195 research outputs found
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=1Ă1014 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 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
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)]
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
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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
Probing Time-Dependent Molecular Dipoles on the Attosecond Time Scale
Photoinduced molecular processes start with the interaction of the
instantaneous electric field of the incident light with the electronic degrees
of freedom. This early attosecond electronic motion impacts the fate of the
photoinduced reactions. We report the first observation of attosecond time
scale electron dynamics in a series of small- and medium-sized neutral
molecules (N2, CO2, and C2H4), monitoring time-dependent variations of the
parent molecular ion yield in the ionization by an attosecond pulse, and
thereby probing the time-dependent dipole induced by a moderately strong near-
infrared laser field. This approach can be generalized to other molecular
species and may be regarded as a first example of molecular attosecond Stark
spectroscopy
Evolution of a Molecular Shape Resonance along a Stretching Chemical Bond
We report experiments on laser-assisted electron recollisions that result from strong-field ionization of photoexcited I2 molecules in the regime of low-energy electron scattering (<25ââeV impact energy). By comparing differential scattering cross sections extracted from the angle-resolved photoelectron spectra to differential scattering cross sections from quantum-scattering calculations, we demonstrate that the electron-scattering dynamics is dominated by a shape resonance. When the molecular bond stretches during the evolution of a vibrational wave packet this shape resonance shifts to lower energies, both in experiment and theory. We explain this behavior by the nature of the resonance wave function, which closely resembles an antibonding molecular orbital of I2
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
Ultrafast modulation of electronic structure by coherent phonon excitations
Femtosecond x-ray absorption spectroscopy with a laser-driven high-harmonic
source is used to map ultrafast changes of x-ray absorption by femtometer-
scale coherent phonon displacements. In LiBH4, displacements along an Ag
phonon mode at 10 THz are induced by impulsive Raman excitation and give rise
to oscillatory changes of x-ray absorption at the Li K edge. Electron density
maps from femtosecond x-ray diffraction data show that the electric field of
the pump pulse induces a charge transfer from the BH4â to neighboring Li+
ions, resulting in a differential Coulomb force that drives lattice vibrations
in this virtual transition state
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