Site-specific probing of charge transfer dynamics in organic photovoltaics

We report the site-specific probing of charge-transfer dynamics in a prototype system for organic photovoltaics (OPV) by picosecond time-resolved X-ray photoelectron spectroscopy. A layered system consisting of approximately two monolayers of C$_{60}$ deposited on top of a thin film of Copper-Phthalocyanine (CuPC) is excited by an optical pump pulse and the induced electronic dynamics are probed with 590 eV X-ray pulses. Charge transfer from the electron donor (CuPC) to the acceptor (C$_{60}$) and subsequent charge carrier dynamics are monitored by recording the time-dependent C 1$s$ core level photoemission spectrum of the system. The arrival of electrons in the C$_{60}$ layer is readily observed as a completely reversible, transient shift of the C$_{60}$ associated C 1$s$ core level, while the C 1$s$ level of the CuPC remains unchanged. The capability to probe charge transfer and recombination dynamics in OPV assemblies directly in the time domain and from the perspective of well-defined domains is expected to open additional pathways to better understand and optimize the performance of this emerging technology

Some aspects of basing the strategic decisions in agriculture by comparative assessment of indicators of eco-efficiency

This paper aims to analyse standard gross margin to business decisions on the use of production resources in the dairy farms. The efficient running the activities in animal farms requires that the production resources, to be provided combined and allocated in such a way as to result in: full capitalization of production capacity of the unit, obtaining increased quantities of product per hectare and animal feed with minimized costs per unit, continuous increase of the efficiency of resources due to the use of modern technologies, improved technical resources and not polluting the environment with residues. For a company to be effective it must be well organized, use various computer programmes, to provide data on the evolution of its activity at any time, to use advanced software to facilitate the work on the farm and increase its productivity. For carrying out the study we used the case study as a major research strategy and for the analysis of performance level of the farm we used the model of determining the standard gross margin made in Excel worksheets. For data collection and analysis, we used techniques and instruments specific for qualitative research, analyses of internal documents, technological records and discussions with the manager of Research and Development Station for Cattle Growth (RDSCG) Dancu, Iasi County, Romania

Dirac states with knobs on: interplay of external parameters and the surface electronic properties of 3D topological insulators

Topological insulators are a novel materials platform with high applications potential in fields ranging from spintronics to quantum computation. In the ongoing scientific effort to demonstrate controlled manipulation of their electronic structure by external means, stoichiometric variation and surface decoration are two effective approaches that have been followed. In ARPES experiments, both approaches are seen to lead to electronic band structure changes. Such approaches result in variations of the energy position of bulk and surface-related features and the creation of two-dimensional electron gases.The data presented here demonstrate that a third manipulation handle is accessible by utilizing the amount of illumination a topological insulator surface has been exposed to under typical experimental ARPES conditions. Our results show that this new, third, knob acts on an equal footing with stoichiometry and surface decoration as a modifier of the electronic band structure, and that it is in continuous competition with the latter. The data clearly point towards surface photovoltage and photo-induced desorption as the physical phenomena behind modifications of the electronic band structure under exposure to high-flux photons. We show that the interplay of these phenomena can minimize and even eliminate the adsorbate-related surface band bending on typical binary, ternary and quaternary Bi-based topological insulators. Including the influence of the sample temperature, these data set up a framework for the external control of the electronic band structure in topological insulator compounds in an ARPES setting. Four external knobs are available: bulk stoichiometry, surface decoration, temperature and photon exposure. These knobs can be used in conjunction to tune the band energies near the surface and consequently influence the topological properties of the relevant electronic states.Comment: 16 pages, 8 figure

Ionisation und Fragmentation von chalkogenen Clustern durch hoch geladene Ionen und Synchrotronstrahlung

Title, Table of contents List of acronyms 1\. Introduction 9 2\. Theoretical Background 11 3. Experimental Setup 33 4\. Results and Discussion 55 5. Conclusions 111 Bibliography 119 Acknowledgements, Erklärung, Publications and Conference Contributions, List of Figures, List of Tables 131The ionization and fragmentation of sulfur and selenium clusters have been investigated via various spectroscopic methods. Various ionization means have also been used in order to achieve a more complete image of the fragmentation mechanisms of the chalcogene clusters. Mass spectrometry methods have been used in order to investigate the clusters as a function of the temperature and as a function of the ionization energy. Thus it has been shown that for the sulfur clusters investigated with highly charged ion beams, on increasing the temperature of the sulfur vapor, the amount of the heavier aggregates S8 and S7 decreases. At the same time, the amount of smaller moieties (S2, and S) has been observed to increase. A similar behavior has also been observed for selenium clusters, where the increase of the temperature of the vapor leads to a higher percentage of the smaller moieties (Se2 and Se). It has also been observed that upon Se-2p ionization multiply charged selenium moieties occur (up to Se4+). However, there is no evidence for singly charged heavier aggregates (such as Se3+), as observed for Se-3d continuum ionization. The COLTRIMS reaction microscope has been for the first time successfully used for the investigation of sulfur clusters. The experiments have been performed at the sulfur 2p and 2s absorption edges. From the experimental data the sulfur coincidence channels have been extracted, and the KER's and CSD's have been inferred. A strong dependence of the kinetic energy distributions has been observed on the angle phi. This exhibits prominent maxima at +90° and -90° and minima at 0° and +180°. This was interpreted as an indication that the fragmentation of the aggregates takes place preferentially in the direction of the polarization of the synchrotron radiation. Ion-ion-coincidence investigations have been performed in order to shed light into the fragmentation mechanisms of sulfur clusters after ionization with highly charged ion beams. The coincidence investigation method has also been chosen in order to ease the link of the present result with the earlier reported results on inner shell excited sulfur clusters. However, the photon- induced inner-shell excitation starts with a well-defined site within the cluster. Auger relaxation leads to holes in the valence shell, which are followed by fission leading eventually to the formation of singly charged products. In the S 2p-regime double ionization is the dominant process. In contrast, ionization by HCI leads to the loss of electrons from the outer valence shell levels, which are expected to be delocalized over the entire molecular system. It is expected that the loss of electrons depends on the charge of the projectile: For low projectile charge state, capture takes place at short distances and is accompanied by electronic excitation whereas for high projectile charges states, capture occurs at large distances and leaves the target further unaffected. In any case, as a result a S8 moiety can break up into several charged fragments. The present work, however, selects only those processes, which lead to correlated pairs of fragments, so that similar product channels as in PEPIPICO experiments are accessed. As a result, it is not entirely surprising that there are similarities between both complementary approaches. The coincidence channels recorded after ionization with HCI have been identified and possible fission paths have been proposed. One important aspect to point out is that subsequent to highly charged ion beams induced fragmentation evidence for the direct two-body dissociation of S8++ into S6+ and S2+ has been observed. This channel has not been observed in earlier reported results on inner-shell excited sulfur clusters. This coincidence channel occurs only when the two charges are localized at almost opposite sites in the doubly charged parent ion, which appears to be a favorable situation upon ionization via HCI. A comparison of the present results indicates that the initial charge localization in the doubly charged parent cluster can be related to the selective charge separation processes into singly charged fragments. Specifically, it has been observed for the Xe5+ ion beam case, that the localization of the charges at opposite sites leads to the occurrence of S3+ in coincidence with other charged moieties. The second ionic species, i.e. S+ or S2+, depends on whether the parent ion has released a neutral prior to charge separation. The localization of the charges in the parent dication at second neighboring positions leads to the appearance of the S2+ / S4+ coincidence channel, with the intermediate formation of the S7++ aggregate. When ionizing with the Xe10+ ion beam, the localization of the charge in the parent ion prior to charge separation at opposite sites can lead to the occurrence of two different coincidence channels (S2+ / S4+ and S2+ / S2+). This depends of whether the doubly charged ion immediately before charge separation is S7++ or S8++. For the ionization with the Xe15+ ion beam, a localization of the two charges at second neighboring positions will lead to the occurrence of the S2+ / S3+ and S+ / S2+ cation pairs. It has also been observed that the localization of charges at opposite position in the parent ion is responsible for the occurrence of the S2+ / S4+ and S2+ / S2+ coincidence channels. The appearance of the two cation pairs depends on whether the first fragmentation process is a charge separation fission (S2+ / S4+) or the release of a neutral moiety (S2+ / S2+). In the case of ionization by Xe20+, it has been observed that when the two charges are located at second neighboring sites before fission, the coincidence channels are S2+ / S3+ and S+ / S2+, irrespective of the parent dication. When the charges are localized at second neighboring sites in the parent dication and it is releasing a neutral moiety prior to charge separation, the fragmentation process will lead to the appearance of the S2+ / S4+ cation pair. It has also been observed that when the charges in the parent ion are localized at adjacent sites, the doubly charged ion can undergo fission via a two-body dissociation mechanism leading to the S2+ / S4+ cation pair. The symmetric S+ / S+ coincidence channel only occurs when the two charges are located in the parent ion at neighboring positions prior to fission, in a similar manner to fission which is followed by S 2p excitation. The occurrence of this channel is irrespective of the charge of the ion beam projectile. PEPIPICO spectroscopy investigations have been also performed on the fragmentation of selenium clusters subsequent to ionization by E = 150 eV photons. The analysis of the experimental data has lead to the identification of several coincidence channels, and possible fragmentation paths have been proposed. Total electron and total ion yields of selenium in the gas phase have been for the first time recorded around the Se-2p absorption edge. The broad resonant feature observed in the 1428 - 1435 eV energy range has been assigned to the transitions into unoccupied molecular orbitals. This assignment was supported by self-consistent field theoretical calculations.Die Ionisierung und die Fragmentierung von Schwefel- und Selenclustern wurde mit verschiedenen spektroskopischen Methoden untersucht. Weiterhin wurden verschiedene Ionisierungmethoden verwendet, um ein kompletteres Bild der Fragmentierungsmechanismen der Chalkogene Cluster zu erhalten. Massenspektrometrische Methoden wurden verwendet, um Cluster als Funktion der Temperatur und als Funktion der Ionisierungenergie zu untersuchen. Es konnte gezeigt werden, daß für Schwefelcluster, die mit hoch geladenen Ionenstrahlen untersucht wurden mit einer Erhöhung der Temperatur des Schwefeldampfes die Menge der schwereren Aggregate S8 und S7 abnimmt. Gleichzeitig nimmt die Menge der kleineren Partikel (S2 und S) zu. Ein ähnliches Verhalten wurde für Selencluster beobachtet. Eine Zunahme der Temperatur des Selendampfes führt zu einem höheren Prozentsatz an kleineren Partikeln (Se2 und Se). Weiterhin konnte nach einer Se-2p Ionisierung das Auftreten von mehrfach geladene Selenanteilen beobachtet werden (bis zu Se4+). Im Gegensatz zur Se-3d Kontinuumsionisation, konnten keine einfach gelandenen schwereren Aggregate (wie z.B. Se3+) nachgewiesen werden. Das COLTRIMS Reaktionsmikroskop wurde erstmals erfolgreich für die Untersuchung von Schwefelclustern eingesetzt. Die Experimente wurden an der Schwefel 2p und 2s Kante durchgeführt. Aus den experimentellen Daten wurden die Schwefelkoinzidenzkanäle extrahiert, woraus schließlich auf die KERs und CSD's geschlussfolgert werden konnte. Es zeigt sich eine starke Anhängigkeit der kinetischen Energieverteilung vom Winkel phi. Die bei +90 und -90 Grad auftretenden Maxima und die Minima bei 0 bzw. + 180 Grad können als Indiz dafür gedeutet werden, dass die Fragmentation der Aggregate vorzugsweise in Richtung der Polarisation der Synchrotronstrahlung stattfindet. Ion-Ion Koinzidenzuntersuchungen wurden durchgeführt, um die Fragmentationsmechanismen von Schwefelclustern nach Ionisation mit einem Strahl von hochgeladenen Ionen aufzuklären. Die Untersuchungsmethode wurde auch augewählt um eine Beziehung zwischen den aktuellen Resultaten und früher veröffentlichten Resultaten an innerschalenangeregten Schwefelclustern herzustellen. Die photoneninduzierte Innerschalenanregung beginnt an einer gut definierten Stelle innerhalb des Clusters. Auger-Zerfall führt zu Löchern im Valenzband, welche zu einer Spaltung führen und schließlich zu einfach geladenen Zerfallsprodukten. Im S 2p-Regime ist Doppelionisierung der dominierende Prozeß. Demgegenüber führt Ionisierung durch hochgeladene Ionen (HCI) zum Verlust von Elektronen der äußeren Valenzniveaus, welche über das gesamte Molekularsystem delokalisiert sind. Es wird erwartet, daß der Verlust der Elektronen von der Ladung des Projektils abhängt: Schwach geladene Projektile fangen Elektronen auf kurze Distanz ein und führen zu Elektronenanregung im Cluster. Hoch geladene Projektile fangen Elektronen auf große Distanzen ein, führen aber zu keiner größeren Beeinflussung des Clusters. In beiden Fällen kann ein S8 Mutterion in mehrere einzelne Fragmente zerfallen. Diese Arbeit betrachtet nur jene Prozesse, welche zu korrellierten Kationenpaaren führen, so dass, wie in PEPIPICO Experimenten, ähnliche Koinzidenzkanäle durchlaufen werden. Infolgedessen ist es nicht überraschend, daß es Ähnlichkeiten zwischen den beiden Ansätzen gibt. Die nach der Ionisation mit HCI auftretenden Koinzidenzkanäle konnten identifiziert werden und die möglichen Aufspaltungen wurden aufgezeigt. Ein wichtiger Aspekt ist, daß nach der durch HCI induzierten Fragmentation ein weiterer Zweikörper-Zerfall von S8++ in S6+ und S2+ beobachtet werden konnte. Dieser Kanal wurde bei früheren Arbeiten zur Innerschalenanregung von Schwefelclustern nicht beobachtet. Der Koinzidenzkanal tritt nur auf, wenn zwei Ladungen an gegenüberliegenden Stellen eines doppelt geladenen Ions lokalisiert sind, was eine bevorzugte Verteilung bei Ionisation mit HCI zu sein scheint. Ein Vergleich mit den aktuellen Ergebnissen zeigt, dass die Trennung in einfach geladene Fragmente von der ursprünglichen Ladungsverteilung im doppelt geladenen Mutterion beeinflusst wird. Insbesondere wurde für den Fall eines Strahls von Xe5+ Ionen beobachtet, dass die Lokalisierung der Ladungen an gegenüberliegenden Seiten zu einem Auftreten von S3+ in Koinzidenz mit anderen einfach geladenen Kationen führt. Die zweite Ionensorte, S+ oder S2+, hängt davon ab, ob das Mutterion vor der Separation ein neutrales Partikel abgegeben hat. Die Lokalisierung der Ladungen im Mutter-Di-Kation an zwei benachbarten Positionen führt zum Auftreten des $S_2^+ / S_4^+$ Koinzidenzkanals, mit dem Zwischenprodukt S7++. Die Lokalisierung der Ladungen im Mutterion an gegenüberliegenden Orten kann bei der Ionisation mit Xe10+ Ionen, zu zwei unterschiedlichen Koinzidenzkanälen (S2+ / S4+ und S2+ / S2+) führen. Dieses hängt von ab, ob das doppelt geladene Ion kurz vor der Trennung ein S7++ oder S8++ -Ion ist. Bei der Ionisierung mit Xe15+ Ionen, führt eine Lokalisierung der zwei Ladungen an benachbarten Plätzen zum Auftreten von S2+ / S3+ und S+ / S2+ Kationenpaaren. Weiterhin wurde beobachtet, daß die Lokalisierung der Ladungen an gegenüberliegenden Positionen im Mutterion für das Auftreten von S2+ / S4+ und S2+ / S2+ Ionenpaaren verantwortlich ist. Das Auftreten der zwei Kationenpaare hängt an ab, ob der erste Prozeß eine Ladungstrennungs (S2+ / S4+) oder die Abspaltung eines neutralen Teilchens (S2+ / S2+) ist. Im Fall der Ionisierung durch Xe20+ wurde beobachtet, daß im Falle zweiter benachbarter Ladungen im Mutterion einer der beiden Koinzidenzkanäle S2+ / S3+ und S+ / S2+ auftreten kann, unabhängig vom Mutterkation. Wenn die Ladungen an zweiten benachbarten Plätzen im Mutter-Di-Kation lokalisiert sind, und ein neutrales Teilchen vor der Ladungsseparierung emittiert wird, resultiert der Trennungsprozess in einem S2+ / S4+ Ionenpaar. Bei direkt benachbarten Ladungen im Mutterion wurde weiterhin beobachtet, daß die Trennung des doppelt geladenen Ions in einem Zweikörperzerfallsprozess zu einem S2+ / S4+ Ionenpaar führt. Der symmetrische S+ / S+ Koinzidenzkanal tritt nur auf, wenn die zwei Ladungen im Mutterion an benachbarten Orten liegen, ähnlich zur Trennung nach der S 2p - Ionisation. Das Auftreten dieses Koinzidenzkanals ist unabhängig von der Ladung des HCI-Strahls. Mittels PEPIPICO Spektroskopie wurden weiterhin Untersuchungen zur Fragmentation von Selenclustern durchgeführt. Die Ionisation erfolgte hier mit E = 150 eV Photonen. Die Analyse der experimentellen Daten führte zur Identifikation von mehreren Koinzidenzkanälen und mögliche Fragentationsprozesse konnten vorgeschlagen werden. Erstmalig konnten totale Elektronenausbeutenkurven sowie totale Ionenausbeutenkurven von Selen in der Gasphase an der Se-2p Absorptionskante aufgenommen werden. Die breite Resonanz zwischen 1428 eV und 1435 eV konnte den Übergängen in unbesetzte molekulare Orbitale zugeordnet werden. Diese Zuordnung wurde durch theoretische Berechnungen auf Basis von Selbstkonsistenten Feldern bestätigt

Angle resolved Photoemission from Ag and Au single crystals: Final state lifetimes in the attosecond range

We present angle resolved photoemission spectra for Ag(111) and Au(111) single crystals in normal emission geometry, taken at closely spaced intervals for photon energies between 8 eV and 160 eV. The most dominant transitions observed are attributed to f-derived final states located about 16–17 eV above E$_F$ for both materials. These transitions exhibit very distinct resonance phenomena and selection rules and are reminiscent of the angular momentum characteristics of the states involved. The excited electron lifetime is in the attosecond (as) range as determined from the energy width of the observed transitions. This serves as an alternate approach to the direct determination of excited electron lifetimes by as laser spectroscopy

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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

Electronic properties and morphology of Cu-phthalocyanine—C60\mathrm{C_{60}} composite mixtures

Phthalocyanines in combination with C$_{60}$ are benchmark materials for organic solar cells. Here we have studied the morphology and electronic properties of co-deposited mixtures (blends) of these materials forming a bulk heterojunction as a function of the concentration of the two constituents. For a concentration of 1:1 of CuPc:C$_{60}$ a phase separation into about 100 nm size domains is observed, which results in electronic properties similar to layered systems. For low C$_{60}$ concentrations (10:1 CuPc:C$_{60}$) the morphology, as indicated by Low-Energy Electron Microscopy (LEEM) images, suggests a growth mode characterized by (amorphous) domains of CuPC, whereby the domain boundaries are decorated with C$_{60}$. Despite of these markedly different growth modes, the electronic properties of the heterojunction films are essentially unchanged

Improving the efficiency of high harmonic generation (HHG) by Ne-admixing into a pure Ar gas medium

Laser-based higher-order harmonic generation has been investigated extensively in the past two decades. The current manuscript deals with the high harmonic generation (HHG) outputs from a gas-filled waveguide when using mixtures of two rare gases (Ar and Ne) as nonlinear media. We find that the efficiency of the HHG process can be optimized by changing the pressure or alternatively the mixing ratio of the two gases. This is attributed to the fact that both of these parameters have an effect on the phase-matching in the waveguide. These observations are especially useful when phase matching in a gas jet is concerned, where the absolute local pressure of the gas media cannot be controlled as readily as in a capillary-based HHG setup

Collision induced fragmentation of free sulfur clusters

Fragmentation of multiply charged sulfur clusters is investigated by ion-ion-coincidence spectroscopy. The experiments were performed at the electron cyclotron resonance (ECR) ion source at the KVI Groningen, where beams of Xeq+ (q = 5, 10, 15, 20) were produced. The Xeq+ ions were accelerated to kinetic energies of 8-10.q keV and collided with a beam of free sulfur clusters. Variable size sulfur clusters are prepared in a two-stage oven source, where the temperature of the oven was used to adjust the cluster size. Most experiments were performed using S-8, the dominant cluster at low oven temperature. Ion-ion-coincidence as well as mass spectra were recorded. Coincidences between singly charged atomic and molecular fragments were studied, where changes of product channels were observed as a function of charge state of the Xeq+ projectile. The mechanisms of cluster fragmentation are discussed. The results are compared to earlier experiments on core-excited sulfur clusters using soft X-rays. (C) 2008 Elsevier B.V. All rights reserve