[en] High-harmonic generation spectroscopy is a promising tool for resolving electron dynamics and structure in atomic and molecular systems. This scheme, commonly described by the strong field approximation, requires a deep insight into the basic mechanism that leads to the harmonic generation. Recently, we have demonstrated the ability to resolve the first stage of the process—field induced tunnel ionization—by adding a weak perturbation to the strong fundamental field. Here we generalize this approach and show that the assumptions behind the strong field approximation are valid over a wide range of tunnel ionization conditions. Performing a systematic study—modifying the fundamental wavelength, intensity and atomic system—we observed a good agreement with quantum path analysis over a range of Keldysh parameters. The generality of this scheme opens new perspectives in high harmonics spectroscopy, holding the potential of probing large, complex molecular systems. (paper)

[en] In this paper a method for the quantitative determination of a morphology descriptor of free clusters with complex nanostructure is presented and applied to transition metal nanoparticles produced by a pulsed vaporization source. The method, which is based on the low-pressure aerodynamic mobility of neutral particles, can be applied as a characterization tool to a broad class of gas-phase nanoparticle sources for on-line investigation of particle growth and for quantifying coalescence versus agglomerate aggregation. We report on the application of this method for the characterization of free titanium clusters produced by a pulsed microplasma cluster source in the size range of approximately 300-6000 atoms. The clusters have an open fractal-like structure, with the fractal dimension depending on their thermal history during growth and evolving towards softer aggregates for longer residence times where lower-temperature conditions characterize the growth environment.

[en] Clusters excited by intense laser pulses are a unique source of warm dense matter, that has been the subject of intensive experimental studies. The majority of these investigations concern atomic clusters, whereas the evolution of molecular clusters excited by intense laser pulses is less explored. In this work we trace the dynamics of CO₂ clusters triggered by a few-cycle 1.45 μm driving pulse through the detection of XUV fluorescence induced by a delayed 800 nm ignition pulse. Striking differences among fluorescence dynamics from different ionic species are observed. (paper)

[en] The aim of this study is probing the multi-electron behavior in xenon by two-color driven high harmonic generation. By changing the relative polarization of the two colors we were able to study different aspects of the multi-electron response. (paper)

[en] Spectral focusing of the recolliding electron in high-order harmonic generation driven by two-color fields is shown to be a powerful tool for isolating and enhancing hidden spectral features of the target under study. In previous works we used this technique for probing multi-electron effects in xenon and we compared our experimental results with time-dependent configuration-interaction singles calculations. We demonstrate here that this technique can be exploited for reconstructing the enhancement factor of the xenon giant dipole resonance and we discuss the sensitivity of this method to macroscopic effects. We then extend the technique to argon in order to test the applicability of this procedure to other targets. (paper)

[en] Synchrotron radiation-based experimental techniques are largely employed for the characterization of the reactivity of finite size systems; in particular, X-ray absorption spectroscopy (XAS) is a suitable tool to shed light on the local electronic structure and chemical status of atoms in nano-objects, as it is very sensitive to the local bonding environment of the probed site. In supported clusters intrinsic properties and reactivity are largely distorted and obscured by the changes imposed by the growth procedure and by the influence of the substrate, so the attainability of experiments on free clusters reacting with species in the gas phase is a primary goal in the development of cluster science. In this paper we report a proof of principle of the applicability of gas phase XAS technique to titanium and titanium oxide, hydride and hydrate systems. Experiments are performed by coupling a pulsed microplasma cluster source (PMCS) with a third generation synchrotron light source, and measuring the intensity of the electron yield coming from the interaction of VUV photons with the clusters seeded in a supersonic beam. (orig.)

[en] The possibility to apply synchrotron radiation-based spectroscopic techniques, and in particular X-ray absorption spectroscopy (XAS), to isolated nanoparticles is expected to bring important insight into the electronic properties, the structural arrangement, and the chemical character of finite size systems. A precise knowledge of such properties has special relevance for a bottom-up approach to the description of nanostructured systems of technological interest. Element specificity, chemical sensitivity and local character are most significant qualities demanded from the characterization tools in this regards. The extremely low density of free particles samples is the main issue limiting the development of such techniques and only very recently first experimental results on systems of strong technological relevance like transition metal nanoparticles. In this paper we describe an experimental setup for core-level absorption investigations on free metal clusters. The most critical issues for experiment feasibility are discussed and the adopted methodology is described in detail. Results from the application of this approach to core-shell photo-ionization studies on free Ti clusters are presented as a case study

[en] We have investigated the ionization of the Ar atom by 51 nm extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan. The angle-resolved photoelectron spectra contain lines due to sequential three-photon double ionization with the second ionization step proceeding via the resonantly enhanced two-photon absorption. The relative intensities of the corresponding photoelectron peaks and their angular dependence are explained in the framework of a three-step model of the process. (fast track communication)

[en] Free electron laser light sources delivering high intensity pulses of short wavelength radiation are opening novel possibilities for the investigation of matter at the nanoscale and for the discovery and understanding of new physical processes occurring at the exotic transient states they make accessible. Strong ionization of atomic constituents of a nano-sized sample is a representative example of such processes and the understanding of ionization dynamics is crucial for a realistic description of the experiments. We report here on multiple ionization experiments on free clusters of titanium, a high cohesive energy metal. The time of flight ion spectra reveal a saturation of the cluster ionization at ∼10"1"6 photons per pulse per cm"2. Our results also show a clear lack of any explosion process, opposite to what is observed for a rare-gas cluster under similar conditions. A simple and generalized multi-step ionization model including Coulomb frustration of the photoemission process effectively reproduces with a good agreement the main features of the experimental observation and points to an interpretation of the data involving a substantial energy deposition into the cluster through electronic system heating upon scattering events within photoemission. (paper)

[en] We present a comprehensive analysis of autoionization processes in Ne clusters (∼5000 atoms) after multiple valence excitations by free electron laser radiation. The evolution from 2-body interatomic Coulombic decay (ICD) to 3-body ICD is demonstrated when changing from surface to bulk Frenkel exciton excitation. Super Coster-Kronig type 2-body ICD is observed at Wannier exciton which quenches the main ICD channel. (paper)