57 research outputs found
Single photon double ionization of the helium dimer
We show that a single photon can ionize the two helium atoms of the helium
dimer in a distance up to 10 {\deg}A. The energy sharing among the electrons,
the angular distributions of the ions and electrons as well as comparison with
electron impact data for helium atoms suggest a knock-off type double
ionization process. The Coulomb explosion imaging of He_2 provides a direct
view of the nuclear wave function of this by far most extended and most diffuse
of all naturally existing molecules.Comment: 10 pages, 5 figure
Subfemtosecond Control of Molecular Fragmentation by Hard X-Ray Photons
Tuning hard x-ray excitation energy along Cl 1s→σ∗ resonance in gaseous HCl
allows manipulating molecular fragmentation in the course of the induced
multistep ultrafast dissociation. The observations are supported by
theoretical modeling, which shows a strong interplay between the topology of
the potential energy curves, involved in the Auger cascades, and the so-called
core-hole clock, which determines the time spent by the system in the very
first step. The asymmetric profile of the fragmentation ratios reflects
different dynamics of nuclear wave packets dependent on the photon energy
A measurement of the evolution of Interatomic Coulombic Decay in the time domain
During the last 15 years a novel decay mechanism of excited atoms has been
discovered and investigated. This so called ''Interatomic Coulombic Decay''
(ICD) involves the chemical environment of the electronically excited atom: the
excitation energy is transferred (in many cases over long distances) to a
neighbor of the initially excited particle usually ionizing that neighbor. It
turned out that ICD is a very common decay route in nature as it occurs across
van-der-Waals and hydrogen bonds. The time evolution of ICD is predicted to be
highly complex, as its efficiency strongly depends on the distance of the atoms
involved and this distance typically changes during the decay. Here we present
the first direct measurement of the temporal evolution of ICD using a novel
experimental approach.Comment: 6 pages, 4 figures, submitted to PR
Interatomic-Coulombic-decay-induced recapture of photoelectrons in helium dimers
We investigate the onset of photoionization shakeup induced interatomic
Coulombic decay (ICD) in He2 at the He+*(n = 2) threshold by detecting two He+
ions in coincidence. We find this threshold to be shifted towards higher
energies compared to the same threshold in the monomer. The shifted onset of
ion pairs created by ICD is attributed to a recapture of the threshold
photoelectron after the emission of the faster ICD electron.Comment: 5 Pages, 2 Figure
Observation of interatomic Coulombic decay induced by double excitation of helium in nanodroplets
Interatomic Coulombic decay (ICD) plays a crucial role in weakly bound
complexes exposed to intense or high-energy radiation. So far, neutral or ionic
atoms or molecules have been prepared in singly excited electron or hole states
which can transfer energy to neighboring centers and cause ionization and
radiation damage. Here we demonstrate that a doubly excited atom, despite its
extremely short lifetime, can decay by ICD; evidenced by high-resolution
photoelectron spectra of He nanodroplets excited to the 2s2p+ state. We find
that ICD proceeds by relaxation into excited HeHe atom-pair states, in
agreement with calculations. The ability of inducing ICD by resonant excitation
far above the single-ionization threshold opens opportunities for controlling
radiation damage to a high degree of element specificity and spectral
selectivity.Comment: 6 pages, 4 figures, to be submitted to PR
Nature and impact of charge transfer to ground-state dications in atomic and molecular environments
Charge transfer processes between weakly bound entities play an important role in various chemical and biological environments. In this combined experimental and theoretical work, we investigate the nature of charge-transfer processes in homogeneous atomic and heterogeneous atomic-molecular clusters. Our results reveal fundamentally different processes to be at play in pure argon clusters compared to mixed argon-nitrogen systems: We demonstrate that the former species decay via photon-mediated charge transfer while a nonradiative direct process is found dominant in the atomic-molecular cases. Our results are of general interest for studies on charge redistribution in more complex and biologically relevant samples where molecules are involved
Potential Energy Surface Reconstruction and Lifetime Determination of Molecular Double-Core-Hole States in the Hard X-Ray Regime
A combination of resonant inelastic x-ray scattering and resonant Auger
spectroscopy provides complementary information on the dynamic response of
resonantly excited molecules. This is exemplified for CH3I, for which we
reconstruct the potential energy surface of the dissociative I 3d−2 double-
core-hole state and determine its lifetime. The proposed method holds a strong
potential for monitoring the hard x-ray induced electron and nuclear dynamic
response of core-excited molecules containing heavy elements, where ab initio
calculations of potential energy surfaces and lifetimes remain challenging
Ultrafast temporal evolution of interatomic Coulombic decay in NeKr dimers
We investigate interatomic Coulombic decay in NeKr dimers after neon inner-valence photoionization [Ne+(2s-1)] using a synchrotron light source. We measure with high energy resolution the two singly charged ions of the Coulomb-exploding dimer dication and the photoelectron in coincidence. By carefully tracing the post-collision interaction between the photoelectron and the emitted ICD electron we are able to probe the temporal evolution of the state as it decays. Although the ionizing light pulses are 80 picoseconds long, we determine the lifetime of the intermediate dimer cation state and visualize the contraction of the nuclear structure on the femtosecond time scale
Acetylacetone photodynamics at a seeded freeelectron laser
The first steps in photochemical processes, such as photosynthesis or animal vision, involve changes in electronic and geometric structure on extremely short time scales. Time-resolved photoelectron spectroscopy is a natural way to measure such changes, but has been hindered hitherto by limitations of available pulsed light sources in the vacuum-ultraviolet and soft Xray spectral region, which have insufficient resolution in time and energy simultaneously. The unique combination of intensity, energy resolution, and femtosecond pulse duration of the FERMI-seeded free-electron laser can now provide exceptionally detailed information on photoexcitation–deexcitation and fragmentation in pump-probe experiments on the 50- femtosecond time scale. For the prototypical system acetylacetone we report here electron spectra measured as a function of time delay with enough spectral and time resolution to follow several photoexcited species through well-characterized individual steps, interpreted using state-of-the-art static and dynamics calculations. These results open the way for investigations of photochemical processes in unprecedented detail
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