118 research outputs found
Imaging Molecular Structure with Photoelectron Diffraction
The possibility to study the structure of polyatomic gas-phase molecules by photoelectron diffraction is investigated with the goal of developing a method capable of imaging ultrafast photochemical reactions with femtosecond temporal and sub-Angström spatial resolution. The fluorine 1s-level of adiabatically laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) molecules was ionized by X-ray pulses from the Linac Coherent Light Source Free-Electron Laser, and the angular distributions of photoelectrons with kinetic energies between 30 and 60 eV were recorded by velocity map imaging. Comparison with density functional theory calculations allows relating the measured distributions to the molecular structure. The results of an IR-pump, X-ray-probe experiment on aligned 1,4-dibromobenzene (C6H4Br2)molecules are presented to explore the potential of photoelectron diffraction for
time-resolved imaging. The influence of the alignment laser pulse on the pumping and probing step is discussed. Laser-alignment is contrasted with determination of the molecular orientation by photoelectron-photoion coincidences for an exemplary data set on 1-ethynyl-4-fluorobenzene molecules recorded at the PETRA III synchrotron. Both methods are evaluated with respect to their applicability to record
time-dependent snapshots of molecular structure. The results obtained in this work indicate possible future avenues for investigating ultrafast molecular dynamics using
X-ray Free-Electron Lasers
Photophysics of indole upon x-ray absorption
A photofragmentation study of gas-phase indole (CHN) upon
single-photon ionization at a photon energy of 420 eV is presented. Indole was
primarily inner-shell ionized at its nitrogen and carbon orbitals.
Electrons and ions were measured in coincidence by means of velocity map
imaging. The angular relationship between ionic fragments is discussed along
with the possibility to use the angle-resolved coincidence detection to perform
experiments on molecules that are strongly oriented in their recoil-frame. The
coincident measurement of electrons and ions revealed
fragmentation-pathway-dependent electron spectra, linking the structural
fragmentation dynamics to different electronic excitations. Evidence for
photoelectron-impact self-ionization was observed.Comment: 11 pages, 6 figure
High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules
Unraveling and controlling chemical dynamics requires techniques to image
structural changes of molecules with femtosecond temporal and picometer spatial
resolution. Ultrashort-pulse x-ray free-electron lasers have significantly
advanced the field by enabling advanced pump-probe schemes. There is an
increasing interest in using table-top photon sources enabled by high-harmonic
generation of ultrashort-pulse lasers for such studies. We present a novel
high-harmonic source driven by a 100 kHz fiber laser system, which delivers
10 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The
combination of record-high photon flux and high repetition rate paves the way
for time-resolved studies of the dissociation dynamics of inner-shell ionized
molecules in a coincidence detection scheme. First coincidence measurements on
CHI are shown and it is outlined how the anticipated advancement of fiber
laser technology and improved sample delivery will, in the next step, allow
pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon
sources. These table-top sources can provide significantly higher repetition
rates than the currently operating free-electron lasers and they offer very
high temporal resolution due to the intrinsically small timing jitter between
pump and probe pulses
Strongly aligned gas-phase molecules at Free-Electron Lasers
We demonstrate a novel experimental implementation to strongly align
molecules at full repetition rates of free-electron lasers. We utilized the
available in-house laser system at the coherent x-ray imaging beamline at the
Linac Coherent Light Source. Chirped laser pulses, i. e., the direct output
from the regenerative amplifier of the Ti:Sa chirped pulse amplification laser
system, were used to strongly align 2,5-diiodothiophene molecules in a
molecular beam. The alignment laser pulses had pulse energies of a few mJ and a
pulse duration of 94 ps. A degree of alignment of
\left = 0.85 was measured, limited by the
intrinsic temperature of the molecular beam rather than by the available laser
system. With the general availability of synchronized chirped-pulse-amplified
near-infrared laser systems at short-wavelength laser facilities, our approach
allows for the universal preparation of molecules tightly fixed in space for
experiments with x-ray pulses.Comment: 10 pages, 5 figure
High Harmonic Generation in Mixed Xuv and Nir Fields at a Free-Electron Laser
We Present the Results of an Experiment Investigating the Generation of High-Order Harmonics by a Femtosecond Near-Infrared (NIR) Laser Pulse in the Presence of an Extreme Ultraviolet (XUV) Field Provided by a Free-Electron Laser (FEL), a Process Referred to as XUV-Assisted High-Order Harmonic Generation (HHG). Our Experimental Findings Show that the XUV Field Can Lead to a Small Enhancement in the Harmonic Yield When the XUV and NIR Pulses overlap in Time, while a Strong Decrease of the HHG Yield and a Red Shift of the HHG Spectrum is Observed When the XUV Precedes the NIR Pulse. the Latter Observations Are in Qualitative Agreement with Model Calculations that Consider the Effect of a Decreased Number of Neutral Emitters but Are at Odds with the Predicted Effect of the Correspondingly Increased Ionization Fraction on the Phase Matching. Our Study Demonstrates the Technical Feasibility of XUV-Assisted HHG Experiments at FELs, Which May Provide New Avenues to Investigate Correlation-Driven Electron Dynamics as Well as Novel Ways to Study and Control Propagation Effects and Phase Matching in HHG
Multiple-core-hole resonance spectroscopy with ultraintense X-ray pulses
Understanding the interaction of intense, femtosecond X-ray pulses with heavy
atoms is crucial for gaining insights into the structure and dynamics of
matter. One key aspect of nonlinear light-matter interaction was, so far, not
studied systematically at free-electron lasers -- its dependence on the photon
energy. Using resonant ion spectroscopy, we map out the transient electronic
structures occurring during the complex charge-up pathways. Massively hollow
atoms featuring up to six simultaneous core holes determine the spectra at
specific photon energies and charge states. We also illustrate how the
influence of different X-ray pulse parameters that are usually intertwined can
be partially disentangled. The extraction of resonance spectra is facilitated
by the fact that the ion yields become independent of the peak fluence beyond a
saturation point. Our study lays the groundwork for novel spectroscopies of
transient atomic species in exotic, multiple-core-hole states that have not
been explored previously.Comment: Supplementary information is include
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Time-resolved site-selective imaging of predissociation and charge transfer dynamics: The CH3I B-band
The predissociation dynamics of the 6s (B2E) Rydberg state of gas-phase CH3I were investigated by time-resolved Coulomb-explosion imaging using extreme ultraviolet (XUV) free-electron laser pulses. Inner-shell ionization at the iodine 4d edge was utilized to provide a site-specific probe of the ensuing dynamics. The combination of a velocity-map imaging (VMI) spectrometer coupled with the pixel imaging mass spectrometry (PImMS) camera permitted three-dimensional ionic fragment momenta to be recorded simultaneously for a wide range of iodine charge states. In accord with previous studies, initial excitation at 201.2 nm results in internal conversion and subsequent dissociation on the lower-lying A-state surface on a picosecond time scale. Examination of the time-dependent yield of low kinetic energy iodine fragments yields mechanistic insights into the predissociation and subsequent charge transfer following multiple ionization of the iodine products. The effect of charge transfer was observed through differing delay-dependencies of the various iodine charge states, from which critical internuclear distances for charge transfer could be inferred and compared to a classical over-the-barrier model. Time-dependent photofragment angular anisotropy parameters were extracted from the central slice of the Newton sphere, without Abel inversion, and highlight the effect of rotation of the parent molecule before dissociation, as observed in previous © 2020 The Author(s). Published by IOP Publishing Ltd Printed in the U
Missense Mutation in Exon 2 of SLC36A1 Responsible for Champagne Dilution in Horses
Champagne coat color in horses is controlled by a single, autosomal-dominant gene (CH). The phenotype produced by this gene is valued by many horse breeders, but can be difficult to distinguish from the effect produced by the Cream coat color dilution gene (CR). Three sires and their families segregating for CH were tested by genome scanning with microsatellite markers. The CH gene was mapped within a 6 cM region on horse chromosome 14 (LOD = 11.74 for θ = 0.00). Four candidate genes were identified within the region, namely SPARC [Secreted protein, acidic, cysteine-rich (osteonectin)], SLC36A1 (Solute Carrier 36 family A1), SLC36A2 (Solute Carrier 36 family A2), and SLC36A3 (Solute Carrier 36 family A3). SLC36A3 was not expressed in skin tissue and therefore not considered further. The other three genes were sequenced in homozygotes for CH and homozygotes for the absence of the dilution allele (ch). SLC36A1 had a nucleotide substitution in exon 2 for horses with the champagne phenotype, which resulted in a transition from a threonine amino acid to an arginine amino acid (T63R). The association of the single nucleotide polymorphism (SNP) with the champagne dilution phenotype was complete, as determined by the presence of the nucleotide variant among all 85 horses with the champagne dilution phenotype and its absence among all 97 horses without the champagne phenotype. This is the first description of a phenotype associated with the SLC36A1 gene
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