An X-ray spectroscopy study of structural stability of superhydrogenated pyrene derivatives

The stability of polycyclic aromatic hydrocarbons (PAHs) upon soft X-ray absorption is of crucial relevance for PAH survival in X-ray dominated regions (XDRs). PAH stability depends on molecular size but also on the degree of hydrogenation that is related to H2 formation in the interstellar medium (ISM). In this project, we intend to reveal the changes of electronic structure caused by hydrogenation and the impact of hydrogenation on the stability of the carbon backbone for cationic pyrene and its hydrogenated derivatives by analysis of near C K-edge soft X-ray photoions. In our experiments, the PAH cations were trapped in a cryogenic radiofrequency (RF) linear ion trap and exposed to monochromatic X-rays with energies from 279 eV to 300 eV. The photo-products were mass-analyzed by means of time-of-flight (TOF) spectroscopy. Partial ion yields (PIYs) were then studied as a function of photon energy. X-ray absorption spectra computed by time-dependent density functional theory (TD-DFT) aided the interpretation of the experimental results. A very good agreement between experimental data and TDDFT with short-range corrected (SRC) functionals for all PAH ions was reached. The near-edge X-ray absorption mass spectra (NEXAMS) exhibit clear peaks due to C 1s transitions to singly occupied molecular orbitals SOMO and to low-lying unoccupied molecular orbitals. In contrast to coronene cations, where hydrogen attachment drastically increases photostability of coronene, the influence of hydrogenation on photostability is substantially weaker for pyrene cations. Here, hydrogen attachment even destabilizes the molecular structure. An astrophysical model describes the half-life of PAH ions in interstellar environments

Exposing the Oxygen-Centered Radical Character of the Tetraoxido Ruthenium(VIII) Cation [RuO4]+

The tetraoxido ruthenium(VIII) radical cation, [RuO4]+, should be a strong oxidizing agent, but has been difficult to produce and investigate so far. In our X-ray absorption spectroscopy study, in combination with quantum-chemical calculations, we show that [RuO4]+, produced via oxidation of ruthenium cations by ozone in the gas phase, forms the oxygen-centered radical ground state. The oxygen-centered radical character of [RuO4]+ is identified by the chemical shift at the ruthenium M3 edge, indicative of ruthenium(VIII), and by the presence of a characteristic low-energy transition at the oxygen K edge, involving an oxygen-centered singly-occupied molecular orbital, which is suppressed when the oxygen-centered radical is quenched by hydrogenation of [RuO4]+ to the closed-shell [RuO4H]+ ion. Hydrogen-atom abstraction from methane is calculated to be only slightly less exothermic for [RuO4]+ than for [OsO4]+

The Highest Oxidation State of Rhodium: Rhodium(VII) in [RhO3]+

Although the highest possible oxidation states of all transition elements are rare, they are not only of fundamental interest but also relevant as potentially strong oxidizing agents. In general, the highest oxidation states are found in the electron‐rich late transition elements of groups 7–9 of the periodic table. Rhodium is the first element of the 4d transition metal series for which the highest known oxidation state does not equal its group number of 9, but reaches only a significantly lower value of +6 in exceptional cases. Higher oxidation states of rhodium have remained elusive so far. In a combined mass spectrometry, X‐ray absorption spectroscopy, and quantum‐chemical study of gas‐phaseRhOn+ (n=1–4), we identify RhO3+ as the 1A1' trioxidorhodium(VII) cation, the first chemical species to contain rhodium in the +7 oxidation state, which is the third‐highest oxidation state experimentally verified among all elements in the periodic table

A comparative laboratory study of soft X-ray-induced ionization and fragmentation of five small PAH cations

The interaction between polycyclic aromatic hydrocarbon (PAH) radical cations and X-rays predominantly leads to photofragmentation, a process that strongly depends on PAH size and geometry. In our experiments, five prototypical PAHs were exposed to monochromatic soft X-ray photons with energies in the C K-edge regime. As a function of soft X-ray photon energy, photoion yields were obtained by means of time-of-flight mass spectrometry. The resulting near-edge X-ray absorption mass spectra were interpreted using time-dependent density functional theory (TD-DFT) with a short-range corrected functional. We found that the carbon backbone of anthracene$$^+$$(C$$_{14}$$H$$_{10}^+$$), pyrene$$^+$$(C$$_{16}$$H$$_{10}^+$$) and coronene$$^+$$(C$$_{24}$$H$$_{12}^+$$) can survive soft X-ray absorption, even though mostly intermediate size fragments are formed. In contrast, for hexahydropyrene$$^+$$(C$$_{16}$$H$$_{16}^+$$) and triphenylene$$^+$$(C$$_{18}$$H$$_{12}^+$$) molecular survival is not observed and the fragmentation pattern is dominated by small fragments. For a given excitation energy, molecular survival evidently does not simply correlate with PAH size but strongly depends on other PAH properties

Magnetic nanodoping: Atomic control of spin states in cobalt doped silver clusters

The interaction of magnetic dopants with delocalized electron states can result in interesting many-body physics. Here, the magnetic properties of neutral and charged finite silver metal host clusters with a magnetic cobalt atom impurity were investigated experimentally by exploiting the complementary methods of Stern- Gerlach molecular beam deflection and x-ray magnetic circular dichroism spectroscopy and are accompanied by density functional theory calculations and charge transfer multiplet simulations. The influence of the number of valence electrons and the consequences of impurity encapsulation were addressed in free size-selected, singly cobalt-doped silver clusters CoAg0,n+ (n = 2–15). Encapsulation of the dopant facilitates the formation of delocalized electronic shells with complete hybridization of the impurity 3d- and the host 5s-derived orbitals, which results in impurity valence electron delocalization, effective spin relaxation, and a low-spin ground state. In the exohedral size regime, spin pairing in the free electron gas formed by the silver 5s electrons is the dominating driving force determining the local 3d occupation of the impurity and therefore, adjusting the spin magnetic moment accordingly

Soft X-ray spectroscopy as a probe for gas-phase protein structure:Electron impact ionization from within

Preservation of protein conformation upon transfer into the gas‐phase is key for structure determination of free single molecules, e.g. using X‐ray free‐electron lasers. In the gas phase, the helicity of melittin decreases strongly as the protein's protonation state increases. We demonstrate the sensitivity of soft X‐ray spectroscopy to the gas phase conformation of melittin cations ([melittin+qH]q+, q=2‐4) in a cryogenic linear radiofrequency ion trap. With increasing helicity we observe a decrease of the dominating carbon 1s‐* transition in the amide C=O bonds for non‐dissociative single ionization and an increase for non‐dissociative double ionization. As the underlying mechanism we identify inelastic electron scattering. Using an independent atom model we show that the more compact nature of the helical protein conformation substantially increases the probability for off‐site intramolecular ionization by inelastic Auger electron scattering

Magnetische Eigenschaften von reinen und gemischten 3d 5 Übergangsmetallclustern

Die Delokalisierung der 3d Elektronen von Übergangsmetallen wurde in Abhängigkeit der atomaren Umgebung untersucht. Dabei wurde die atomare Umgebung durch das Manipulieren der Zusammensetzung und der Größe von atomaren Clustern variiert. Die elektronische Struktur und die magnetischen Eigenschaften wurden mittels Röntgenabsorptionsspektroskopie an freien, größenselektierten Cr+n, Mn+n, CrMn+ und MnSi+n (n <= 14) Clustern untersucht. In den molekularen Dimeren Cr+2, Mn+2, und CrMn+ wurden lokalisierte magnetische Momente beobachtet. Während diese Momente in Cr+2 und Mn+2 durch eine ferromagnetische indirekte Austauschwechselwirkung gekoppelt sind, ist im Fall von CrMn+ der Kopplungsmechanismus der antiferromagnetische direkte Austausch. Die elektronischen Grundzustände dieser drei Moleküle sind damit zum ersten Mal experimentell eindeutig identifiziert: Cr+2 (12Sigma+u), Mn+2 (12Sigma+g), und CrMn+ (1Sigma). In größeren Chromclustern wird das magnetische Moment aufgrund der Bildung von Molekülorbitalen unter Beteiligung von 3d Zuständen auf null reduziert. Im Gegensatz dazu bleiben die atomaren magnetischen Momente im Mangantrimerkation Mn+3 erhalten. Dies resultiert aus der elektronischen Konfiguration des Manganatoms mit halbgefüllten 3d und gefüllten 4s Orbitalen, welche eine Partizipation der 3d Elektronen an der Bildung der Molekülarorbitale verhindert. Im Mangantrimer wird die ferromagnetische Kopplung der atomaren Spinmomente der einzelnen Manganatome durch indirekte Austauschwechselwirkung vermittelt. Mit steigender Größe der Mangancluster werden die 3d Orbitale zunehmend delokalisiert und eine graduelle Reduzierung des magnetischen Moments findet statt. Zusätzlich erfolgt der vorhergesagte graduelle Übergang von ferro- zu antiferromagnetischer Kopplung. Eine starke Lokalisierung der 3d Orbitale wurde auch in MnSi+n Clustern mit n <= 10 beobachtet. Diese exohedral dotierten Cluster tragen ein magnetisches Moment von 4 µB. Mit mehr als zehn Siliziumatomen wird die endohedrale Dotierung energetisch günstiger. Infolgedessen delokalisieren die 3d Orbitale des Manganatoms vollständig und das magnetische Moment wird auf null reduziert. Der hier identifizierte Übergang von magnetischem zu nichtmagnetischem Verhalten in Abhängigkeit der gewichteten Koordination des Fremdatoms, ist ein allgemein gültiger Zusammenhang. Mit seiner Hilfe lassen sich strukturelle Parameter in MnxSi(1-x) Materialien vorhersagen, welche die Erhaltung des magnetischen Moments des Fremdatoms zur Folge haben.The delocalization of transition metal 3d orbitals was studied experimentally as a function of the atomic environment, which was controlled by varying the size and composition of atomic clusters. The electronic structure and magnetic properties were probed by applying x-ray absorption and magnetic circular dichroism spectroscopy to size-selected, free Cr+n, Mn+n, CrMn+ and MnSi+n clusters (n <= 14). The dimers Cr+2, Mn+2, and CrMn+ were all found to have localized magnetic moments and to be magnetically ordered due to ferromagnetic indirect exchange in Cr+n and Mn+n, and antiferromagnetic direct exchange in CrMn+. The electronic ground state has been identified to be 12Sigma+u, 12Sigma+g, and 1Sigma, respectively. In larger chromium clusters, the participation of the 3d orbitals in molecular bonding leads to the quenching of the magnetic moment. In contrast, the electronic closed 3d and 4s subshell configuration of atomic manganese was found to prevent participation of the 3d orbitals in bonding causing the trimer to have atomic-like magnetic moments per atom which are ferromagnetically coupled due to indirect exchange. The weak delocalization of the 3d orbitals with increasing manganese cluster size leads to only a gradual quenching of the magnetic moments and to the predicted transition from ferro- to antiferromagnetic ordering. The strong localization of 3d orbitals was also observed in MnSi+n clusters with n <= 10 which are exohedrally doped species and posses unquenched magnetic moments of 4 µB. With more than ten silicon atoms, the encapsulation of the manganese impurity becomes energetically favorable and is accompanied by a delocalization of the manganese 3d orbitals and a complete quenching of the magnetic moment. The newly identified coordination-driven magnetic-to-nonmagnetic transition reveals a general behavior which could be used to predict favorable structural parameters in MnxSi(1-x) materials for the stabilization of the manganese impurity's magnetic moment