Dynamique de relaxation de molécules excitées en couche interne

L'excitation d'orbitales de cœur par un rayonnement X-mou porte un système moléculaire dans des états électroniques dont les surfaces de potentiel sont très hautes en énergie. Les mécanismes de relaxation de ces états intermédiaires très instables sont complexes et font intervenir, sur une même échelle de temps, le mouvement des noyaux et la réorganisation du nuage électronique. J'illustre à travers les exemples d'H-2O et de CO-2 excités de manière résonante selon les transitions respectives O1s^(-1)- 2b-2 et C1s^(-1)-2p-u, comment, grâce à des mesures de coïncidences entre un électron Auger et un ion, nous pouvons mettre en évidence l'influence d'un changement de géométrie de l'état intermédiaire sur la dissociation. Les développements techniques apportés au montage expérimental EPICEA II, qui ont comme objectif la détermination complète des vecteurs quantité de mouvement des ions produits, ont nécessité la mise au point d'un nouveau spectromètre de masse par temps de vol couplé à un détecteur sensible en position. Les performances sont illustrées par les premiers résultats obtenus sur N-2 et par une comparaison des énergies cinétiques des ions O+ et CO+ issus de la fragmentation de CO-2 excité en couche interne. Une étude de photochimie sélective réalisée sur la molécule d'hexaméthyldisiloxane (CH-3)_6Si-2O ionisée en couche 2p du silicium ou 1s du carbone est présentée. La corrélation des états électroniques les plus bas du dication, avec les fragments créés montre que la dissociation est dépendante du site initialement ionisé. Des expériences complémentaires de coïncidences de paires d'électrons de seuil-ion ainsi que des calculs théoriques ont permis d'expliquer la production quasi exclusive du système ((CH3)2SiOSi(CH3)2)^(2+) pour les états d'énergie interne les plus bas.The excitation of core orbitals by a soft X-ray radiation carries a molecular system in electronic states of which the potential surfaces are very high in energy. The mechanisms of relaxation of these very unstable intermediate states are complex and bring in, on the same time scale, the nuclear motion and the reorganization of the electronic cloud. The dissociation of H2O and CO2 resonantly excited on the respective transitions 01s^(-1)-2b-2 and C1s^(-1)-2p-u are illustrating the evidence of the influence of geometry change in the intermediate state by the measurement in coincidence of an Auger electron with an ion. In order to obtain the complete orientation of the ions momenta, we have developed a new time of flight mass spectrometer coupled with a position sensitive detector. The performances are illustrated by the first results obtained on N2 and by the comparison of the kinetic energy released in the O+ and CO+ ions created after core excitation of CO2. A photoselective fragmentation of the hexaméthyldisiloxane molecule (CH3)6Si2O after silicon 2p or carbon 1s inner-shell ionization has been obtained. The correlation of the lowest electronic states of the dication, with the created fragments shows that the dissociation is dependent on the initially ionized site. Additional experiments of two threshold electrons-ion coincidences as well as theoretical calculations allowed to explain the almost exclusive production of the ((CH3)2SiOSi (CH3)2)^2+ for the lowest internal energy states of the doubly charged ion.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Resonant Auger spectroscopy on solid xenon on gold, silver, and copper substrates

An investigation of the radiationless decay of core excited Xe atoms in the region of Xe L3M4,5M4,5 Auger electron kinetic energies (using x-ray energies in the vicinity of the L3 threshold) is presented for Xe adsorbed on Cu, Ag, and Au. The intensity distribution of the decay channels is different compared with Xe in the gas phase. Charge transfer of the core excited electron occurs within tens of attoseconds in all systems for excitation energies approaching the ionization threshold of the condensed system, whereas charge transfer times are substrate-dependent for lower excitation energies. The determination of partial yields into the decay channels allows for the observation of a decay channel present in the Xe/Cu and Xe/Ag systems but not in the case of Xe/Au. Theoretical calculations allow us to interpret this difference as emanating from varying amounts of the ground state hybridization between Xe and the substrates, which impacts the energy of the Auger final states enabling identification of these states giving rise to system specific features in the experimental data

Operando hard X-ray photoelectron spectroscopy study of the Pt/Ru/PbZr0.52_{0.52}Ti0.48_{0.48}O3_3 interface

International audienceWe have used hard X-ray photoelectron spectroscopy to probe the Pt/Ru/PbZr$_{0.52}$Ti${0.48}$O$_3$ (PZT) interface in a Pt/Ru/PZT(220 nm)/Pt/TiO$_2$/SiO$_2$/Si stack. A customized sample-holder allows in-situ photoemission analysis while applying bias to the capacitor. Hard X-rays probe the buried interface between the top electrode and the ferroelectric PZT. The use of operando conditions reveals a polarization-dependent electronic response, most probably due to imperfect screening of the depolarizing field. There is evidence for an additional core level component related to the electrode-PZT interface. Zr oxide nanostructures at the surface of the sol-gel layer may form a ferroelectric dead layer at the interface, affecting device performance

Radiation damage by extensive local water ionization from two-step electron-transfer-mediated decay of solvated ions

Biomolecular radiation damage is largely mediated by radicals and low-energy electrons formed by water ionization rather than by direct ionization of biomolecules. It was speculated that such an extensive, localized water ionization can be caused by ultrafast processes following excitation by core-level ionization of hydrated metal ions. In this model, ions relax via a cascade of local Auger–Meitner and, importantly, non-local charge- and energy-transfer processes involving the water environment. Here, we experimentally and theoretically show that, for solvated paradigmatic intermediate-mass Al$^{3+}$ ions, electronic relaxation involves two sequential solute–solvent electron transfer-mediated decay processes. The electron transfer-mediated decay steps correspond to sequential relaxation from Al$^{5+}$ to Al$^{3+}$ accompanied by formation of four ionized water molecules and two low-energy electrons. Such charge multiplication and the generated highly reactive species are expected to initiate cascades of radical reactions

Photo-induced ultrafast dissociation following deep-core-electron excitation

Creation of deep core holes leads to extensive nuclear dynamics on a few femtosecond timescale despite the very short (tau <= 1 fs) lifetime of such states. This is because the 1st steps of the relaxation processes (i.e. both radiative and non-radiative decays) generate intermediate states with one and multiple holes in core orbitals. As an example, ultrafast dissociation is observed in three well-distinguishable LVV Auger decay channels for HCl following Cl1s ->sigma* excitation

Argon 1s(-2) Auger hypersatellites

The 1s(-2) Auger hypersatellite spectrum of argon is studied experimentally and theoretically. In total, three transitions to the final states 1s(-1)2p(-2)(S-2(e),D-2(e)) and 1s(-1)2s(-1)(S-1)2p(-1)(P-2(o)) are experimentally observed. The lifetime broadening of the 1s(-2) -> 1s(-1)2p(-2)(S-2(e),D-2(e)) states is determined to be 2.1(4) eV. For the used photon energy of h nu = 7500 eV a KK/K ionisation ratio of 2.5(3) x 10(-4) is derived. Generally, a good agreement between the experimental and present theoretical energy positions, linewidths, and intensities is obtained

Direct measurements of interfacial photovoltage and band alignment in perovskite solar cells using hard X-ray photoelectron spectroscopy

A heterojunction is the key junction for charge extraction in many thin film solar cell technologies. However, the structure and band alignment of the heterojunction in the operating device are often difficult to predict from calculations and, due to the complexity and narrow thickness of the interface, are difficult to measure directly. In this study, we demonstrate a technique for direct measurement of the band alignment and interfacial electric field variations of a fully functional lead halide perovskite solar cell structure under operating conditions using hard X-ray photoelectron spectroscopy (HAXPES). We describe the design considerations required in both the solar cell devices and the measurement setup and show results for the perovskite, hole transport, and gold layers at the back contact of the solar cell. For the investigated design, the HAXPES measurements suggest that 70% of the photovoltage was generated at this back contact, distributed rather equally between the hole transport material/gold interface and the perovskite/hole transport material interface. In addition, we were also able to reconstruct the band alignment at the back contact at equilibrium in the dark and at open circuit under illumination

Probing aqueous ions with non-local Auger relaxation

Non-local analogues of Auger decay are increasingly recognized as important relaxation processes in the condensed phase. Here, we explore non-local autoionization, specifically Intermolecular Coulombic Decay (ICD), of a series of aqueous-phase isoelectronic cations following 1s core-level ionization. In particular, we focus on Na$^+$, Mg$^{2+}$, and Al$^{3+}$ ions. We unambiguously identify the ICD contribution to the K-edge Auger spectrum. The different strength of the ion–water interactions is manifested by varying intensities of the respective signals: the ICD signal intensity is greatest for the Al$^{3+}$ case, weaker for Mg$^{2+}$, and absent for weakly-solvent-bound Na$^+$. With the assistance of ab initio calculations and molecular dynamics simulations, we provide a microscopic understanding of the non-local decay processes. We assign the ICD signals to decay processes ending in two-hole states, delocalized between the central ion and neighbouring water. Importantly, these processes are shown to be highly selective with respect to the promoted water solvent ionization channels. Furthermore, using a core-hole-clock analysis, the associated ICD timescales are estimated to be around 76 fs for Mg$^{2+}$ and 34 fs for Al$^{3+}$. Building on these results, we argue that Auger and ICD spectroscopy represents a unique tool for the exploration of intra- and inter-molecular structure in the liquid phase, simultaneously providing both structural and electronic information

4D Multimodal Nanomedicines Made of Nonequilibrium Au-Fe Alloy Nanoparticles

Several examples of nanosized therapeutic and imaging agents have been proposed to date, yet for most of them there is a low chance of clinical translation due to long-term in vivo retention and toxicity risks. The realization of nanoagents that can be removed from the body after use remains thus a great challenge. Here, we demonstrate that nonequilibrium gold-iron alloys behave as shape-morphing nanocrystals with the properties of self-degradable multifunctional nanomedicines. DFT calculations combined with mixing enthalpy-weighted alloying simulations predict that Au-Fe solid solutions can exhibit self-degradation in an aqueous environment if the Fe content exceeds a threshold that depends upon element topology in the nanocrystals. Exploiting a laser-assisted synthesis route, we experimentally confirm that nonequilibrium Au-Fe nanoalloys have a 4D behavior, that is, the ability to change shape, size, and structure over time, becoming ultrasmall Au-rich nanocrystals. In vivo tests show the potential of these transformable Au-Fe nanoalloys as efficient multimodal contrast agents for magnetic resonance imaging and computed X-ray absorption tomography and further demonstrate their self-degradation over time, with a significant reduction of long-term accumulation in the body, when compared to benchmark gold or iron oxide contrast agents. Hence, Au-Fe alloy nanoparticles exhibiting 4D behavior can respond to the need for safe and degradable inorganic multifunctional nanomedicines required in clinical translation