Electronic properties of Mn-Phthalocyanine - C60_{60} bulk heterojunctions: combining photoemission and electron energy-loss spectroscopy

The electronic properties of co-evaporated mixtures (blends) of manganese phthalocyanine and the fullerene C$_{60}$ (MnPc:C$_{60}$) have been studied as a function of the concentration of the two constituents using two supplementary electron spectroscopic methods, photoemission spectroscopy (PES) as well as electron energy-loss spectroscopy (EELS) in transmission. Our PES measurements provide a detailed picture of the electronic structure measured with different excitation energies as well as different mixing ratios between MnPc and C$_{60}$. Besides a relative energy shift, the occupied electronic states of the two materials remain essentially unchanged. The observed energy level alignment is different compared to that of the related CuPc:C$_{60}$ bulk heterojunction. Moreover, the results from our EELS investigations show that despite of the rather small interface interaction the MnPc related electronic excitation spectrum changes significantly by admixing C$_{60}$ to MnPc thin films

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

Femtosekunden-Analyse und Rückkoppelungskontrolle von molekularen Prozessen in organometallischen und alkalischen Systemen

Title 1 1\. Introduction 7 2\. Femtosecond Laser Pulses 13 2.1 Generation of Femtosecond Laser Pulses 13 2.2 Diagnostic Techniques 25 3 Femtosecond Pulse Shaping with Liquid Crystal Modulators 35 3.1 Shaped Femtosecond Laser Pulses 35 3.2 The Pulse Shaper 39 3.3 Examples of Other Pulse Shaping Techniques 52 4 Experimental Details 55 4.1 Molecular Beam Apparatus 55 4.2 The Laser Systems 63 4.3 Acquisition of the Experimental Data 71 5 Theoretical Considerations 73 5.1 Molecular Dynamics - A Short Introduction 73 5.2 Control of Molecular Reactions 86 6 Analysis and Control of Ultrafast Processes in CpMn(CO)3 97 6.1 Choice of the System 98 6.2 Experimental Preparations 100 6.3 Femtosecond Analysis of CpMn(CO)3+ 102 6.4 Control of CpMn(CO)2+ Ionization Process 124 6.5 Summary and Outlook 129 7 Femtosecond Time-Resolved Spectroscopy of Na2F 133 7.1 Choice of the System 134 7.2 Experimental Preparations 135 7.3 Mass-spectrometric Studies 137 7.4 Pump-Probe Experiments on Na2F 139 7.5 Summary and Outlook 143 8 Frequency-Dependent Optimization of the NaK Ionization Process 145 8.1 Choice of the System 146 8.2 Generation of NaK Dimers in Molecular Beams 149 8.3 Frequency-Dependent Optimization Factors 149 8.4 Analysis of the Optimal Pulse Forms 154 8.5 Summary and Outlook 159 9 Isotope Ratio Control of K2+ 161 9.1 Choice of the System 162 9.2 Experimental Preparations 164 9.3 Acquisition of the Experimental Data 165 9.4 Two-Pulse Optimization of the 39,39K2+ / 39,41K2+ Ratio 167 9.5 Single-Pulse Optimization of the 39,39K2+ / 39,41K2+ Ratio 174 9.6 Summary and Outlook 204 10 Concluding Remarks 209 11 Kurzzusammenfassung 213 Bibliography 216 Appendix A Alignment of the Pulse Shaping Apparatus 231 Appendix B Phase Retrieval Software: FROGGUI 233 Lebenslauf 235 Acknowledgments 243The goal of this work was to analyze and control ultrafast laser-induced processes occurring in model species, like the organometallic molecule CpMn(CO)3 and alkali systems. The control experiments were performed by employing the adaptive feedback-loop scheme. The analysis of the optimal pulses demonstrated that intrinsic information about the control process itself can be gained. The pump-probe experiments on CpMn(CO)3 together with ab initio quantum mechanical analysis allowed us to explain for the first time the ionization mechanism performed by the optimal pulse obtained for maximization of CpMn(CO)3+. The optimized laser field avoided intelligently any fragmentation channels favorizing exclusively the ionization pathway. A method of producing sodium-fluoride clusters in a molecular beam was elaborated, as well. For the Na2F trimer, geometric rearrangement between the linear and triangular configurations with a period of 185 fs was successfully observed. Characteristic motions of the involved wave packets were proposed as an attempt to explain the optimized dynamics in NaK. The proposed scheme is in good agreement with the developed theoretical model for the ionization mechanism. Furthermore, a technique advanced in this work allowed the control of isotope ionization ratio. This approach was first demonstrated for the potassium dimer isotopes. One was able to learn about the optimized process (the transitions involved on the way to the ion states) from the acquired optimal pulses, in particular their spectra. The studies were extended to the comparison between phase and amplitude modulation and phase-only or amplitude- only modulation. he control of isotope ratio demonstrated for K2 can be very well seen as a new spectroscopic approach of a distinct frequency pattern on femtosecond time scales due to the superposition of the spectral components.Das Ziel dieser Arbeit war die Analyse und Steuerung ultraschneller photoinduzierter molekularer Prozesse in Modellsystemen, wie dem organometallischen Molekül CpMn(CO)3 und Alkali Systemen. Die Kontrollexperimente wurden unter Anwendung des Schemas der adaptiven Rückkopplungsschleife durchgeführt. Die Analyse der optimalen Pulsformen hat gezeigt, dass intrinsische Informationen über den Kontrollprozess selbst gewonnen werden können. Die Pump-Probe Experimente an CpMn(CO)3 zusammen mit der ab initio quantenmechanischen Analyse haben erstmalig den Ionisierungsmechanismus erklärt, der von der optimalen Pulsform für die Maximierung des CpMn(CO)3+ Ionensignals durchgeführt worden ist. Das optimierte Laserfeld vermeidet intelligent jeden Fragmentationskanal und nur der Ionisationspfad wird favorisiert. Eine Methode zur Erzeugung von Natriumfluorid-Clustern in Molekularstrahlen wurde ebenfalls entwickelt. Für das Na2F Trimer wurde mit Hilfe der Pump-Probe Spektroskopie die geometrische Veränderung zwischen linearer und gewinkelter Struktur mit einer Periode von 185 fs erfolgreich aufgelöst. Charakteristische Bewegungen von Wellenpaketen wurden vorgeschlagen als ein Erklärungsversuch für die optimierte Dynamik in NaK. Das vorgeschlagene Schema stimmt mit dem theoretischen Modell für den Ionisierungsmechanismus überein. Darüber hinaus hat eine während dieser Arbeit entwickelte Technik die Steuerung des Isotopenionisierungsverhältnisses ermöglicht. Diese Vorgehensweise wurde für die Kaliumdimer Isotope gezeigt. Die optimierten Pulsspektren haben es ermöglicht, über den optimierten Prozess (die Übergänge, die vom Grundzustand bis zum Ionenzustand involviert sind) zu lernen. Die Untersuchungen wurden um einen Vergleich zwischen Phasen- und Amplitudenmodulation mit reiner Phasen-, bzw. reiner Amplitudenmodulation erweitert. Die Steuerung des Isotopenverhältnisses für K2 könnte als ein neuer spektroskopischer Ansatz angesehen werden von eindeutigen Frequenzmustern auf einer Femtosekundenzeitskala, aufgrund der Überlagerung der Spektralkomponenten

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

Irradiation-induced degradation of PTB7 investigated by valence band and S 2 p photoelectron spectroscopy

Monochromatic radiation with known absolute radiant power from an undulator at the electron storage ring Metrology Light Source (MLS) was used to irradiate PTB7 (a thieno[3, 4-b]thiophene-alt-benzodithiophene polymer) thin films at wavelengths (photon energies) of 185 nm (6.70 eV), 220 nm (5.64 eV), 300 nm (4.13 eV), 320 nm (3.88 eV), 356 nm (3.48 eV) and 675 nm (1.84 eV) under ultra-high vacuum conditions for the investigation of radiation-induced degradation effects. The characterization of the thin films is focused at ultraviolet photoelectron spectroscopy (UPS) of valence bands and is complemented by S 2p x-ray photoelectron spectroscopy (S 2p XPS) before and after the irradiation procedure. The radiant exposure was determined for each irradiation by means of photodiodes traceably calibrated to the international system of units SI. The valence band spectra show the strongest changes for the shortest wavelengths and no degradation effect at 356 nm and 675 nm even with the highest radiant exposure applied. In the spectral range where the Sun appears bright on the Earth's surface, no degradation effects are observed

New insight into the Auger decay process in O-2 : The coincidence perspective

Photoelectron-Auger electron coincidence spectroscopy is a powerful tool for the investigation of Auger decay processes with different core-ionized intermediate states. In this paper we describe an investigation into the Auger decay of the O-2 molecule, with the purpose of bringing new insight into the dynamics of the core hole decay mechanism. Using a novel experimental approach to measuring such coincidence spectra we report the highest resolution Auger spectrum of O-2 recorded hitherto. In our approach, we have combined the advantages of these coincidence spectra with the high resolution and excellent signal-to-noise ratios of non-coincident Auger spectra and a state-of-the-art fit analysis. In this way we have derived information about the potential energy curves of the final states W-3 Delta(u), B-3 Pi(g), and B' (3)Sigma(-)(u) and concluded that the corresponding Auger transitions are formed to a large part by strongly overlapping vibrational progressions. The present findings are compared to earlier results reported in the literature confirming some theoretical predictions