64 research outputs found
Chiral photoelectron angular distributions from ionization of achiral atomic and molecular species
We show that the combination of two achiral components - atomic or molecular
target plus a circularly polarized photon - can yield chirally structured
photoelectron angular distributions. For photoionization of CO, the angular
distribution of carbon K-shell photoelectrons is chiral when the molecular axis
is neither perpendicular nor (anti-)parallel to the light propagation axis. In
photo-double-ionization of He, the distribution of one electron is chiral, if
the other electron is oriented like the molecular axis in the former case and
if the electrons are distinguishable by their energy. In both scenarios, the
circularly polarized photon defines a plane with a sense of rotation and an
additional axis is defined by the CO molecule or one electron. This is
sufficient to establish an unambiguous coordinate frame of well-defined
handedness. To produce a chirally structured electron angular distribution,
such a coordinate frame is necessary, but not sufficient. We show that
additional electron-electron interaction or scattering processes are needed to
create the chiral angular distribution
Radiationless decay spectrum of O 1s double core holes in liquid water
We present a combined experimental and theoretical investigation of the radiationless decay spectrum of an O 1s double core hole in liquid water. Our experiments were carried out using liquid-jet electron spectroscopy from cylindrical microjets of normal and deuterated water. The signal of the double-core-hole spectral fingerprints (hypersatellites) of liquid water is clearly identified, with an intensity ratio to Auger decay of singly charged O 1s of 0.0014(5). We observe a significant isotope effect between liquid H2O and D2O. For theoretical modeling, the Auger electron spectrum of the central water molecule in a water pentamer was calculated using an electronic-structure toolkit combined with molecular-dynamics simulations to capture the influence of molecular rearrangement within the ultrashort lifetime of the double core hole. We obtained the static and dynamic Auger spectra for H2O, (H2O)5, D2O, and (D2O)5, instantaneous Auger spectra at selected times after core-level ionization, and the symmetrized oxygen-hydrogen distance as a function of time after double core ionization for all four prototypical systems. We consider this observation of liquid-water double core holes as a new tool to study ultrafast nuclear dynamics
How to measure work functions from aqueous solutions
The recent application of concepts from condensed-matter physics to
photoelectron spectroscopy (PES) of volatile, liquid-phase systems has enabled
the measurement of electronic energetics of liquids on an absolute scale.
Particularly, vertical ionization energies, VIEs, of liquid water and aqueous
solutions, both in the bulk and at associated interfaces, can now be routinely
determined. These IEs are referenced to the local vacuum level, which is the
appropriate quantity for condensed matter with associated surfaces, including
liquids. Here, we connect this newly accessible energy level to another
important surface property, namely, the solution work function, e.
We lay out the prerequisites for and unique challenges of determining e
of aqueous solutions and liquids in general. We demonstrate - for a model
aqueous solution with a tetra-n-butylammonium iodide (TBAI) surfactant solute -
that concentration-dependent work functions, associated with the surface
dipoles generated by the segregated interfacial layer of TBA and Iions,
can be accurately measured under controlled conditions. We detail the nature of
surface potentials, uniquely tied to the nature of the flowing-liquid sample,
which must be eliminated or quantified to enable such measurements. This allows
us to refer measured spectra of aqueous solutions to the Fermi level and
quantitatively assign surfactant concentration-dependent spectral shifts to
competing work function and electronic-structure effects, the latter
determining, e.g., (electro)chemical reactivity. We describe the extension of
liquid-jet PES to quantitatively access concentration-dependent surface
descriptors that have so far been restricted to solid-phase measurements. These
studies thus mark the beginning of a new era in the characterization of the
interfacial electronic structure of aqueous solutions and liquids more
generally.Comment: Main manuscript: 26 pages, 7 figures. Supporting information: 5
pages, 5 figure
X-Ray absorption spectroscopy of H3O+
We report the X-ray absorption of isolated H3O+ cations at the O 1s edge. The molecular ions were prepared in a flowing afterglow ion source which was designed for the production of small water clusters, protonated water clusters, and hydrated ions. Isolated H2O+ cations have been analyzed for comparison. The spectra show significant differences in resonance energies and widths compared to neutral H2O with resonances shifting to higher energies by as much as 10 eV and resonance widths increasing by as much as a factor of 5. The experimental results are supported by time-dependent density functional theory calculations performed for both molecular cations, showing a good agreement with the experimental data. The spectra reported here could enable the identification of the individual molecules in charged small water clusters or liquid water using X-ray absorption spectroscopy
Kinematically complete experimental study of Compton scattering at helium atoms near the ionization threshold
Compton scattering is one of the fundamental interaction processes of light
with matter. Already upon its discovery [1] it was described as a billiard-type
collision of a photon kicking a quasi-free electron. With decreasing photon
energy, the maximum possible momentum transfer becomes so small that the
corresponding energy falls below the binding energy of the electron. Then
ionization by Compton scattering becomes an intriguing quantum phenomenon. Here
we report a kinematically complete experiment on Compton scattering at helium
atoms below that threshold. We determine the momentum correlations of the
electron, the recoiling ion, and the scattered photon in a coincidence
experiment finding that electrons are not only emitted in the direction of the
momentum transfer, but that there is a second peak of ejection to the backward
direction. This finding links Compton scattering to processes as ionization by
ultrashort optical pulses [2], electron impact ionization [3,4], ion impact
ionization [5,6], and neutron scattering [7] where similar momentum patterns
occur.Comment: 7 pages, 4 figure
Enabling time-resolved 2D spatial-coherence measurements using the Fourier-analysis method with an integrated curved-grating beam monitor
Direct 2D spatial-coherence measurements are increasingly gaining importance at synchrotron beamlines, especially due to present and future upgrades of synchrotron facilities to diffraction-limited storage rings. We present a method to determine the 2D spatial coherence of synchrotron radiation in a direct and particularly simple way by using the Fourier-analysis method in conjunction with curved gratings. Direct photon-beam monitoring provided by a curved grating circumvents the otherwise necessary separate determination of the illuminating intensity distribution required for the Fourier-analysis method. Hence, combining these two methods allows for time-resolved spatial-coherence measurements. As a consequence, spatial-coherence degradation effects caused by beamline optics vibrations, which is one of the key issues of state-of-the-art X-ray imaging and scattering beamlines, can be identified and analyzed. © 2020 Optical Society of America
Observation of enhanced chiral asymmetries in the inner-shell photoionization of uniaxially oriented methyloxirane enantiomers
Most large molecules are chiral in their structure: they exist as two
enantiomers, which are mirror images of each other. Whereas the rovibronic
sublevels of two enantiomers are almost identical, it turns out that the
photoelectric effect is sensitive to the absolute configuration of the ionized
enantiomer - an effect termed Photoelectron Circular Dichroism (PECD). Our
comprehensive study demonstrates that the origin of PECD can be found in the
molecular frame electron emission pattern connecting PECD to other fundamental
photophysical effects as the circular dichroism in angular distributions
(CDAD). Accordingly, orienting a chiral molecule in space enhances the PECD by
a factor of about 10
Revealing the two-electron cusp in the ground states of He and H2 via quasifree double photoionization
We report on kinematically complete measurements and ab initio nonperturbative calculations of double ionization of He and
H 2 by a single 800 eV circularly polarized photon. We confirm the quasifree mechanism of photoionization for
H 2 and show how it originates from the two-electron cusp in the ground state of a two-electron target. Our approach establishes a method for mapping electrons relative to each other and provides valuable insight into photoionization beyond the electric-dipole approximation.We acknowledge support by DFG and
BMBF
Observation of Photoion Backward Emission in Photoionization of He and N2
We experimentally investigate the effects of the linear photon momentum on
the momentum distributions of photoions and photoelectrons generated in
one-photon ionization in an energy range of 300 eV 40 keV.
Our results show that for each ionization event the photon momentum is imparted
onto the photoion, which is essentially the system's center of mass.
Nevertheless, the mean value of the ion momentum distribution along the light
propagation direction is backward-directed by times the photon momentum.
These results experimentally confirm a 90 year old prediction.Comment: 5 pages, 3 figure
- …