59 research outputs found
Strong-field ionization of complex molecules
Strong-field photoelectron momentum imaging of the prototypical biomolecule
indole was disentangled in a combined experimental and computational approach.
Experimentally, strong control over the molecules enabled the acquisition of
photoelectron momentum distributions in the molecular frame for a well-defined,
narrow range of incident intensities. A novel, highly efficient semiclassical
simulation setup based on the adiabatic tunneling theory quantitatively
reproduced these results. Jointly, experiment and computations revealed
holographic structures in the asymptotic momentum distributions, which were
found to sensitively depend on the alignment of the molecular frame. We
identified the essential molecular properties that shape the photoelectron
wavepacket in the first step of the ionization process and employ a
quantum-chemically exact description of the cation during the subsequent
continuum dynamics. The detailed modeling of the molecular ion, which accounts
for its polarization by the laser-electric field, enables the simulation of
laser-induced electron diffraction off large and complex molecules and provides
full insight into the photoelectron's dynamics in terms of semiclassical
trajectories. This provides the computational means to unravel strong-field
diffractive imaging of biomolecular systems on femtosecond time scales
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
Setting the photoelectron clock through molecular alignment
The interaction of strong laser fields with matter intrinsically provides
powerful tools to image transient dynamics with an extremely high
spatiotemporal resolution. Here, we study strong-field ionisation of
laser-aligned molecules and show a full real-time picture of the photoelectron
dynamics in the combined action of the laser field and the molecular
interaction. We demonstrate that the molecule has a dramatic impact on the
overall strong-field dynamics: it sets the clock for the emission of electrons
with a given rescattering kinetic energy. This result represents a benchmark
for the seminal statements of molecular-frame strong-field physics and has
strong impact on the interpretation of self-diffraction experiments.
Furthermore, the resulting encoding of the time-energy relation in
molecular-frame photoelectron momentum distributions shows the way of probing
the molecular potential in real-time and accessing a deeper understanding of
electron transport during strong-field interactions.Comment: Final version. Added appendixes and supplementary display item
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
Picosecond pulse-shaping for strong three-dimensional field-free alignment of generic asymmetric-top molecules
We demonstrate three-dimensional (3D) field-free alignment of the
prototypical non-rotation-symmetric molecule indole using elliptically
polarized, shaped, off-resonant laser pulses. A truncated laser pulse is
produced using a combination of extreme linear chirping and controlled phase
and amplitude shaping using a spatial-light-modulator (SLM) based pulse shaper
of a broadband laser pulse. The angular confinement is detected through
velocity-map imaging of H and C fragments resulting from
strong-field ionization and Coulomb explosion of the aligned molecules by
intense femtosecond laser pulses. The achieved three-dimensional alignment is
characterized by comparing the result of ion-velocity-map measurements for
different alignment directions and for different times during and after the
alignment laser pulse to accurate computational results. The achieved strong
three-dimensional field-free alignment of
demonstrates the feasibility of both, strong three-dimensional alignment of
generic complex molecules and its quantitative characterization
Molecular sunscreen: water protects pyrrole from radiation damage
Radiation-induced damage of biological matter is an ubiquitous problem in
nature. The influence of the hydration environment is widely discussed, but its
exact role remains elusive. We present the experimental observation of a
hydrogen-bonded water molecule acting as a radiation protection agent for
ionized pyrrole, a prototypical aromatic biomolecule. Pure samples of pyrrole
and pyrrole(HO) were outer-valence ionized and the subsequent damage and
relaxation processes were studied. Bare pyrrole fragmented through the breaking
of the C-C or N-C covalent bonds. However, for pyrrole(HO), we observed a
strong protection of the pyrrole ring through the dissociative release of
neutral water or by transferring an electron or proton across the hydrogen
bond. Furthermore, for pyrrole(HO) a smaller probability for double
ionization was observed. Overall, a single water molecule strongly reduces the
fragmentation probability and thus the persistent radiation damage of ionized
pyrrole.Comment: 10 pages, 8 figure
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
The fate of steroid estrogens: Partitioning during wastewater treatment and onto river sediments
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ 2010 Springer Science+Business Media B.V.The partitioning of steroid estrogens in wastewater treatment and receiving waters is likely to influence their discharge to, and persistence in, the environment. This study investigated the partitioning behaviour of steroid estrogens in both laboratory and field studies. Partitioning onto activated sludge from laboratory-scale Husmann units was rapid with equilibrium achieved after 1 h. Sorption isotherms and Kd values decreased in the order 17α-ethinyl estradiol > 17α-estradiol > estrone > estriol without a sorption limit being achieved (1/n >1). Samples from a wastewater treatment works indicated no accumulation of steroid estrogens in solids from primary or secondary biological treatment, however, a range of steroid estrogens were identified in sediment samples from the River Thames. This would indicate that partitioning in the environment may play a role in the long-term fate of estrogens, with an indication that they will be recalcitrant in anaerobic conditions.EPSR
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