Tracing transient charges in expanding clusters

We study transient charges formed in methane clusters following ionization by intense near-infrared laser pulses. Cluster ionization by 400-fs (I=1×1014 W/cm2) pulses is highly efficient, resulting in the observation of a dominant C3+ ion contribution. The C4+ ion yield is very small but is strongly enhanced by applying a time-delayed weak near-infrared pulse. We conclude that most of the valence electrons are removed from their atoms during the laser-cluster interaction and that electrons from the nanoplasma recombine with ions and populate Rydberg states when the cluster expands, leading to a decrease of the average charge state of individual ions. Furthermore, we find clear bound-state signatures in the electron kinetic energy spectrum, which we attribute to Auger decay taking place in expanding clusters. Such nonradiative processes lead to an increase of the final average ion charge state that is measured in experiments. Our results suggest that it is crucial to include both recombination and nonradiative decay processes for the understanding of recorded ion charge spectra

Ionization avalanching in clusters ignited by extreme-ultraviolet driven seed electrons

We study the ionization dynamics of Ar clusters exposed to ultrashort near-infrared (NIR) laser pulses for intensities well below the threshold at which tunnel ionization ignites nanoplasma formation. We find that the emission of highly charged ions up to Ar$^{8+}$ can be switched on with unit contrast by generating only a few seed electrons with an ultrashort extreme ultraviolet (XUV) pulse prior to the NIR field. Molecular dynamics simulations can explain the experimental observations and predict a generic scenario where efficient heating via inverse bremsstrahlung and NIR avalanching are followed by resonant collective nanoplasma heating. The temporally and spatially well-controlled injection of the XUV seed electrons opens new routes for controlling avalanching and heating phenomena in nanostructures and solids, with implications for both fundamental and applied laser-matter science.Comment: 5 pages, 4 figure

Rabi oscillations in extreme ultraviolet ionization of atomic argon

We demonstrate Rabi oscillations in nonlinear ionization of argon by an intense femtosecond extreme ultraviolet (XUV) laser field produced by high- harmonic generation. We monitor the formation of Ar2+ as a function of the time delay between the XUV pulse and an additional near-infrared (NIR) femtosecond laser pulse, and show that the population of an Ar+* intermediate resonance exhibits strong modulations both due to an NIR laser-induced Stark shift and XUV-induced Rabi cycling between the ground state of Ar+ and the Ar+* excited state. Our experiment represents a direct experimental observation of a Rabi-cycling process in the XUV regime

Observation of correlated electronic decay in expanding clusters triggered by near-infrared fields

When an excited atom is embedded into an environment, novel relaxation pathways can emerge that are absent for isolated atoms. A well-known example is interatomic Coulombic decay, where an excited atom relaxes by transferring its excess energy to another atom in the environment, leading to its ionization. Such processes have been observed in clusters ionized by extreme- ultraviolet and X-ray lasers. Here, we report on a correlated electronic decay process that occurs following nanoplasma formation and Rydberg atom generation in the ionization of clusters by intense, non-resonant infrared laser fields. Relaxation of the Rydberg states and transfer of the available electronic energy to adjacent electrons in Rydberg states or quasifree electrons in the expanding nanoplasma leaves a distinct signature in the electron kinetic energy spectrum. These so far unobserved electron-correlation-driven energy transfer processes may play a significant role in the response of any nano- scale system to intense laser light

Recombination dynamics of clusters in intense extreme-ultraviolet and near- infrared fields

We investigate electron-ion recombination processes in clusters exposed to intense extreme-ultraviolet (XUV) or near-infrared (NIR) pulses. Using the technique of reionization of excited atoms from recombination (REAR), recently introduced in Schütte et al (2014 Phys. Rev. Lett. 112 253401), a large population of excited atoms, which are formed in the nanoplasma during cluster expansion, is identified under both ionization conditions. For intense XUV ionization of clusters, we find that the significance of recombination increases for increasing cluster sizes. In addition, larger fragments are strongly affected by recombination as well, as shown for the case of dimers. We demonstrate that for mixed Ar–Xe clusters exposed to intense NIR pulses, excited atoms and ions are preferentially formed in the Xe core. As a result of electron-ion recombination, higher charge states of Xe are efficiently suppressed, leading to an overall reduced expansion speed of the cluster core in comparison to the shell

Recombination dynamics of clusters in intense extreme-ultraviolet and near-infrared fields

We investigate electron-ion recombination processes in clusters exposed to intense extreme-ultraviolet (XUV) or near-infrared (NIR) pulses. Using the technique of reionization of excited atoms from recombination (REAR), recently introduced in Schütte et al (2014 Phys. Rev. Lett. 112 253401), a large population of excited atoms, which are formed in the nanoplasma during cluster expansion, is identified under both ionization conditions. For intense XUV ionization of clusters, we find that the significance of recombination increases for increasing cluster sizes. In addition, larger fragments are strongly affected by recombination as well, as shown for the case of dimers. We demonstrate that for mixed Ar–Xe clusters exposed to intense NIR pulses, excited atoms and ions are preferentially formed in the Xe core. As a result of electron-ion recombination, higher charge states of Xe are efficiently suppressed, leading to an overall reduced expansion speed of the cluster core in comparison to the shell

Compact realization of all-attosecond pump-probe spectroscopy

The ability to perform attosecond-pump attosecond-probe spectroscopy (APAPS) is a longstanding goal in ultrafast science. While first pioneering experiments demonstrated the feasibility of APAPS, the low repetition rates (10-120 Hz) and the large footprints of existing setups have so far hindered the widespread exploitation of APAPS. Here we demonstrate two-color APAPS using a commercial laser system at 1 kHz, straightforward post-compression in a hollow-core fiber and a compact high-harmonic generation (HHG) setup. The latter enables the generation of intense extreme-ultraviolet (XUV) pulses by using an out-of-focus HHG geometry and by exploiting a transient blueshift of the driving laser in the HHG medium. Near-isolated attosecond pulses are generated, as demonstrated by one-color and two-color XUV-pump XUV-probe experiments. Our concept allows selective pumping and probing on extremely short timescales and permits investigations of fundamental processes that are not accessible by other pump-probe techniques.Comment: 8 pages, 5 figure

Correlated electronic decay following intense near-infrared ionization of clusters

We report on a novel correlated electronic decay process following extensive Rydberg atom formation in clusters ionized by intense near-infrared fields. A peak close to the atomic ionization potential is found in the electron kinetic energy spectrum. This new contribution is attributed to an energy transfer between two electrons, where one electron decays from a Rydberg state to the ground state and transfers its excess energy to a weakly bound cluster electron in the environment that can escape from the cluster. The process is a result of nanoplasma formation and is therefore expected to be important, whenever intense laser pulses interact with nanometer-sized particles