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

Construction and Characterization of a Cs-Sputter Ion Source for theGeneration of Intense Beams of Negatively Charged Atomic Ions

In this thesis the construction and a first characterization of a new cesium-sputterion source for the generation of intense beams of negatively charged atomic ions ispresented. Such an ion source was manufactured by the mechanical workshop of thephysics department at Justus-Liebig-University Giessen. The source is in operationsince january 2017 at the ion source testbench of the group for atomic and moleculephysics.Ion beams were generated from different sputter targets. The generated currentswere satisfying compared to the currents which have been generated with an ECR ionsource. However, the here produced ion currents are lower than those from a similarsource operated at the Max-Planck-Institute of Nuclear Physics in Heidelberg. Thisist because of a currently still too high base-pressure in the source. Moreover, theinfluence of the sputter potential as well as the ionizer current on the generated ioncurrent have been investigated. In addition, a manual for assembly, operation anddisassembly of the ion source has been written

Near K-edge Photoionization and Photoabsorption of Singly, Doubly, and Triply Charged Silicon Ions

Experimental and theoretical results are presented for double, triple, and quadruple photoionization of Si$^+$ and Si$^{2+}$ ions and for double photoionization of Si$^{3+}$ ions by a single photon. The experiments employed the photon-ion merged-beams technique at a synchrotron light source. The experimental photon-energy range 1835--1900~eV comprises resonances associated with the excitation of a $1s$ electron to higher subshells and subsequent autoionization. Energies, widths, and strengths of these resonances are extracted from high-resolution photoionization measurements, and the core-hole lifetime of K-shell ionized neutral silicon is inferred. In addition, theoretical cross sections for photoabsorption and multiple photoionization were obtained from large-scale Multi-Configuration Dirac-Hartree-Fock (MCDHF) calculations. The present calculations agree with the experiment much better than previously published theoretical results. The importance of an accurate energy calibration of laboratory data is pointed out. The present benchmark results are particularly useful for discriminating between silicon absorption in the gaseous and in the solid component (dust grains) of the interstellar medium

Near K-edge Photoionization and Photoabsorption of Singly, Doubly, and Triply Charged Silicon Ions

Experimental and theoretical results are presented for double, triple, and quadruple photoionization of Si$^+$ and Si$^{2+}$ ions and for double photoionization of Si$^{3+}$ ions by a single photon. The experiments employed the photon-ion merged-beams technique at a synchrotron light source. The experimental photon-energy range 1835--1900~eV comprises resonances associated with the excitation of a $1s$ electron to higher subshells and subsequent autoionization. Energies, widths, and strengths of these resonances are extracted from high-resolution photoionization measurements, and the core-hole lifetime of K-shell ionized neutral silicon is inferred. In addition, theoretical cross sections for photoabsorption and multiple photoionization were obtained from large-scale Multi-Configuration Dirac-Hartree-Fock (MCDHF) calculations. The present calculations agree with the experiment much better than previously published theoretical results. The importance of an accurate energy calibration of laboratory data is pointed out. The present benchmark results are particularly useful for discriminating between silicon absorption in the gaseous and in the solid component (dust grains) of the interstellar medium

Role of L -shell single and double core-hole production and decay in m -fold ( 1 ⩽\leqslant m ⩽\leqslant 6 ) photoionization of the Ar+^+ ion

Multiple ionization of the Ar$^{+}(3s^2 3p^5)$ ion by a single photon has been investigated in the photon-energy range 250-1800~eV employing the photon-ion merged-beams technique. Absolute partial cross sections were measured for all Ar$^{(1+m)+}$ product-ion channels with 1 $\leqslant$ m $\leqslant$ 6 covering a size range from several tens of Mb down to a few b. Narrow $2p$-subshell excitation resonances were observed in all channels up to quadruple ionization at a photon-energy bandwidth of 52~meV. Double excitations involving a $2p$ and a $3s$ or $3p$ electron were also studied at high resolution and the measurements of the broad $2s$ excitation resonances directly showed their natural widths. Contributions of direct photo double ionization (PDI) to the production of the highest final Ar ion charge states are revealed, with PDI of the $2s$ subshell being mainly responsible for the production of Ar$^{7+}$. The experiment made use of the PIPE setup installed at beamline P04 of the PETRA III synchrotron light source of DESY in Hamburg. The measurements were supported by theoretical calculations to identify the main contributions to the observed cross sections. Comparisons of theory and experiment show remarkable agreement but also hint to additional ionization mechanisms that are not considered in the theoretical models such as core ionization accompanied by excitations with subsequent Auger decays leading to net $m$-fold ionization with m $\leqslant$ 4

Cover Feature: Vibrationally Resolved Inner‐Shell Photoexcitation of the Molecular Anion C2−C_{2}^− (ChemPhysChem 11/2023)

The cover illustrates the application of the photon-ion merged-beams method to vibrationally-resolved inner-shell excitation of carbon dimer anions by synchrotron radiation. Cover design by Elisa Monte. More information can be found in the Research Article by Stefan Schippers, Pierre-Michel Hillenbrand, Alexander Perry-Sassmannshausen and coworkers (https://doi.org/10.1002/cphc.202300061)

Multiple Photodetachment of Carbon Anions via Single and Double Core-Hole Creation

We report on new measurements of m-fold photodetachment (m = 2 − 5) of carbon anions via K-shellexcitation and ionization. The experiments were carried out employing the photon-ion merged-beamstechnique at a synchrotron light source. While previous measurements were restricted to doubledetachment (m = 2) and to just the lowest-energy K-shell resonance at about 282 eV, our absoluteexperimental m-fold detachment cross sections at photon energies of up to 1000 eV exhibit a wealth of newthresholds and resonances. We tentatively identify these features with the aid of detailed atomic-structurecalculations. In particular, we find unambiguous evidence for fivefold detachment via double K-holeproduction

Disentangling the Photodissociation Dynamics of the HF+HF^{+} Molecular Radical via Kinetic-Energy-Release-Resolved F 1s Core Excitation and Ionization

The F 1s core level photoionization of the ionic molecular radical HF+ has been studied using the photon–ion merged-beams technique at a synchrotron radiation source. Upon analyzing kinetic energy release (KER) dependent photoion yield spectra, complex ultrafast dissociation dynamics of the F 1s core hole excited σ* state can be revealed. By means of configuration–interaction electronic structure calculations of the excited molecular potential energy curves, this complex process can be attributed to a spin-dependent dissociation of the excited σ* biradical state

Soft X-Ray-induced Dimerization of Methane

Carbon 1s excitation of methane, CH4, has been studied in the gas phase using the ion trap integrated with the photon–ion instrument at PETRA III/DESY and soft X-rays from the beamline P04. The created photoions are stored within the ion trap so that in further steps the photoions can undergo reactions with neutral methane molecules. The ionic photoproducts as well as reaction products created thereby are mass-over-charge analyzed by an ion time-of-flight spectrometer. Besides the photoions, product ions with up to three carbon atoms are found. In contrast to experiments using vacuum ultraviolet radiation, especially highly reactive product ions with a small number of hydrogen atoms such as ${{\rm{C}}}_{2}{{\rm{H}}}_{2}^{+}$ and ${{\rm{C}}}_{2}{{\rm{H}}}_{3}^{+}$ are found, which are important precursors for larger hydrocarbons such as C6H6. Possible production routes of the product ions are analyzed on the basis of a model that considers the probabilities for photofragmentation and the first subsequent chemical reaction step. The model indicates that the high degree of fragmentation by photons with energies around 280 eV is favoring these products. The results of the measurements show that the products like ${{\rm{C}}}_{2}{{\rm{H}}}_{2}^{+}$ and ${{\rm{C}}}_{2}{{\rm{H}}}_{3}^{+}$ can be generated by a single collision of the ionization product with neutral methane. The results suggest that soft X-rays might be important for chemical reactions in planetary atmospheres, which has usually not been taken into account. However, due to the high degree of fragmentation and large cross sections involved, they can have a large influence even when the corresponding photon flux is rather small