Competition between proton transfer and intermolecular Coulombic decay in water

Intermolecular Coulombic decay (ICD) is a ubiquitous relaxation channel of electronically excited states in weakly bound systems, ranging from dimers to liquids. As it is driven by electron correlation, it was assumed that it will dominate over more established energy loss mechanisms, for example fluorescence. Here, we use electron–electron coincidence spectroscopy to determine the efficiency of the ICD process after 2a1 ionization in water clusters. We show that this efficiency is surprisingly low for small water clusters and that it gradually increases to 40–50% for clusters with hundreds of water units. Ab initio molecular dynamics simulations reveal that proton transfer between neighboring water molecules proceeds on the same timescale as ICD and leads to a configuration in which the ICD channel is closed. This conclusion is further supported by experimental results from deuterated water. Combining experiment and theory, we infer an intrinsic ICD lifetime of 12–52 fs for small water clusters

Untersuchung von nicht-lokalen Autoionisationsprozessen in Edelgasclustern

Gegenstand dieser Arbeit ist die Untersuchung von nichtlokalen Autoionisationsprozessen in gemischten Edelgasclustern nach der Photoionisation mit Synchrotronstrahlung. Insbesondere ist das Ziel der Arbeit der experimentelle Nachweis von ETMD(3) (electron transfer mediated decay). Nach der Photoionisation der Cluster wurden die Flugzeiten der resultierenden Photoelektronen und der Sekundärelektronen mit Hilfe eines magnetische Flasche Elektronenflugzeitspektrometers in Koinzidenz gemessen. Nach der Anregung von Ar-Kr Clustern mit Synchrotronstrahlung mit einer Energie von 32eV konnten zwei Signale im Koinzidenzspektrum identifiziert werden. Das erste Signal tritt nur in Koinzidenz mit der Ar 3s Clusterlinie auf und ist das gesuchte ETMD(3) Signal. Ein Spektrum der kinetischen Energieverteilung des ETMD(3) Elektrons wird gezeigt. Das zweite Signal stammt von Elektron-Elektron Stößen im Cluster. In Ar-Xe Clustern sind sowohl ETMD(3) als auch ICD (interatomic coulombic decay) energetisch erlaubt. Mit Hilfe einer detaillierten Analyse der Valenzspektren von Ar-Xe Clustern, konnten die Struktur und die Zusammensetzung der Cluster bestimmt werden. Das Spektrum der Sekundärelektronen, die in Koinzidenz zu dem Ar 3s Band aufgenommen wurden, zeigt Beiträge von ICD und von ETMD. Die Intensität dieser Beiträge ist abhängig von der Zusammensetzung und Größe der untersuchten Cluster. Weiterhin wurden Struktur und Form der Photoelektronenspektren der Außenvalenzen homogener Argon und Krypton Cluster mit unterschiedlichen Größen untersucht. Beide Spezies zeigen in einem kleinen Bereich von Anregungsenergien ein dispergierendes Feature, was dem Außenvalenzband überlagert ist. Dieses Feature tritt ab Clustergrößen von ca. 230 Atomen bei Argon und unter ca. 270 Atomen bei Krypton auf. Es wird gezeigt, dass es auf Dispersion des Valenzbandes aufgrund der Kristallstruktur der untersuchten Cluster zurückzuführen ist. Es ist somit ein Indikator für die Entstehung von Festkörpereigenschaften.The main topic of this thesis is the investigation of non-local autoionization processes in mixed rare gas clusters after photoionization using synchrotron radiation. In particular, the aim of the work is the experimental detection of ETMD(3) (electron transfer mediated decay). After photoionization and subsequent autoionization of the clusters, the flight times of the resulting photo-electrons and secondary electrons were measured in coincidence using a magnetic bottle electron time-of-flight spectrometer. After excitation of Ar-Kr clusters using synchrotron radiation with an energy of 32eV, two signals are identified in the coincidence spectrum. The first signal occurs only in coincidence with the Ar 3s derived cluster band and is the ETMD(3) signal. A spectrum of the kinetic energy distribution of the ETMD(3) electron is shown. The second signal comes from electron-electron collisions in the cluster. In Ar-Xe clusters, both ETMD(3) and ICD (interatomic coulombic decay) are energetically allowed. It was possible to determine the structure and the composition of the Ar-Xe clusters through detailed analysis of their valence spectra. The spectrum of secondary electrons, recorded in coincidence with the Ar 3s derived cluster band, shows contributions from ICD and ETMD(3). The intensity of these contributions depends on the composition and size of the clusters under study. Furthermore, the structure and shape of the outer valence spectra of homogeneous argon and krypton clusters of different sizes were examined. Both species show a dispersing feature in a small range of excitation energies, which is superimposed on the outer valence cluster band. This feature is observed at cluster sizes starting at about 230 atoms, in the case of argon, and about 270 atoms, in the case of krypton. This results from dispersion of the valence band due to the crystal structure of the clusters investigated. It is therefore an indicator of the development of bulk-like properties of the clusters

Microhydration of substituted diamondoid radical cations of biological relevance: infrared spectra of amantadine+-(H2O)n = 1–3 clusters

Hydration of biomolecules and pharmaceutical compounds has a strong impact on their structure, reactivity, and function. Herein, we explore the microhydration structure around the radical cation of the widespread pharmaceutical drug amantadine (C16H15NH2, Ama) by infrared photodissociation (IRPD) spectroscopy of mass-selected Ama+Wn = 1–3 clusters (W = H2O) recorded in the NH, CH, and OH stretch range of the cation ground electronic state. Analysis of the size-dependent frequency shifts by dispersion-corrected density functional theory calculations (B3LYP-D3/cc-pVTZ) provides detailed information about the acidity of the protons of the NH2 group of Ama+ and the structure and strength of the NH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. The preferred sequential cluster growth begins with hydration of the two acidic NH protons of the NH2 group (n = 1–2) and continues with an extension of the H-bonded hydration network by forming an OH⋯O H-bond of the third W to one ligand in the first hydration subshell (n = 3), like in the W2 dimer. For n = 2, a minor population corresponds to Ama+W2 structures with a W2 unit attached to Ama+via a NH⋯W2 H-bond. Although the N–H proton donor bonds are progressively destabilized by gradual microhydration, no proton transfer to the Wn solvent cluster is observed in the investigated size range (n ≤ 3). Besides the microhydration structure, we also obtain a first impression of the structure and IR spectrum of bare Ama+, as well as the effects of both ionization and hydration on the structure of the adamantyl cage. Comparison of Ama+ with aliphatic and aromatic primary amine radical cations reveals differences in the acidity of the NH2 group and the resulting interaction with W caused by substitution of the cycloalkyl cage.TU Berlin, Open-Access-Mittel – 202

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The angular distribution of photoelectrons emitted from water clusters has been measured by linearly polarized synchrotron radiation of 40 and 60 eV photon energy. Results are given for the three outermost valence orbitals. The emission patterns are found more isotropic than for isolated molecules. While a simple scattering model is able to explain most of the deviation from molecular behavior, some of our data also suggest an intrinsic change of the angular distribution parameter. The angular distribution function was mapped by rotating the axis of linear polarization of the synchrotron radiation. [http://d

Competition between proton transfer and intermolecular Coulombic decay in water

Interatomic or intermolecular Coulombic decay is responsible for the generation of slow electrons in clusters and biological samples. Here the authors use electron–electron coincidence detection to find the competitive roles of proton transfer and ICD that occur on similar time scales in water clusters