262 research outputs found
Attosecond streaking in a nano-plasmonic field
A theoretical study of the application of attosecond streaking spectroscopy to
time-resolved studies of the plasmonic fields surrounding isolated, resonantly
excited spherical nanoparticles is presented. A classification of the
different regimes in attosecond streaking is proposed and identified in our
results that are derived from Mie calculations of plasmon fields, coupled to
classical electron trajectory simulations. It is shown that in an attosecond
streaking experiment, the electrons are almost exclusively sensitive to the
component of the field parallel to the direction in which they are detected.
This allows one to probe the different components of the field individually by
resolving the angle of emission of the electrons. Finally, simulations based
on fields calculated by finite-difference time-domain (FDTD) are compared with
the results obtained using Mie fields. The two are found to be in good
agreement with each other, supporting the notion that FDTD methods can be used
to reliably investigate non-spherical structures
Attosecond time-resolved photoelectron holography
Ultrafast strong-field physics provides insight into quantum phenomena that evolve on an attosecond time scale, the most fundamental of which is quantum tunneling. The tunneling process initiates a range of strong field phenomena such as high harmonic generation (HHG), laser-induced electron diffraction, double ionization and photoelectron holography—all evolving during a fraction of the optical cycle. Here we apply attosecond photoelectron holography as a method to resolve the temporal properties of the tunneling process. Adding a weak second harmonic (SH) field to a strong fundamental laser field enables us to reconstruct the ionization times of photoelectrons that play a role in the formation of a photoelectron hologram with attosecond precision. We decouple the contributions of the two arms of the hologram and resolve the subtle differences in their ionization times, separated by only a few tens of attoseconds
Attosecond control of electron dynamics in carbon monoxide
Laser pulses with stable electric field waveforms establish the opportunity
to achieve coherent control on attosecond timescales. We present experimental
and theoretical results on the steering of electronic motion in a
multi-electron system. A very high degree of light-waveform control over the
directional emission of C+ and O+ fragments from the dissociative ionization of
CO was observed. Ab initio based model calculations reveal contributions to the
control related to the ionization and laser-induced population transfer between
excited electronic states of CO+ during dissociation
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
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 BH4− to neighboring Li+
ions, resulting in a differential Coulomb force that drives lattice vibrations
in this virtual transition state
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
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Signatures of Light-Induced Potential Energy Surfaces in H2+
Using theory and Cold Target Recoil Ion Momentum Spectroscopy we find signatures of light-induced molecular potential energy surfaces in the 3-dimensional proton momentum distributions of dissociating H+2. © 2020 Journal of Physics: Conference Series. All rights reserved
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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
Visualizing the Coupling between Red and Blue Stark States Using Photoionization Microscopy
In nonhydrogenic atoms in a dc electric field, the finite size of the ionic
core introduces a coupling between quasibound Stark states that leads to
avoided crossings between states that would otherwise cross. Near an avoided
crossing, the interacting states may have decay amplitudes that cancel each
other, decoupling one of the states from the ionization continuum. This well-
known interference narrowing effect, observed as a strongly electric field-
dependent decrease in the ionization rate, was previously observed in several
atoms. Here we use photoionization microscopy to visualize interference
narrowing in helium atoms, thereby explicitly revealing the mechanism by which
Stark states decay. The interference narrowing allows measurements of the
nodal patterns of red Stark states, which are otherwise not observable due to
their intrinsic short lifetime
Attosecond investigation of extreme-ultraviolet multi-photon multi-electron ionization
Multi-electron dynamics in atoms and molecules very often occur on sub- to few-femtosecond time scales. The available intensities of extreme-ultraviolet (XUV) attosecond pulses have previously allowed the time-resolved investigation of two-photon, two-electron interactions. Here we study double and triple ionization of argon atoms involving the absorption of up to five XUV photons using a pair of intense attosecond pulse trains (APTs). By varying the time delay between the two APTs with attosecond precision and the spatial overlap with nanometer precision, we obtain information on complex nonlinear multi-photon ionization pathways. Our experimental and numerical results show that Ar2+ is predominantly formed by a sequential two-photon process, whereas the delay dependence of the Ar3+ ion yield exhibits clear signatures of the involvement of a simultaneous two-photon absorption process. Our experiment suggests that it is possible to investigate multi-electron dynamics using attosecond pulses for both pumping and probing the dynamics
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