LOW-ENERGY ELECTRON DAMAGE OF DPPC MOLECULES – A NEXAFS STUDY

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Collective Autoionization in Multiply-Excited Systems: A novel ionization process observed in Helium Nanodroplets

Free electron lasers (FELs) offer the unprecedented capability to study reaction dynamics and image the structure of complex systems. When multiple photons are absorbed in complex systems, a plasma-like state is formed where many atoms are ionized on a femtosecond timescale. If multiphoton absorption is resonantly-enhanced, the system becomes electronically-excited prior to plasma formation, with subsequent decay paths which have been scarcely investigated to date. Here, we show using helium nanodroplets as an example that these systems can decay by a new type of process, named collective autoionization. In addition, we show that this process is surprisingly efficient, leading to ion abundances much greater than that of direct single-photon ionization. This novel collective ionization process is expected to be important in many other complex systems, e.g. macromolecules and nanoparticles, exposed to high intensity radiation fields

Chemical Bond and Charge Transfer Dynamics of a Dye-Hierarchical TiO2 Hybrid Interface

The adsorption of Zn-Tetra-Phenyl-Porphyrin (ZnTPP) on nanoporous hierarchically organized anatase TiO2 structures, and the properties of the corresponding hybrid interface were studied by synchrotron radiation experiments. The molecular structure, electronic properties and the bonding with nanostructured TiO2 surfaces were analyzed by photoemission (XPS and UPS) and x-ray absorption spectroscopy (XAS). The charge transfer at the interface was investigated by means of valence band resonant photoemission experiments (ResPES) at the C K edge. We show that the charge transfer dynamics between the photo-excited ZnTPP and TiO2 is strongly influenced by the presence of defects on the TiO2 surface. On a stoichiometric anatase nanostructure, ZnTPP bonding occurs primarily via carbon atoms belonging to the molecular phenyl rings and this creates a preferential channel for the charge transfer. This phenomenon is reduced in the case of defective TiO2 surface, where ZnTPP interacts mainly through the molecule macrocycle. Our results represent a surface science study of the dye molecule behavior on a nanoporous TiO2 photoanode relevant to dye-sensitized or hybrid solar cell applications and it shows the importance of the surface oxidation state for the charge transfer proces

Two-photon resonant excitation of interatomic coulombic decay in neon dimers

The recent availability of intense and ultrashort extreme ultraviolet sources opens up the possibility of investigating ultrafast electronic relaxation processes in matter in an unprecedented regime. In this work we report on the observation of two-photon excitation of interatomic Coulombic decay (ICD) in neon dimers using the tunable intense pulses delivered by the free electron laser FERMI. The unique characteristics of FERMI (narrow bandwidth, spectral stability, and tunability) allow one to resonantly excite specific ionization pathways and to observe a clear signature of the ICD mechanism in the ratio of the ion yield created by Coulomb explosion. The present experimental results are explained by ab initio electronic structure and nuclear dynamics calculations

SOFT X-RAY SPECTROSCOPY OF GLYCYL-GLYCINE ADSORBED ON Cu(110) SURFACE

Author Institution: Sincrotrone Trieste, Basovizza (Trieste), Italy; Institute of Physics, Prague, Czech Republic; Charles University, Prague, Czech Republic; CNR-Institute of Chemical Physical Processes, Pisa, ItalyStudies of the interaction between organic compounds and surfaces are motivated by their application as bio sensors, and their relevance to biocompatibility of implants and the origin of life. In the present work interaction of the simplest peptide, glycyl-glycine, with the Cu surface has been studied. Multilayer, monolayer and sub-monolayer films of this dipeptide on the clean and oxygen modified Cu(110) surface were prepared by thermal evaporation in high vacuum. The techniques used were soft X-ray photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy and density functional theory calculations. By comparing the experimental and theoretical spectra, detailed models of the electronic structure and adsorption geometry for each coverage have been proposed, which are in good agreement with the theoretical calculations. The carboxylic acid group of glycyl-glycine loses hydrogen and the molecule is coordinated via the carboxylate oxygen atoms to the surface. At low coverage the amino group bonds to the surface via a hydrogen atom, while at higher coverage the bonding is via the nitrogen lone pair. The peptide group is not involved in the bonding to the surface

Photoionization and Velocity Map Imaging spectroscopy of atoms, molecules and clusters with Synchrotron and Free Electron Laser radiation at Elettra

Advances in laser and synchrotron radiation instrumentation are continuously boosting fundamental research on the electronic structure of matter. At Elettra the collaboration between several groups active in the field of atomic, molecular and cluster physics and the Instrumentation and Detector Laboratory has resulted in an experimental set-up that successfully tackles the challenges posed by the investigation of the electronic structure of isolated species in the gas phase. The use of Synchrotron Radiation (SR) and Free Electron Laser (FEL) light, allows to cover a wide spectrum of targets from energetic to dynamics. We developed a VMI that allows to perform as well SR as FEL experiments, just by changing part of the detection system. In SR experiments, at the GasPhase beamline of Elettra, a cross delay line detector is used, coupled to a 4-channel time-to-digital converter that reconstructs the position of the electrons. Simultaneously, a Time-of-Flight (TOF) mass spectrometer is used to acquire photoion spectra. Such a system allows PhotoElectron-PhotoIon-Coincidence (PEPICO) spectroscopy of atoms, molecules and clusters. In FEL experiments (notably differing from SR experiments in the much higher rate of events produced and detected, which forces one to forfeit coincidence detection), at the LDM beamline of FERMI, a Micro Channel Plate (MCP) a phosphor screen and a CCD camera are used instead, capable of shot-by-shot collection of practically all events, albeit without time resolution

Migration of surface excitations in highly-excited nanosystems probed by intense resonant XUV radiation

Ionization dynamics of resonantly excited helium nanodroplets have been studied by intense XUV light. By doping the nanodroplets with atoms that either attach to the surface or submerge into the center of the droplet, one can study the dynamics of excitation and ionization through the droplet. When resonantly exciting the droplet, we observe a strong ionization enhancement for atoms attached to the surface. On the other hand, atoms embedded inside the nanodroplet are less efficiently ionized. We attribute this eect to an ultrafast energy transfer to the surface of the droplet and subsequent Penning ionization of the surface-bound dopant