108 research outputs found
Attosecond Streaking in the Water Window: A New Regime of Attosecond Pulse Characterization
We report on the first streaking measurement of water-window attosecond
pulses generated via high harmonic generation, driven by sub-2-cycle,
CEP-stable, 1850 nm laser pulses. Both the central photon energy and the energy
bandwidth far exceed what has been demonstrated thus far, warranting the
investigation of the attosecond streaking technique for the soft X-ray regime
and the limits of the FROGCRAB retrieval algorithm under such conditions. We
also discuss the problem of attochirp compensation and issues regarding much
lower photo-ionization cross sections compared with the XUV in addition to the
fact that several shells of target gases are accessed simultaneously. Based on
our investigation, we caution that the vastly different conditions in the soft
X-ray regime warrant a diligent examination of the fidelity of the measurement
and the retrieval procedure.Comment: 14 Pages, 12 figure
exploding clusters dynamics probed by XUV fluorescence
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 those investigations concerns atomic clusters, whereas the
evolution of molecular clusters excited by intense laser pulses is less
explored. In this work we trace the dynamics of clusters
triggered by a few-cycle 1.45-m 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
EUV ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization and electron energy-loss spectra
The ionization dynamics of pure He nanodroplets irradiated by EUV radiation
is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence
(VMI-PEPICO) spectroscopy. We present photoelectron energy spectra and angular
distributions measured in coincidence with the most abundant ions He+, He2+,
and He3+. Surprisingly, below the autoionization threshold of He droplets we
find indications for multiple excitation and subsequent ionization of the
droplets by a Penning-like process. At high photon energies we evidence
inelastic collisions of photoelectrons with the surrounding He atoms in the
droplets
High-order harmonic spectroscopy for molecular imaging of polyatomic molecules
High-order harmonic generation is a powerful and sensitive tool for probing
atomic and molecular structures, combining in the same measurement an
unprecedented attosecond temporal resolution with a high spatial resolution, of
the order of the angstrom. Imaging of the outermost molecular orbital by
high-order harmonic generation has been limited for a long time to very simple
molecules, like nitrogen. Recently we demonstrated a technique that overcame
several of the issues that have prevented the extension of molecular orbital
tomography to more complex species, showing that molecular imaging can be
applied to a triatomic molecule like carbon dioxide. Here we report on the
application of such technique to nitrous oxide (N2O) and acetylene (C2H2). This
result represents a first step towards the imaging of fragile compounds, a
category which includes most of the fundamental biological molecules
Penning ionization of doped helium nanodroplets following EUV excitation
Helium nanodroplets are widely used as a cold, weakly interacting matrix for
spectroscopy of embedded species. In this work we excite or ionize doped He
droplets using synchrotron radiation and study the effect onto the dopant atoms
depending on their location inside the droplets (rare gases) or outside at the
droplet surface (alkali metals). Using photoelectron-photoion coincidence
imaging spectroscopy at variable photon energies (20-25 eV), we compare the
rates of charge-transfer to Penning ionization of the dopants in the two cases.
The surprising finding is that alkali metals, in contrast to the rare gases,
are efficiently Penning ionized upon excitation of the (n=2)-bands of the host
droplets. This indicates rapid migration of the excitation to the droplet
surface, followed by relaxation, and eventually energy transfer to the alkali
dopants
Ultrafast charge carrier dynamics in quantum confined 2D perovskite
We studied the charge carrier dynamics in 2D perovskite NBT2PbI4 by ultrafast optical pump-THz probe spectroscopy. We observed a few ps long relaxation dynamics that can be ascribed to the band to band carrier recombination, in the absence of any contribution from many-body and trap assisted processes. The transient conductivity spectra show that the polaron dynamics is strongly modulated by the presence of a rich exciton population. The polarization field resulting from the exciton formation acts as the source of a restoring force that localizes polarons. This is revealed by the presence of a negative imaginary conductivity. Our results show that the dynamics of excitons in 2D perovskites at room temperature can be detected by monitoring their effect on the conductivity of the photoinduced polaronic carrier
Collective Autoionization in Multiply-Excited Systems: A novel ionization process observed in Helium Nanodroplets
Free electron lasers (FELs) offer the unprecedented capability to study
reaction dynamics and image the structure of complex systems. When multiple
photons are absorbed in complex systems, a plasma-like state is formed where
many atoms are ionized on a femtosecond timescale. If multiphoton absorption is
resonantly-enhanced, the system becomes electronically-excited prior to plasma
formation, with subsequent decay paths which have been scarcely investigated to
date. Here, we show using helium nanodroplets as an example that these systems
can decay by a new type of process, named collective autoionization. In
addition, we show that this process is surprisingly efficient, leading to ion
abundances much greater than that of direct single-photon ionization. This
novel collective ionization process is expected to be important in many other
complex systems, e.g. macromolecules and nanoparticles, exposed to high
intensity radiation fields
Interatomic Coulombic decay in helium nanodroplets
Interatomic Coulombic decay (ICD) is induced in helium nanodroplets by photoexciting the
n
=
2
excited state of
He
+
using XUV synchrotron radiation. By recording multiple-coincidence electron and ion images we find that ICD occurs in various locations at the droplet surface, inside the surface region, or in the droplet interior. ICD at the surface gives rise to energetic
He
+
ions as previously observed for free He dimers. ICD deeper inside leads to the ejection of slow
He
+
ions due to Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formation of
He
k
+
complexes
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