995 research outputs found
Wave packet dynamics in triplet states of Na2 attached to helium nanodroplets
The dynamics of vibrational wave packets excited in Na2 dimers in the triplet
ground and excited states is investigated by means of helium nanodroplet
isolation (HENDI) combined with femtosecond pump-probe spectroscopy. Different
pathways in the employed resonant multi-photon ionization scheme are
identified. Within the precision of the method, the wave packet dynamics
appears to be unperturbed by the helium droplet environment
Sequential and direct ionic excitation in the strong-field ionization of 1-butene molecules
We study the Strong-Field Ionization (SFI) of the hydrocarbon 1-butene as a function of wavelength using photoion-photoelectron covariance and coincidence spectroscopy. We observe a striking transition in the fragment-associated photoelectron spectra: from a single Above Threshold Ionization (ATI) progression for photon energies less than the cation D0–D1 gap to two ATI progressions for a photon energy greater than this gap. For the first case, electronically excited cations are created by SFI populating the ground cationic state D0, followed by sequential post-ionization excitation. For the second case, direct sub-cycle SFI to the D1 excited cation state contributes significantly. Our experiments access ionization dynamics in a regime where strong-field and resonance-enhanced processes can interplay
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Molecular orbital imprint in laser-driven electron recollision
Electrons released by strong-field ionization from atoms and molecules or in solids can be accelerated in the oscillating laser field and driven back to their ion core. The ensuing interaction, phase-locked to the optical cycle, initiates the central processes underlying attosecond science. A common assumption assigns a single, welldefined return direction to the recolliding electron. We study laser-induced electron rescattering associated with two different ionization continua in the same, spatially aligned, polyatomic molecule. We show by experiment and theory that the electron return probability is molecular frame-dependent and carries structural information on the ionized orbital. The returning wave packet structure has to be accounted for in analyzing strong-field spectroscopy experiments that critically depend on the interaction of the laser-driven continuum electron, such as laser-induced electron diffraction
Photoelectron imaging of XUV photoionization of CO2 by 13-40 eV synchrotron radiation
Valence band photoionization of CO2 has been studied by photoelectron
spectroscopy using a velocity map imaging spectrometer and synchrotron
radiation. The measured data allow retrieving electronic and vibrational
branching ratios, vibrationally resolved asymmetry parameters, and the total
electron yield which includes multiple strong resonances. Additionally, the
spectrum of low kinetic energy electrons has been studied in the resonant
region, and the evolution with photon energy of one of the forbidden
transitions present in the slow photoelectrons spectrum has been carefully
analyzed, indicating that in the presence of auto-ionizing resonances the
vibrational populations of the ion are significantly redistributed
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Strong field ionization of small hydrocarbon chains with full 3D momentum analysis
Strong field ionization of small hydrocarbon chains is studied in a kinematic complete experiment using a reaction microscope. By coincidence detection of ions and electrons different ionization continua populated during the ionization process are identified. In addition, photoelectron momentum distributions from laser-aligned molecules allow to characterize the electron wavepackets emerging from different Dyson orbitals
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Generation and characterisation of few-pulse attosecond pulse trains at 100 kHz repetition rate
The development of attosecond pump-probe experiments at high repetition rate requires the development of novel attosecond sources maintaining a sufficient number of photons per pulse. We use 7 fs, 800 nm pulses from a non-collinear optical parametric chirped pulse amplification laser system to generate few-pulse attosecond pulse trains (APTs) with a flux of >106 photons per shot in the extreme ultraviolet at a repetition rate of 100 kHz. The pulse trains have been fully characterised by recording frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) traces with a velocity map imaging spectrometer. For the pulse retrieval from the FROG-CRAB trace a new ensemble retrieval algorithm has been employed that enables the reconstruction of the shape of the APTs in the presence of carrier envelope phase fluctuations of the few-cycle laser system. © 2020 The Author(s). Published by IOP Publishing Ltd
Generation and characterisation of few-pulse attosecond pulse trains at 100 kHz repetition rate
The development of attosecond pump–probe experiments at high repetition rate requires the development of novel attosecond sources maintaining a sufficient number of photons per pulse. We use 7 fs, 800 nm pulses from a non-collinear optical parametric chirped pulse amplification laser system to generate few-pulse attosecond pulse trains (APTs) with a flux of >106 photons per shot in the extreme ultraviolet at a repetition rate of 100 kHz. The pulse trains have been fully characterised by recording frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) traces with a velocity map imaging spectrometer. For the pulse retrieval from the FROG-CRAB trace a new ensemble retrieval algorithm has been employed that enables the reconstruction of the shape of the APTs in the presence of carrier envelope phase fluctuations of the few-cycle laser system
Generation and characterization of isolated attosecond pulses at 100  kHz repetition rate
The generation of coherent light pulses in the extreme ultraviolet (XUV) spectral region with attosecond pulse durations constitutes the foundation of the field of attosecond science. Twenty years after the first demonstration of isolated attosecond pulses, they continue to be a unique tool enabling the observation and control of electron dynamics in atoms, molecules, and solids. It has long been identified that an increase in the repetition rate of attosecond light sources is necessary for many applications in atomic and molecular physics, surface science, and imaging. Although high harmonic generation (HHG) at repetition rates exceeding 100 kHz, showing a continuum in the cutoff region of the XUV spectrum, was already demonstrated in 2013, the number of photons per pulse was insufficient to perform pulse characterization via attosecond streaking, let alone to perform a pump-probe experiment. Here we report on the generation and full characterization of XUV attosecond pulses via HHG driven by near-single-cycle pulses at a repetition rate of 100 kHz. The high number of 106 XUV photons per pulse on target enables attosecond electron streaking experiments through which the XUV pulses are determined to consist of a dominant single attosecond pulse. These results open the door for attosecond pump-probe spectroscopy studies at a repetition rate 1 or 2 orders of magnitude above current implementations
Ion-ion coincidence imaging at high event rate using an in-vacuum pixel detector
A new ion-ion coincidence imaging spectrometer based on a pixelated
complementary metal-oxide-semiconductor detector has been developed for the
investigation of molecular ionization and fragmentation processes in strong
laser fields. Used as a part of a velocity map imaging spectrometer, the
detection system is comprised of a set of microchannel plates and a Timepix
detector. A fast time-to-digital converter (TDC) is used to enhance the ion
time-of-flight resolution by correlating timestamps registered separately by
the Timepix detector and the TDC. In addition, sub-pixel spatial resolution
(<6 ÎĽm) is achieved by the use of a center-of-mass centroiding algorithm. This
performance is achieved while retaining a high event rate (104 per s). The
spectrometer was characterized and used in a proof-of-principle experiment on
strong field dissociative double ionization of carbon dioxide molecules (CO2),
using a 400 kHz repetition rate laser system. The experimental results
demonstrate that the spectrometer can detect multiple ions in coincidence,
making it a valuable tool for studying the fragmentation dynamics of molecules
in strong laser fields
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