Photon-electron coincidence experiments at synchrotron radiation facilities with arbitrary bunch modes

We report the adaptation of an electron–photon coincidence detection scheme to the multibunch hybrid mode of the synchrotron radiation source BESSY II (Helmholtz-Zentrum Berlin). Single-event-based data acquisition and evaluation, combined with the use of relative detection times between the coincident particles, enable the acquisition of proper coincidence signals from a quasi-continuous excitation pattern. The background signal produced by accidental coincidences in the time difference representation is modeled using the non-coincident electron and photon spectra. We validate the method by reproducing previously published results, which were obtained in the single bunch mode, and illustrate its usability for the multibunch hybrid mode by investigating the photoionization of CO2 into CO+2 B satellite states, followed by subsequent photon emission. The radiative lifetime obtained and the electron binding energy are in good agreement with earlier publications. We expect this method to be a useful tool to extend the versatility of coincident particle detection to arbitrary operation modes of synchrotron radiation facilities and other excitation sources without the need for additional experimental adjustments

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

Photoelectron circular dichroism of O 1ss-photoelectrons of uniaxially oriented trifluoromethyloxirane: Energy dependence and sensitivity to molecular configuration

The photoelectron circular dichroism (PECD) of the O 1s-photoelectrons of trifluoromethyloxirane(TFMOx) is studied experimentally and theoretically for different photoelectron kinetic energies. The experiments were performed employing circularly polarized synchrotron radiation and coincidentelectron and fragment ion detection using Cold Target Recoil Ion Momentum Spectroscopy. The corresponding calculations were performed by means of the Single Center method within the relaxed-core Hartree-Fock approximation. We concentrate on the energy dependence of the differential PECD of uniaxially oriented TFMOx molecules, which is accessible through the employed coincident detection. We also compare results for differential PECD of TFMOx to those obtained for the equivalent fragmentation channel and similar photoelectron kinetic energy of methyloxirane (MOx), studied in our previous work. Thereby, we investigate the influence of the substitution of the methyl-group by the trifluoromethyl-group at the chiral center on the molecular chiral response. Finally, the presently obtained angular distribution parameters are compared to those available in literature.Comment: 6 fig

Properties of radiative charge transfer in heterogeneous noble gas clusters

Radiative charge transfer RCT from cationic to neutral atoms is a fundamental and frequently occurring process in ion neutral collisions and in van der Waals clusters. In contrast to collisions with only two collision partners, RCT in clusters is more involved as it may depend on the cluster size, the cluster stoichiometry, and the local arrangement of atoms in the cluster. Here, we present a systematic investigation of an RCT photon spectrum as a function of cluster size, stoichometry, and local atom arrangement. For this purpose, we utilize RCT in heterogeneous NeKr and NeXe clusters after Ne 2p photoionization. Our results confirm that Ne dimer ions form prior to RCT if enough Ne is available and we observe that different features dominate the photon spectrum depending on the cluster production parameters. Additionally, we find that the lifetime of the radiative decay is sensitive to the cluster stoichiometry, which we explain and interpret by theoretical calculations on the RCT decay width for different local geometric structures of the involved atoms. We conclude that RCT properties such as photon spectrum and lifetime in turn exhibit information on the mean size and stoichiometry of a cluster jet and the arrangement of the involved atom

Core level interatomic Coulombic decay in van der Waals clusters

We report on the experimental observation of the direct decay of a core vacancy in van der Waals clusters byemission of a fast electron from a neighboring atom. The process can be regarded as an interatomic Coulombicdecay of core holes (core-level ICD). We identify it unambiguously by electron-electron and electron-electron-photon coincidence spectroscopy of the decay of 2pvacancies in Ar clusters. While several earlier works reportedthe absence of this channel, we find core-level ICD to be of considerable significance and quantify the branchingratio of this nonlocal electron emission to conventional local Auger decay as (0.8±0.2)%. Our results aresupported by calculations on smaller clusters and show a reasonable agreement. This report on a successfullyperformed electron-electron-photon coincidence experiment provides a perspective for explorations of matterexposed to ionizing radiation. The observed core-level ICD is proposed to be of general importance for studieson charge redistribution after core-level photoionization where van der Waals clusters are often used as prototypesystems

Search for the interatomic Auger effect in Nitrous Oxide

The interatomic Auger effect following O 1s ionization in N2O has been experimentally investigated using multi-electron coincidence spectroscopy. The expected transition energies have been established by comparison to the measured N 1s−1v−1 core-valence double ionization energies. We describe a procedure to eliminate the background of two competing processes contributing spectroscopic signatures to the same energy range, namely double Auger decay of the O 1s vacancy and direct single-photon double ionization into the N 1s−1v−1 states. While the interatomic Auger transitions could not be successfully isolated, we provide an upper boundary of the transition probability of 0.07% with respect to the dominant single Auger decay after O 1s ionization

Recovery of High Energy Photoelectron Circular Dichroism through Fano Interference

It is commonly accepted that the magnitude of a photoelectron circular dichroism (PECD) is governed by the ability of an outgoing photoelectron wave packet to probe the chiral asymmetry of a molecule. To be able to accumulate this characteristic asymmetry while escaping the chiral ion, photoelectrons need to have relatively small kinetic energies of up to a few tens of electron volts. Here, we demonstrate a substantial PECD for very fast photoelectrons above 500 eV kinetic energy released from methyloxirane by a participator resonant Auger decay of its lowermost O 1s excitation. This effect emerges as a result of the Fano interference between the direct and resonant photoionization pathways, notwithstanding that their individual effects are negligibly small. The resulting dichroic parameter has an anomalous dispersion: It changes its sign across the resonance, which can be considered as an analogue of the Cotton effect in the x-ray regime