extreme ultraviolet plasma emission from liquid microsized argon droplets exposed to intense near-infrared laser pulses has been investigated. Emission from the warm dense matter targets is recorded in a spectral range in between 16 and 30 nm at laser intensities of 10^14 W/cm^2. Above the emission threshold, soft x-ray radiation exponentially increases with the pulse energy whereby a strong dependence of the yields on the pulse duration is observed, which points at an effective electron collisional heating of the microplasma by inverse bremsstrahlung. Accompanying hydrodynamic simulations reveal the temporal and spatial development of the microplasma conditions. The good agreement in between the measured and calculated emission spectra as well as the extracted electron temperatures confirm that hydrodynamic simulations can be applied in the analysis of strongly excited droplets., Comment: 13 pages, 10 figures

Momentum-resolved photoelectron emission from xenon in colinearly polarized two-color laser fields at above-threshold ionization conditions is studied both experimentally and theoretically. We utilize phase-of-the-phase spectroscopy as recently introduced by Skruszewicz et al., Phys. Rev. Lett. 115, 043001 (2015) to analyze the dependence of the yields on the relative phase $\varphi$ between the fundamental and second harmonic laser fields. The resulting phase-of-phase spectra feature a characteristic checkerboard pattern, which can analytically be described within the strong-field approximation., Comment: 6 pages, 5 figures, RevTeX 4.1

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 $\omega$ and the $2\omega$ 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 function of 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., Comment: 5 pages, REVTeX4, 5 figures

Magnesium atoms embedded in superfluid helium nanodroplets have been identified to arrange themselves in a metastable network, refered to as foam. In order to investigate the ionization dynamics of this unique structure with respect to a possible light-induced collapse the femtosecond dual-pulse spectroscopy technique is applied. Around zero optical delay a strong feature is obtained which represents a direct probe of the foam response. We found that upon collapse, ionization is reduced. A particlar intensity ratio of the pulses allows to address either direct ionization or photoactivation of the neutral complexes, thus affecting reaction pathways. A simplified excitation scheme visualizes possible scenarios in accordance with the experimental observations., Comment: 12 pages, 6 figures

Optimal control techniques combined with femtosecond laser pulse shaping are applied to steer and enhance the strong-field induced emission of highly charged atomic ions from silver clusters embedded in helium nanodroplets. With light fields shaped in amplitude and phase we observe a substantial increase of the Ag$^{q+}$ yield for $q>10$ when compared to bandwidth-limited and optimally stretched pulses. A remarkably simple double-pulse structure, containing a low-intensity prepulse and a stronger main pulse, turns out to produce the highest atomic charge states up to Ag$^{20+}$. A negative chirp during the main pulse hints at dynamic frequency locking to the cluster plasmon. A numerical optimal control study on pure silver clusters with a nanoplasma model converges to a similar pulse structure and corroborates, that the optimal light field adapts to the resonant excitation of cluster surface plasmons for efficient ionization., Comment: 7 pages, 5 figures, text revised

The generation of highly charged Xe$^{q+}$ ions up to {$q=24$} is observed in Xe clusters embedded in helium nanodroplets and exposed to intense femtosecond laser pulses ($\lambda$=800 nm). Laser intensity resolved measurements show that the high-$q$ ion generation starts at an unexpectedly low threshold intensity of about {10$^{14}$ W/cm$^{2}$}. Above threshold, the Xe ion charge spectrum saturates quickly and changes only weakly for higher laser intensities. Good agreement between these observations and a molecular dynamics analysis allows us to identify the mechanisms responsible for the highly charged ion production and the surprising intensity threshold behavior of the ionization process., Comment: 4 pages, 4 figures, content revised