Photoemission electron microscopy for nanoscale imaging and attosecond control of light-matter interaction at metal surfaces

Electron dynamics at solid surfaces unfold on the nanometer length and attosecond timescale when driven by electromagnetic fields at optical frequencies, enabling vast scientific and technological applications in the field of nano-optics and nanoplasmonics. Direct imaging of the electrons upon interaction with light is a highly desirable tool for understanding and control of the dynamics, which requires ultrahigh spatiotemporal resolution. This thesis explores the combination of photoemission electron microscopy (PEEM) with few-cycle femtosecond laser pulses and attosecond extreme ultraviolet (XUV) pulses for studying ultrafast electron dynamics from metallic surfaces and nanosystems. The work involves development and implementation of new experimental tools including detection, data acquisition and analysis techniques for PEEM measurements. The first approach is using a combination of PEEM with attosecond streaking spectroscopy (atto-PEEM) for direct, non-invasive probing of nanoplasmonic fields from supported nanostructures. As a first step towards the implementation of the atto-PEEM concept, PEEM imaging on lithographically fabricated gold structures employing 93 eV attosecond XUV pulses from a 1 kHz high-harmonic generation (HHG) source is performed. The spatial resolution is limited to ~200 nm due to space charge effects when working with such a low-repetition-rate HHG source and chromatic aberrations caused by the large energy bandwidth of XUV-generated photoelectrons. Nevertheless, we show that microspectroscopic imaging of core-level and valence band electrons is achievable using our energy-resolved PEEM despite the aforementioned issues. Most importantly, we find that the fast photoelectrons from the valence band, which carry the attosecond temporal structure of the plasmonic field, are not affected by space charge effects. The currently developed megahertz-repetition-rate attosecond XUV sources are therefore expected to enable the experimental realization of nanoplasmonic streaking with ultrahigh spatiotemporal resolution in the near future. Second, PEEM is coupled with a single-shot stereographic above-threshold ionization phase meter, which allows carrier-envelope phase (CEP) tagging for studying attosecond control of photoemission. First experiments performed on gold nanospheres on a gold plane and on a random rough gold surface using few-cycle near-infrared pulses show a CEP artefact with a modulation period of π. The artefact is found to be caused by a laser intensity dependence of both the photoelectron spectra and the CEP measurement. Intensity tagging is therefore added to the current CEP tagging technique to correct this intensity-dependent artefact. As a result, a very weak CEP modulation (~1% amplitude) of the photoemission yield from a bulk tungsten surface with a 2π modulation period (as expected from solids) is successfully detected in the above-threshold photoemission regime after applying appropriate corrections based on the intensity tagging. Entering the tunneling regime, the CEP modulation increases to ~7% despite the presence of space charge effects due to high laser peak intensity. We also apply this technique to investigate the CEP dependence on gold nanotriangles and find no apparent CEP modulation within an accuracy of ~0.6% as given by our experimental conditions, which constitutes an upper limit for a possible CEP modulation from this nanostructure.Elektronendynamik an Festkörperoberflächen, die von elektromagnetischen Feldern mit optischen Frequenzen getrieben wird, findet auf einer Längen- und Zeitskala im Bereich von Nanometern bzw. Attosekunden statt und ermöglicht eine Vielzahl wissenschaftlicher und technischer Anwendungen auf dem Gebiet der Nanooptik und Nanoplasmonik. Die direkte Visualisierung der Elektronen in Folge ihrer Wechselwirkung mit Licht, was eine ultrahohe räumlich-zeitliche Auflösung erfordert, ist ein sehr nützliches Instrument zum Verständnis dieser Dynamik und ihrer Kontrolle. In dieser Dissertation wird eine Kombination aus Photoemissionselektronenmikroskopie (PEEM) mit Femtosekundenlaserpulsen von wenigen Zyklen Dauer sowie extrem ultravioletten (XUV) Attosekundenpulsen erforscht, um ultraschnelle Elektronendynamik an Metalloberflächen und in Nanosystemen zu untersuchen. Diese Arbeit beinhaltet die Entwicklung und Implementierung neuer Messinstrumente und Methoden für PEEM-Experimente, insbesondere Detektion, Datenerfassung und Datenanalyse. Der erste Ansatz für eine direkte, nichtinvasive Untersuchung nanoplasmonischer Felder an ortsfesten Nanostrukturen ist eine Kombination von PEEM mit Attosekunden-Streaking (Atto-PEEM). Als eine Voraussetzung für die Implementierung des Atto-PEEM-Konzepts wird eine PEEM-Abbildung von lithographisch hergestellten Goldstrukturen mittels 93 eV XUV Attosekundenpulsen aus einer 1 kHz Quelle für die Erzeugung hoher Harmonischer realisiert. Wegen Raumladungseffekten, die durch die niedrige Repetitionsrate der hohen Harmonischen zustande kommen, sowie chromatischer Aberrationen aufgrund der hohen Energiebandbreite der durch die XUV-Strahlung erzeugten Photoelektronen, ist die räumliche Auflösung auf ~200 nm begrenzt. Dennoch wird gezeigt, dass trotz dieser Schwierigkeiten eine mikrospektroskopische Abbildung von inneren Elektronen und Valenzelektronen mittels unserer energieaufgelöster PEEM möglich ist. Unsere wichtigste Erkenntnis ist, dass die schnellen Photoelektronen aus dem Valenzband, die die zeitliche Struktur der plasmonischen Felder auf der Attosekundenskala abtasten, nicht durch Raumladungseffekte beeinträchtigt werden. Die sich derzeit in Entwicklung befindenden Quellen für Attosekunden-XUV-Pulse mit Megahertz Repetitionsraten sind daher vielversprechend für die experimentelle Realisierung von nanoplasmonischem Streaking mit ultrahoher räumlicher und zeitlicher Auflösung in naher Zukunft. Zweitens wird PEEM mit einem stereographischen, auf Above-Threshold-Ionisation basierenden Einzelschuss-Phasenmessgerät verbunden, was eine Zuordnung (Tagging) der Träger-Einhüllenden-Phase (carrier-envelope phase, CEP) erlaubt und dadurch ermöglicht, die Kontrolle der Photoemission auf der Attosekundenskala zu erforschen. Erste Experimente an Goldnanosphären auf einer Goldebene sowie an einer rauen Goldoberfläche mit wenige Zyklen kurzen Laserpulsen im Nah-Infraroten weisen ein CEP-Artefakt mit einer Modulationsperiode von π auf. Es wird gezeigt, dass dieses Artefakt durch eine Abhängigkeit sowohl der Photoelektronenspektra als auch der CEP-Messung von der Laserintensität hervorgerufen wird. Die bisherige CEP-Tagging-Technik wird deshalb um Intensitäts-Tagging erweitert, um dieses intensitätsabhängige Artefakt zu korrigieren. Als Resultat wird nach angemessenen Korrekturen basierend auf dem Intensitäts-Tagging eine schwache CEP-Modulation (~1% Amplitude) der Photoemissionsergiebigkeit von einer unstrukturierten Wolframoberfläche mit einer Modulationsperiode von 2π (wie bei Festkörpern erwartet) im Above-Threshold-Photoemissionsregime erfolgreich nachgewiesen. Im Tunnelregime wächst die CEP-Modulation auf ~7% trotz aufkommender Raumladungseffekte aufgrund der starken Spitzenintensität der Laserpulse. Es werden ebenfalls Goldnanodreiecke mit dieser Technik untersucht, jedoch kann keine CEP-Modulation innerhalb der experimentellen Genauigkeit von ~0.6% gefunden werden. Dies stellt eine Obergrenze für eine mögliche CEP-Modulation an dieser Nanostruktur dar

An Attention-Based View of Strategic Human Resource Management

In spite of the designation, research in strategic human resource management (SHRM) has largely focused on tactical rather than strategic issues. These studies have attempted to explain whether firm performance is associated with “best-practice” or “best-fit” human resource management (HRM) practices. The focus on internally consistent (horizontal fit) HRM practices is better characterized as tactical implementation than as strategic formulation. In the latter, HRM leads rather than follows the firm’s long-term responses to shifts in the firm’s competitive and market environment. In this paper, I propose an attention-based view of the firm to reframe SHRM research and policy to support a strategy formulation based on environment conditions (vertical fit). From an organizational policy perspective, this approach elevates the role of the HR executive from that of a functional specialist to that of a strategic generalist

Control of Agrilus ruficollis (Coleoptera: Buprestidae) With Insecticides and Identifying Visual Attractants for Use in a Monitoring Trap

The rednecked cane borer, Agrilus ruficollis (F.), is a pest of cultivated and wild blackberries in the Midwestern and Eastern parts of the United States. Feeding, mating, egg laying and development of A. ruficollis from larvae to adult only occurs on primocane, the first year vegetative growth stage of blackberries, and not on the second year fruiting stage called floricanes that die after fruiting. Damage from this pest is caused by the larva girdling the primocane and tunneling in the pith, causing the formation of a gall. Gall formation on the primocane increases the chance of winter injury and can also potentially reduce yields the following season. There is currently only one class of insecticide (imidacloprid) approved for use in managing the pest and no trap available for monitoring this pest. The research presented in this dissertation was to determine if other chemical classes of insecticides would provide equal adequate control of A. ruficollis as achieved by an application of imidacloprid and determine what visual and chemical cues act as stimulants for attracting A. ruficollis. An efficacy study of several insecticides only found that paraffinic oil (JMS Stylet Oil) provided a level of control of A. ruficollis similar to that achieved by the industry standard (imidacloprid). Paints that mimicked the spectral reflectance of blackberry leaves and canes of both primocane and floricane growth stages were applied to four dowel sizes (ranged from 0.3 to 2.5 cm diameters), covered with sticky Tanglefoot® and field evaluated for attractiveness to A. ruficollis for three years, with modifications to the trap each year. Commercially available green or purple plastic funnel traps covered with fluon were evaluated for attractiveness to A. ruficollis in 2014. In 2011, the greatest numbers of A. ruficollis adults were captured on one inch prism-shaped, vertical primocane mimic traps that reflected light at a peak wavelength between 540-560 nm (green). In 2012 and 2013, field tests demonstrated that the most A. ruficollis adults were captured on traps painted the same green color of traps used to monitor for emerald ash borer, Agrilus planipennis Fairmaire. The funnel trap testing in 2014 reinforced the previous findings that A. ruficollis is most attracted to the green color of emerald ash borer traps. In 2013, it was noted that colored traps usually captured significantly more A. ruficollis males than females (\u3e 2.4 males: 1 female ratio). This indicated a need to determine if there was a chemical cue used by A. ruficollis adult females to find and select only blackberry primocanes and not floricanes. However, no differences were found in volatile compounds collected from blackberry primocanes and floricanes. Although peaks were identified, these collected volatiles did not stimulate antennae of A. ruficollis adults. Overall, this dissertation was able to show that certain green colors attracted A. ruficollis. Further research should be conducted to optimize trap design and determine if there is a pheromone or primocane plant odor that enhances colored trap catch of female and/or male A. ruficollis. A baited colored trap will improve the monitoring for this insect and timing of insecticide applications and lead to development of a mass trapping tactic that reduces the local density of A. ruficollis and lessen need for insecticide application

Corporate Governance and Management Succession in Family Businesses

Family businesses carry the weight of economic wealth creation in most economies. In the U.S. alone, family businesses account for 80 to 90 percent of the 18-million business enterprises in the United States, and 50 percent of the employment and GNP. In many ways, the family business is synonymous with the entrepreneurial organization as many were started as a means to provide for the financial well being of the founder's family. Founders who went on to build family empires started many of today's large corporations (e.g., Anheuser-Busch, Dupont, and Seagrams). Still, we know relatively little about the issues peculiar to a family business, such as the process and impact of succession planning. Yet, no recurring event in the life of the family firm is more critical to survival than the transfer of power from the incumbent to the successor. Organizations are especially susceptible to loss of vision and purpose during periods of CEO transition, as the leaders who helped shape the vision are replaced by others who may not share the same values and abilities. This study addresses the importance of understanding business succession planning by proposing and empirically verifying a model of succession planning and firm effectiveness in the family business. It links aspects of succession planning and successor preparation to the effectiveness of transition and from performance. The model depicts multiple interactive relationships, with emphasis placed not only on the planning and process-specific but also on successor-specific factors that lead to effectiveness.corporate governance, family businesses, management succession, firm performance, successor characteristics

Zeolitic Imidazolate Frameworks as H_2 Adsorbents: Ab Initio Based Grand Canonical Monte Carlo Simulation

We report the H_2 uptake behavior of 10 zeolitic−imidazolate frameworks (ZIFs), based on grand canonical Monte Carlo (GCMC) simulations. The force fields (FFs) describing the interactions between H_2 and ZIF in the GCMC were based on ab initio quantum mechanical (QM) calculations (MP2) aimed at correctly describing London dispersion (van der Waals attraction). Thus these predictions of H_2 uptake are based on first principles (non empirical) and hence applicable to new framework materials for which there is no empirical data. For each of these 10 ZIFs we report the total and excess H_2 adsorption isotherms up to 100 bar at both 77 and 300 K. We report the hydrogen adsorption sites in the ZIFs and the relationships between H_2 uptake amount, isosteric heat of adsorption (Q_(st)), surface area, and free volume. Our simulation shows that various ZIFs lead to a variety of H_2 adsorption behaviors in contrast to the metal−organic frameworks (MOFs). This is because ZIFs leads to greater diversity in the adsorption sites (depending on both organic linkers and zeolite topologies) than in MOFs. In particular, the ZIFs uptake larger amounts of H_2 at low pressure because of the high H_2 adsorption energy, and ZIFs have a variety of H_2 adsorption sites. For example, ZIF-11 has an initial Q_(st) value of ~15 kJ/mol, which is higher than observed for MOFs. Moreover, the preferential H_2 adsorption site in ZIFs is onto the organic linker, not nearby the metallic joint as is the case for MOFs

Improved H_2 Storage in Zeolitic Imidazolate Frameworks Using Li^+, Na^+, and K^+ Dopants, with an Emphasis on Delivery H_2 Uptake

We use grand canonical Monte Carlo simulations with first principles based force fields to show that alkali metal (Li^+, Na^+, and K^+)-doped zeolitic imidazolate frameworks (ZIFs) lead to significant improvement of H_2 uptake at room temperature. For example, at 298 K and 100 bar, Li-ZIF-70 totally binds to 3.08 wt % H_2, Na-ZIF-70 to 2.19 wt % H_2, and K-ZIF-70 to 1.62 wt % H_2, much higher than 0.74 wt % H_2 for pristine ZIF-70. Thus, the dopant effect follows the order of Li-ZIF > Na-ZIF > K-ZIF, which correlates with the H_2 binding energies to the dopants. Moreover, the total H_2 uptake is higher at lower temperatures: 243 K > 273 K > 298 K. On the other hand, delivery H_2 uptake, which is the difference between the total adsorption at the charging pressure (say 100 bar) and the discharging pressure (say 5 bar), is the important factor for practical on-board hydrogen storage in vehicles. We show that delivery H_2 uptake leads to Na-ZIF-70 (1.37 wt %) > K-ZIF-70 (1.25 wt %) > Li-ZIF-70 (1.07 wt %) > ZIF-70 (0.68 wt %), which is different from the trend from the total and excess uptake. Moreover, the delivery uptake increases with increasing temperatures (i.e., 298 K > 273 K > 243 K)! To achieve high delivery H_2 uptake at room temperature, the large free volume of ZIFs is required. We find that higher H_2 binding energy needs not always lead to higher delivery H_2 uptake