91 research outputs found
Femtosecond Photoelectron Imaging of Anions
Several recent results of a time-resolved photoelectron imaging experiment are presented. Following a broad introduction into the area of femtochemistry and time-resolved photoelectron spectroscopy, a detailed description of the spec- trometer is given. This utilises an electrospray ionisation source, coupled to an electrostatic ion trap. Ions are mass selected using time-of-flight methods and investigated using photoelectron imaging in a velocity-mapping geometry. Ultrafast dynamics are investigated by pump-probe spectroscopy with femtosec- ond laser pulses.
Recent results are separated into three distinct projects:
(i) The investigation of electron acceptor radical anions based on the quinone backbone. These commonly exhibit electron transfer rates exceeding those pre- dicted by Marcus theory by orders of magnitude. We show that an alternative pathway to electron transfer could involve the participation of electronic excited states, as these couple strongly to the anion ground state. Specifically, for p- Benzoquinone we show that electronic resonances located in the detachment continuum primarily undergo internal conversion via a number of conical inter- sections.
(ii) Several polyanions have been investigated in the gas-phase. These systems exhibit unusual electronic properties, due to the presence of multiple excess charges, leading to the formation of a repulsive Coulomb barrier to photode- tachment. We investigate the effect of excess internal energy on this barrier and how it affects outgoing photoelectrons. We show that the trajectories of electrons are strongly influenced by this potential and demonstrate its use as a probe for large amplitude structural dynamics in polyanions.
(iii) The isolated chromophore of the green fluorescent protein (GFP) has been studied, and the vertical and adiabatic detachment energies determined for the first time. Using time-resolved spectroscopy the excited state dynamics are in- vestigated. We show that the first singlet excited state of the anion primarily decays through internal conversion, explaining the absence of fluorescence in the gas-phase. Using high level quantum chemistry calculations we show the specific motion involved and hence confirm the function of the protein back- bone in GFP.
This thesis is concluded with a few suggested experimental improvements and ideas for future studies of anions using the presented spectrometer
Characterizing gas flow from aerosol particle injectors
A novel methodology for measuring gas flow from small orifices or nozzles
into vacuum is presented. It utilizes a high-intensity femtosecond laser pulse
to create a plasma within the gas plume produced by the nozzle, which is imaged
by a microscope. Calibration of the imaging system allows for the extraction of
absolute number densities. We show detection down to helium densities of
~cm with a spatial resolution of a few micrometer. The
technique is used to characterize the gas flow from a convergent-nozzle aerosol
injector [Struct.\ Dyn.~2, 041717 (2015)] as used in single-particle
diffractive imaging experiments at free-electron laser sources. Based on the
measured gas-density profile we estimate the scattering background signal under
typical operating conditions of single-particle imaging experiments and
estimate that fewer than 50 photons per shot can be expected on the detector
Optimizing aerodynamic lenses for single-particle imaging
A numerical simulation infrastructure capable of calculating the flow of gas
and the trajectories of particles through an aerodynamic lens injector is
presented. The simulations increase the fundamental understanding and predict
optimized injection geometries and parameters. Our simulation results were
compared to previous reports and also validated against experimental data for
500 nm polystyrene spheres from an aerosol-beam- characterization setup. The
simulations yielded a detailed understanding of the radial phase-space
distribution and highlighted weaknesses of current aerosol injectors for
single-particle diffractive imaging. With the aid of these simulations we
developed new experimental implementations to overcome current limitations
Spatially separated polar samples of the cis and trans conformers of 3-fluorophenol
We demonstrate the spatial separation of the cis- and trans-conformers of
3-fluorophenol in the gas phase based on their distinct electric dipole
moments. For both conformers we create very polar samples of their
lowest-energy rotational quantum states. A >95 % pure beam of
trans-3-fluorophenol and a >90 % pure beam of the lowest-energy rotational
states of the less polar cis-3-fluorophenol were obtained for helium and neon
supersonic expansions, respectively. This is the first demonstration of the
spatial separation of the lowest-energy rotational states of the least polar
conformer, which is necessary for strong alignment and orientation of all
individual conformers.Comment: 5 pages, 5 figure
Spatially-controlled complex molecules and their applications
The understanding of molecular structure and function is at the very heart of
the chemical and molecular sciences. Experiments that allow for the creation of
structurally pure samples and the investigation of their molecular dynamics and
chemical function have developed tremendously over the last few decades,
although "there's plenty of room at the bottom" for better control as well as
further applications.
Here, we describe the use of inhomogeneous electric fields for the
manipulation of neutral molecules in the gas-phase, \ie, for the separation of
complex molecules according to size, structural isomer, and quantum state. For
these complex molecules, all quantum states are strong-field seeking, requiring
dynamic fields for their confinement. Current applications of these controlled
samples are summarised and interesting future applications discussed.Comment: Accepted by Int. Rev. Phys. Che
Development and characterization of a laser-induced acoustic desorption source
A laser-induced acoustic desorption source, developed for use at central
facilities, such as free-electron lasers, is presented. It features prolonged
measurement times and a fixed interaction point. A novel sample deposition
method using aerosol spraying provides a uniform sample coverage and hence
stable signal intensity. Utilizing strong-field ionization as a universal
detection scheme, the produced molecular plume is characterized in terms of
number density, spatial extend, fragmentation, temporal distribution,
translational velocity, and translational temperature. The effect of desorption
laser intensity on these plume properties is evaluated. While translational
velocity is invariant for different desorption laser intensities, pointing to a
non-thermal desorption mechanism, the translational temperature increases
significantly and higher fragmentation is observed with increased desorption
laser fluence.Comment: 8 pages, 7 figure
Characterizing and optimizing a laser-desorption molecular beam source
The design and characterization of a new laser-desorption molecular beam
source, tailored for use in x-ray-free-electron-laser and
ultrashort-pulse-laser imaging experiments, is presented. It consists of a
single mechanical unit containing all source components, including the
molecular-beam valve, the sample, and the fiber-coupled desorption laser, which
is movable in five axes, as required for experiments at central facilities.
Utilizing strong-field ionization, we characterize the produced molecular beam
and evaluate the influence of desorption laser pulse energy, relative timing of
valve opening and desorption laser, sample bar height, and which part of the
molecular packet is probed on the sample properties. Strong-field ionization
acts as a universal probe and allows to detect all species present in the
molecular beam, and hence enables us to analyze the purity of the produced
molecular beam, including molecular fragments. We present optimized
experimental parameters for the production of the purest molecular beam,
containing the highest yield of intact parent ions, which we find to be very
sensitive to the placement of the desorbed-molecules plume within the
supersonic expansion
Visualizing aerosol-particle injection for diffractive-imaging experiments
Delivering sub-micrometer particles to an intense x-ray focus is a crucial
aspect of single-particle diffractive-imaging experiments at x-ray
free-electron lasers. Enabling direct visualization of sub-micrometer aerosol
particle streams without interfering with the operation of the particle
injector can greatly improve the overall efficiency of single-particle imaging
experiments by reducing the amount of time and sample consumed during
measurements. We have developed in-situ non-destructive imaging diagnostics to
aid real-time particle injector optimization and x-ray/particle-beam alignment,
based on laser illumination schemes and fast imaging detectors. Our diagnostics
are constructed to provide a non-invasive rapid feedback on injector
performance during measurements, and have been demonstrated during diffraction
measurements at the FLASH free-electron laser.Comment: 15 page
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