13 research outputs found
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Phase cycling of extreme ultraviolet pulse sequences generated in rare gases
The development of schemes for coherent nonlinear time-domain spectroscopy in the extreme-ultraviolet regime (XUV) has so far been impeded by experimental difficulties that arise at these short wavelengths. In this work we present a novel experimental approach, which facilitates the timing control and phase cycling of XUV pulse sequences produced by harmonic generation in rare gases. The method is demonstrated for the generation and high spectral resolution characterization of narrow-bandwidth harmonics (˜14 eV) in argon and krypton. Our technique simultaneously provides high phase stability and a pathway-selective detection scheme for nonlinear signals - both necessary prerequisites for all types of coherent nonlinear spectroscopy. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft
Phase cycling of extreme ultraviolet pulse sequences generated in rare gases
The development of schemes for coherent nonlinear time-domain spectroscopy in the extreme-ultraviolet regime (XUV) has so far been impeded by experimental difficulties that arise at these short wavelengths. In this work we present a novel experimental approach, which facilitates the timing control and phase cycling of XUV pulse sequences produced by harmonic generation in rare gases. The method is demonstrated for the generation and high spectral resolution characterization of narrow-bandwidth harmonics (≈14 eV) in argon and krypton. Our technique simultaneously provides high phase stability and a pathway-selective detection scheme for nonlinear signals—both necessary prerequisites for all types of coherent nonlinear spectroscopy
High-Gain Harmonic Generation with temporally overlapping seed pulses and application to ultrafast spectroscopy
Collinear double-pulse seeding of the High-Gain Harmonic Generation (HGHG)
process in a free-electron laser (FEL) is a promising approach to facilitate
various coherent nonlinear spectroscopy schemes in the extreme ultraviolet
(XUV) spectral range. However, in collinear arrangements using a single
nonlinear medium, temporally overlapping seed pulses may introduce nonlinear
mixing signals that compromise the experiment at short time delays. Here, we
investigate these effects in detail by extending the analysis described in a
recent publication (Wituschek et al., Nat. Commun., 11, 883, 2020). High-order
fringe-resolved autocorrelation and wave-packet interferometry experiments at
photon energies > eV are performed, accompanied by numerical simulations.
It turns out that both the autocorrelation and the wave-packet interferometry
data are very sensitive to saturation effects and can thus be used to
characterize saturation in the HGHG process. Our results further imply that
time-resolved spectroscopy experiments are feasible even for time delays
smaller than the seed pulse duration.Comment: This is accepted version of the article. The Version of Record is
available online at https://doi.org/10.1364/OE.40124
Improved stabilization scheme for extreme ultraviolet quantum interference experiments
Interferometric pump-probe experiments in the extreme ultraviolet (XUV)
domain are experimentally very challenging due to the high phase stability
required between the XUV pulses. Recently, an efficient phase stabilization
scheme was introduced for seeded XUV free electron lasers (FELs) combining
shot-to-shot phase modulation with lock-in detection. This method stabilized
the seed laser beampath on the fundamental ultraviolet wavelength to a high
degree. Here, we extend this scheme including the stabilization of the XUV
beampath, incorporating phase fluctuations from the FEL high gain harmonic
generation process. Our analysis reveals a clear signal improvement with the
new method compared to the previous stabilization scheme
High-gain harmonic generation with temporally overlapping seed pulses and application to ultrafast spectroscopy
Collinear double-pulse seeding of the High-Gain Harmonic Generation (HGHG) process in a free-electron laser (FEL) is a promising approach to facilitate various coherent nonlinear spectroscopy schemes in the extreme ultraviolet (XUV) spectral range. However, in collinear arrangements using a single nonlinear medium, temporally overlapping seed pulses may introduce nonlinear mixing signals that compromise the experiment at short time delays. Here, we investigate these effects in detail by extending the analysis described in a recent publication (Wituschek et al., Nat. Commun., 11, 883, 2020). High-order fringe-resolved autocorrelation and wave packet interferometry experiments at photon energies > 23 eV are performed, accompanied by numerical simulations. It turns out that both the autocorrelation and the wave-packet interferometry data are very sensitive to saturation effects and can thus be used to characterize saturation in the HGHG process. Our results further imply that time-resolved spectroscopy experiments are feasible even for time delays smaller than the seed pulse duration
Time-resolved Ultrafast Interatomic Coulombic Decay in Superexcited Sodium-doped Helium Nanodroplets
The autoionization dynamics of superexcited superfluid He nanodroplets doped
with Na atoms is studied by extreme-ultraviolet (XUV) time-resolved electron
spectroscopy. Following excitation into the higher-lying droplet absorption
band, the droplet relaxes into the lowest metastable atomic S
states from which Interatomic Coulombic Decay (ICD) takes places either between
two excited He atoms or between an excited He atom and a Na atom attached to
the droplet surface. Four main ICD channels are identified and their time
constants are determined by varying the delay between the XUV pulse and a UV
pulse that ionizes the initial excited state and thereby quenches ICD. The time
constants for the different channels all fall in the range 1~ps
indicating that the ICD dynamics are mainly determined by the droplet
environment. A periodic modulation of the transient ICD signals is tentatively
attributed to the oscillation of the bubble forming around the localized He
excitation. The ICD efficiency depends on the total number of excited states in
a droplet rather than the density of excited states pointing to a collective
enhancement of ICD
Unravelling the full relaxation dynamics of superexcited helium nanodroplets
The relaxation dynamics of superexcited superfluid He nanodroplets is thoroughly investigated by means of extreme-ultraviolet (XUV) femtosecond electron and ion spectroscopy complemented by time- dependent density functional theory (TDDFT). Three main paths leading to the emission of electrons and ions are identified: droplet autoionization, pump-probe photoionization, and autoionization induced by re-excitation of droplets relaxing into levels below the droplet ionization threshold. The most abundant product ions are He2+, generated by droplet autoionization and by photoionization of droplet-bound excited He atoms. He+ appear with some pump-probe delay as a result of the ejection He atoms in their lowest excited states from the droplets. The state-resolved time-dependent photoelectron spectra reveal that intermediate excited states of the droplets are populated in the course of the relaxation, terminating in the lowest-lying metastable singlet and triplet He atomic states. The slightly faster relaxation of the triplet state compared to the singlet state is in agreement with the simulation showing faster formation of a bubble around a He atom in the triplet state