Lutz Marder

We report on the shift of energies released during Auger cascades in rare-gas ions, when the ions are not isolated but embedded in a homogeneous cluster. By combining high-resolution electron spectroscopy with multielectron coincidence spectroscopy of small Ar and Xe clusters, we investigate the dependence of Auger electron emission energies on the charge state of the emitting ion in the polarizable cluster medium up to charge states of + 4 . Our results show deviations from the predictions of the established electrostatic Born model by a factor of ≈ 2 for the final step of the cascades.

Excited electronic states of fenchone, thiofenchone, and selenofenchone are characterized and assigned with different gas‐phase spectroscopic methods and ab initio quantum chemical calculations. With an increasing atomic number of the chalcogen, increasing bathochromic (red) shifts are observed, which vary in strength for Rydberg states, valence‐excited states, and ionization energies. The spectroscopic insight is used to state‐resolve the contributions in multiphoton photoelectron circular dichroism with femtosecond laser pulses. This is shown to be a sensitive observable of molecular chirality in all studied chalcogenofenchones. This work contributes new spectroscopic information, particularly on thiofenchone and selenofenchone. It may open a perspective for future coherent control experiments exploiting resonances in the visible and near‐ultraviolet spectral regions.

Highly charged cations are omnipresent species after the interaction of high-energy or high-intensity light with matter. When embedded in environments, the mechanism and outcome of the redistribution of the cation’s charge are crucial for the further fate of the whole system. Generally, ground-state cations can decay by charge transfer, proceeding radiatively, through nuclear dynamics, or by electron-transfer-mediated decay (ETMD). ETMD causes electron emission from a remote neighbor and appears ubiquitous in loosely bound systems. It remained unclear how multiply charged ions decay if conventional ETMD channels are closed. Here, we show that a yet undiscovered variant of ETMD is possible, where multiple electrons are collectively transferred. Explicitly, we observe ETMD in which two electrons from two distinct neighbors (partially) neutralize a multiply charged ion, and an electron from a fourth site is emitted. According to the established nomenclature, we suggest naming the process ETMD(4).

The two-dimensional photon-excitation photon-emission (PhexPhem) map of the archetypal H 2 molecule, constructed from dispersed-fluorescence measurements after energy-scanning excitation by small-bandwidth photons, displays complete systems of its Condon diffraction bands. They originate from spontaneous radiative dissociation of individual bound electronically excited rovibronic levels, one of the most relevant processes for the destruction of H 2 in space. For the B − X system of electronic states, we show that specific measured spectral characteristics of individual bands are extremely sensitive to the difference Δ R of the internuclear distances, where the two potential energy curves each have their minimum. Using data from recorded H 2 -PhexPhem maps, it is possible to experimentally validate the calculations of Δ R down to an accuracy of at least 50 f m . This accuracy may be used as a sensitive experimental test to calculational accuracy. It is well feasible to improve the currently achieved experimental accuracy, for an even more accurate benchmark of advanced potential energy curve calculations.

X-ray-induced damage is one of the key topics in radiation chemistry. Substantial damage is attributed to low-energy electrons and radicals emerging from direct inner-shell photoionization or produced by subsequent processes. We apply multi-electron coincidence spectroscopy to X-ray-irradiated aqueous solutions of inorganic ions to investigate the production of low-energy electrons (LEEs) in a predicted cascade of intermolecular charge- and energy-transfer processes, namely electron-transfer-mediated decay (ETMD) and interatomic/intermolecular Coulombic decay (ICD). An advanced coincidence technique allows us to identify several LEE-producing steps during the decay of 1s vacancies in solvated Mg²⁺ ions, which escaped observation in previous non-coincident experiments. We provide strong evidence for the predicted recovering of the ion’s initial state. In natural environments the recovering of the ion’s initial state is expected to cause inorganic ions to be radiation-damage hot spots, repeatedly producing destructive particles under continuous irradiation.

High-resolution absorption, dispersed fluorescence emission, and photoionization cross sections are presented for gas-phase adamantane excited by synchrotron radiation in the exciting-photon energy range of 6–30 eV. Relative and absolute absorption cross sections of so-far unmatched resolution of down to 0.27 cm⁻¹ line width in the region from 6.4–28 eV are shown along with newly discovered vibronic substructures around the HOMO–LUMO transition. Absorption line positions are provided with very high accuracy and listed in tabular form to be used as spectral fingerprints for the detection of adamantane in interstellar media, where its column density may be determined via the absolute cross sections. The fluorescence emission lies in the ultraviolet range from 190–250 nm and is excited starting at the HOMO–LUMO transition at 6.49 eV, which corresponds to the highest fluorescence emission energy. Hitherto unreported fluorescence in the same spectral range and relative photoionization cross sections in the exciting-photon energy range up to 30 eV are also presented along with lifetime measurements for differentiation of the involved electronic states.

Interatomic Coulombic Decay (ICD) and related interatomic and intermolecular autoionization mechanisms are ubiquitous decay processes of excited atoms and molecules in an environment. It is commonly accepted that the efficiency of ICD of an ionized atom in a cluster increases with an increasing number of nearest neighbors. Here, we present a method for experimental validation of this assumption by a site-specific and quantitative comparison of ICD and its main competitor, Auger decay, in core-level ionized Kr clusters. Our results are in quantitative agreement with scaled theoretical calculations on Kr₂.

Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d_3/2 and 4d_5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.