Pour la réalisation des transistors FDSOI 22 nm et 3D FinFET 10 nm, la gravure de couches ultraminces de quelques nanomètres d’épaisseur doit être réalisée sans endommagement de la couche sous-jacente et n’est plus envisageable avec les procédés reposant sur les plasmas continus à haute densité. Une nouvelle technologie de gravure est étudiée dans cette thèse : elle consiste à modifier la surface d’un matériau sous l’action d’un plasma et à retirer par voie chimique le matériau modifié sélectivement par rapport au matériau non modifié. Nous nous focalisons ici sur la compréhension de la modification du matériau SiN induite par les plasmas de H2 et He, suivie d’une gravure chimique réalisée en solution de HF. Tout d’abord, un dépôt de conditionnement est développé pour prévenir la contamination du substrat et la dégradation des parois. Des diagnostics en plasmas de H2 et He sont ensuite réalisés pour déterminer la nature des ions, leurs flux et leurs énergies. Après exposition du SiN au plasma d’hélium, les caractérisations de surface (FTIR, SIMS) indiquent premièrement que la composition chimique du SiN est inchangée. De plus, la bonne corrélation entre les vitesses de gravure en HF avec les simulations de l’implantation des ions sous SRIM permet de conclure que l’augmentation de la vitesse de gravure est due aux dégâts induits par les ions dans le matériau. Après exposition au plasma d’hydrogène, la vitesse de gravure du SiN en HF dépend essentiellement de la concentration en hydrogène dans le matériau. Une synergie a lieu entre les radicaux H du plasma et le bombardement ionique : les ions créent des liaisons pendantes qui sont indispensables à la formation de liaisons Si-H et N-H par les radicaux. En outre, nous montrons que le temps de plasma de H2 et la dose d’ions ont une importance capitale dans la formation de la couche modifiée qui n’atteint parfois un état stationnaire qu’au bout d’un temps relativement long. / To fabricate 22 nm FDSOI and 10 nm 3D FinFET transistors, ultrathin layers of several nanometers thickness must be etched without damaging the under layer, which can no longer be managed using processes based on high density continuous plasmas. To meet those new challenges, we study in this thesis a new etching technology where the surface of the material is modified under plasma exposure and then removed chemically selectively with respect to the non modified material. We focus here on the understanding of the modification of the SiN material induced by H2 and He plasmas, followed by a chemical etching in HF aqueous solution. First, a protective coating is developed to prevent the contamination of the substrate and the degradation of the wall. Diagnostics in H2 and He plasmas were then carried out to determine the nature of the ions, their fluxes and their energies. After He plasma exposure of the SiN, surface characterizations (FTIR, SIMS) first show that the SiN chemical composition is unchanged. Moreover, the good correlation between the etch rates in HF and the ion implantation profiles calculated by SRIM allows to conclude the increase of the etch rate is due to the ion-induced damages on the material. After H2 plasma exposure, the etch rate of SiN in HF mainly depends on the hydrogen concentration of the film. A synergetic effect occurs between H radicals and the ionic bombardment: the ions induce dangling bonds which are unavoidable to form Si-H and N-H bonds with the radicals. In addition, we show the plasma exposure time and the ion dose play a key role in the formation of the modified layer who sometimes only reaches a steady state after a relatively long time.

Ce travail de thèse aborde l’un des défis technologiques liés au développement de nouvelles générations de transistors (FinFET, FDSOI), pour lesquels la gravure de couches ultraminces révèle plusieurs problèmes. En particulier, la gravure des espaceurs nitrure (SiN) doit être réalisée avec une précision nanométrique sans endommager les couches sous-jacentes, étape qui ne peut plus être réalisée par des plasmas conventionnels continus. Afin de dépasser cette limitation, une approche innovante a été récemment développée (dite Smart-Etch), qui s’appuie sur l'implantation d’ions légers et se déroule en deux étapes. Premièrement, le matériau à graver est exposé à un plasma ICP ou CCP d’hydrogène (H2) ou d’hélium (He); dans une deuxième étape, la couche modifiée est retirée sélectivement par gravure humide ou exposition à des réactifs gazeux. Afin d’appréhender les mécanismes fondamentaux de la première étape et assister le développement de cette nouvelle technologie, des simulations de dynamique moléculaire (MD) ont été réalisées pour étudier l'interaction des plasmas H2/He avec des couches de Si/SiN. La MD a été utilisée pour examiner comment la modification de ces substrats est affectée par l’énergie ionique, la dose ionique, la composition ionique ou le rapport flux de radicaux/ flux d’ions (dans le cas d’un plasma H2). En accord avec les expériences, les simulations de bombardement ionique He+ ou Hx+ (x = 1-3) sur Si/SiN montrent que l’implantation ionique est auto-limitée, et que l’évolution de la surface se déroule en deux étapes : une rapide modification en volume (sans gravure) suivie d'une saturation lente et de la formation d'une couche implantée stable en régime permanent (état stationnaire). Les mécanismes d'endommagement induit par les ions (rupture des liaisons Si-Si ou Si-N, piégeage/désorption d’He ou H2, formation de groupes SiHx (x = 1-3) en profondeur), sont étudiés et permettent d’apporter de nouveaux éléments de compréhension aux technologies Smart-Cut et Smart-Etch. L’exposition de substrats Si/SiN à un plasma H2 (impacts d’ions Hx+ et de radicaux H) a également été étudiée pour différentes conditions plasma. Dans ce cas, une transformation auto-limitée est observée mais les couches modifiées/hydrogénées sont simultanément gravées pendant l'implantation ionique, à un taux 10 fois inférieur pour SiN par rapport à Si. Les simulations montrent que modifier des substrats Si/SiN avec une précision nanométrique nécessite un contrôle prudent de l’énergie et du flux des ions incidents. En particulier, les faibles doses ioniques doivent être évitées car l’évolution de la surface ne peut pas être contrôlée précisément en régime transitoire (modification rapide). Dans les plasmas H2, les énergies ioniques élevées induisent des couches modifiées plus épaisses mais des taux d'hydrogénation plus faibles et moins homogènes. La composition ionique et le rapport flux de radicaux/ flux ions (Γ) doivent également être controllés avec précaution, notamment car la vitesse de gravure du matériau augmente avec Γ, ce qui empêche entre-autre la possibilité du Smart-Etch pour le silicium. Les simulations MD réalisées dans cette thèse permettent de clarifier divers phénomènes inexpliqués observés dans le Smart-Etch expérimentalement, et de révéler quelques problèmes possibles dans ce nouveau procédé. Finalement, une gamme de paramètres plasma est proposée pour optimiser cette première étape de Smart-Etch et contrôler la modification de SiN avec une précision sous-nanométrique. / This PhD thesis focuses on technological challenges related to the development of advanced transistors (FinFET, FDSOI), where the etching of thin films reveals several issues. In particular, the etching of silicon nitride spacers should be achieved with a nanoscale precision without damaging the underlayers, a step which cannot be addressed by conventional CW plasmas. To overpass this limitation, an innovative approach was recently developed (so-called Smart Etch), which is based on light ion implantation and composed of two steps. First, the material to be etched is modified by exposure to a hydrogen (H2) or helium (He) ICP or CCP plasma; in a second step, the modified layer is selectively removed using wet etching or gaseous reactants only. To support the fundamental understanding of the first step and assist the development of this new technology, molecular dynamics (MD) simulations were performed to study the interaction between silicon/silicon nitride films and hydrogen/helium plasmas. MD was used to investigate how the substrates modification is affected by the ion energy, the ion dose, the ion composition or the radical-to-ion flux ratio (in the case of a H2 plasma). In agreement with experiments, simulations of He+ or Hx+ (x=1-3) ion bombardment of Si/SiN show that a self-limited ion implantation takes place with a surface evolution composed of two stages: a rapid volume modification (with no etching) followed by a slow saturation and the formation of a stable He- or H- implanted layer at steady state. The mechanisms of ion-induced damage (Si-Si or Si-N bond breaking, He or H2 trapping/desorption, SiHx (x=1-3) complex creation) are investigated and allow to bring new insights to both the Smart Cut and Smart Etch technologies. Si/SiN exposure to various H2 plasma conditions (with both Hx+ ions and H radicals) was then studied. In this case, a self-limited transformation is observed but the H-modified layers are simultaneously etched during the ion implantation, at a rate ~10 times smaller for SiN compared to Si. Simulations show that to modify Si/SiN thin films with a nanoscale precision by H2 or He plasmas, both the ion energy and the ion flux have to be controlled very cautiously. In particular, low ion doses, where the substrate evolution is in rapid modification stage, must be avoided since the substrate evolution cannot be precisely controlled. In H2 plasmas, high ion energies induce thicker modified layers but smaller and less homogeneous hydrogenation rates. The ion composition and the radical-to-ion flux ratio Γ must be considered as well, since the etch rate increases with Γ, compromising even the possibility to achieve a Smart Etch of silicon. The MD simulations performed in this thesis enable to clarify various unexplained phenomena seen in the Smart-Etch experimentally, and reveal some possible issues in this new process. In the end, a range for plasma parameters is proposed to optimize this first step of the Smart Etch process and to control the modification of SiN with a sub-nanoscale precision.

AN ABSTRACT OF THE DISSERTATION OF Mohamed Zaneer Segu Mohideen, for the Doctor of Philosophy degree in Chemistry, presented on November 3 2008, at Southern Illinois University Carbondale. TITLE: TARGET MODIFICATION FOR ENHANCED PERFORMANCE MATRIX ASSISTED LASER DESORPTION IONIZATION (MALDI) MASS SPECTROMETRY MAJOR PROFESSOR: Dr. Gary R Kinsel MALDI MS, a powerful tool for the analysis of biomolecules, has undergone major advancement in instrumentation to yield improvements in robustness, sensitivity and throughput since its invention. Despite these developments in instrumentation, the performance of MALDI is in question when it comes to the analysis of complex protein/peptide mixtures. For these types of mixtures the performance of MALDI can be improved by either simplifying the sample complexity, modifying the sample preparation approach to increase the ionization efficiency of mixture components or seeking further enhancements to instrument performance. In this work these improvements are pursued through modifications to the MALDI target itself. In the MALDI analysis of high MW proteins a primary limitation is thought to be related to inefficient desorption of these compounds as proteins are expected to experience relatively stronger interaction with the MALDI target surface. This insight led to investigations of the use of various sublayers, deposited directly on the MALDI target, as a means to improve high molecular weight protein MALDI ion signals. In the first approach the protein / matrix mixture is applied on a laser desorbable polyaromatic hydrocarbon layer which serves as a barrier to protein surface binding interactions. These sublayers are also shown to be useful for on probe sample purification from salts that are known to interfere with MALDI performance. In the second approach the sublayer is formed from bovine serum albumin, a protein that is known to have strong binding affinity for surfaces and is also expected to form a barrier to protein surface binding interactions. Enhancements in MALDI performance and reductions in the limit of detection for proteins on these albumin precoated probes clearly demonstrate the influence of surface-protein interaction in the analysis of these species by MALDI MS. In further studies, methods to improve on-MALDI-target approaches to the simplification of sample complexity are investigated. These on-target separation approaches have been previously developed and shown to be successful for reducing sample complexity in the Kinsel Research Group. However, one significant limitation to this separation approach is the limited surface binding capacity of the MALDI probe. This limitation led to theoretical and experimental studies of methods to improve the surface protein binding capacity. Studies performed show that the surface binding capacity can be improved significantly through attachment of gold beads and through physical / chemical roughening of the target surface. Both approaches are shown to yield higher performance MALDI probes with lowered limits of detection for deposited / affinity captured proteins.

Artificial surfaces are increasingly more common in a number of sports including football, rugby and hockey. Each specific sport has mechanical properties designed to suit the requirements of the sport which can be achieved through appropriate selection of surface specification, as well as the appropriate selection of footwear. In player-surface interactions, traction is a key system property that needs to be measured for comfort, performance and any potential injury risk. Many of the current industry tests used to measure traction are simplistic and have limitations when used in tests. The aim of the thesis was to make a contribution to knowledge with regard to the mobilisation of traction and apply this to the understanding of shoe-surface interactions. This was achieved by completing a number of objectives. These included reviewing current knowledge of player-surface interaction behaviour in relation to traction and obtaining relevant human boundary conditions for biofidelic mechanical test development. The mechanisms of traction were then investigated and the variables in the mobilisation of traction identified. The traction forces developed were quantified with appropriate measurement systems. Mechanical test equipment was then developed along with protocols to replicate the translational and rotational lower limb behaviour during sport specific behaviour. This included the standard FIFA rotational device being modified to include two sensors which record continuous data throughout a trial to allow for more than a peak torque value to be analysed. In addition, a piece of equipment to measure translational traction was developed and constructed to support the rotational traction device and help to understand the mobilisation of traction. The device pulled a tray containing a surface sample, with a shoe/plate placed on the sample. The horizontal force was measured, as well as the amount of stud penetration into the surface. It was also necessary to characterise the state of the surface and the effects that any changes may have on the traction that is mobilised. Testing completed involved repeated testing on both the rotational and translational to allow for comparison. Changes in the surface properties were made such as the number of fibres in a set area and the rubber infill density as well as shoe properties such as stud spacing, stud type and number of studs. In the results, the initial stiffness response of the surface was often focussed on as it was stated that this may be a better indicator of the mechanisms involved in the traction mobilised by subjects, compared to peak torque. This is due to actual foot rotation measured in subject testing being observed to be much smaller than the rotation/distance required to produce the peak force. The larger angles/displacements were also considered to help inform the mechanisms of traction. The final objective was to refine the mechanisms based on the experimental design. This all adds to the contribution of knowledge regarding the mobilisation of traction. A key outcome from the thesis is the effect the surface and shoe properties have on traction, therefore it is essential to state the specification when reporting results otherwise comparisons are not able to be made. The mechanism of traction has not previously been fully understood, with this thesis beginning to understand the details of how the change in surface or shoe properties affect how the surface reacts during shoe-surface interactions.

In dieser Arbeit wird die Erzeugung permanenter Nanostrukturen durch den Beschuss mit langsamen (v < 5x105m/s) hochgeladenen (q < 40) Ionen auf den Oberflächen der Ionenkristalle CaF2 sowie KBr untersucht. Die systematische Analyse der Probenoberfläche mittels Raster-Kraft-Mikroskopie liefert detaillierte Informationen über den Einfluss von potentieller und kinetischer Projektilenergie auf den Prozess der Strukturerzeugung. Der individuelle Einfall hochgeladener Ionen auf der KBr(001)-Oberfläche kann die Erzeugung monoatomar tiefer, lochartiger Strukturen -Nanopits- mit einer lateralen Ausdehnung von wenigen 10nm initiieren. Das Volumen dieser Löcher und damit die Anzahl gesputterter Sekundärteilchen zeigt eine lineare Abhängigkeit von der potentiellen Energie der Projektile. Für das Einsetzen der Locherzeugung konnte ein von der Projektilgeschwindigkeit abhängiger Grenzwert der potentiellen Energie E_grenz^pot (Ekin) gefunden werden. Auf der Basis der defekt-induzierten Desorption durch Elektronen wurde unter Einbeziehung von Effekten der Defektagglomeration ein konsistentes mikroskopisches Modell für den Prozess der Locherzeugung konzipiert. Für die CaF2(111)-Oberfläche kann die aus jüngsten Studien bekannte, individuelle Erzeugung hügelartiger Nanostrukturen -Nanohillocks- durch hochgeladene Ionen in dieser Arbeit auch für kleinste kinetische Energien (E_kin < 150eVxq) verifiziert werden. Die potentielle Energie der einfallenden Ionen wird damit erstmalig zweifelsfrei als alleinige Ursache der Nanostrukturerzeugung identifiziert. Zudem zeigt sich bei geringer Projektilgeschwindigkeit eine Verschiebung der potentiellen Grenzenergie zur Hillock-Erzeugung. Im Rahmen einer Kooperation an der Technischen Universität Wien durchgeführte Simulationsrechnungen auf der Grundlage des inelastischen thermal spike-Modells zeigen, dass die individuelle Hillock-Erzeugung durch hochgeladene Ionen mit einer lokalen Schmelze des Ionenkristalls verknüpft werden kann. Dem essentiellen Einfluss der Elektronenemission während der Wechselwirkung des hochgeladenen Ions mit der Oberfläche auf den Prozess der Nanostrukturerzeugung wird in komplementären Untersuchungen zur Sekundärelektronenstatistik Rechnung getragen. Erstmalig werden dabei Gesamtelektronenausbeuten für Isolatoroberflächen bei kleinsten Projektilgeschwindigkeiten (v < 1x10^5 m/s) bestimmt. Für Geschwindigkeiten v < 5x10^4 m/s findet sich für die Isolatoroberfläche in starkem Kontrast zu Metallen ein signifikanter Abfall der Elektronenausbeute mit sinkender kinetischer Energie. Mögliche Ursachen dieses Effektes werden auf der Grundlage unterschiedlicher Modelle diskutiert.

Studying the interactions of solid particles and deformable gas<br />bubbles in viscous liquids is very important in many applications,<br />especially in mining and chemical industries. These interactions<br />involve liquid-solid-air multiphase flows and an<br />arbitrary-Lagrangian-Eulerican (ALE) approach is used for the direct<br />numerical simulations. In the system of rigid particles and<br />deformable gas bubbles suspended in viscous liquids, the<br />Navier-Stokes equations coupled with the equations of motion of the<br />particles and deformable bubbles are solved in a finite-element<br />framework. A moving, unstructured, triangular mesh tracks the<br />deformation of the bubble and free surface with adaptive refinement.<br />In this dissertation, we study four problems. In the first three<br />problems the flow is assumed to be axisymmetric and two dimensional<br />(2D) in the fourth problem.<br /><br />Firstly, we study the interaction between a rising deformable bubble<br />and a solid wall in highly viscous liquids. The mechanism of the<br />bubble deformation as it interacts with the wall is described in<br />terms of two nondimensional groups, namely the Morton number (Mo)<br />and Bond number (Bo). The film drainage process is also<br />considered. It is found that three modes of bubble-rigid wall<br />interaction exist as Bo changes at a moderate Mo.<br />The first mode prevails at small Bo where the bubble deformation<br />is small. For this mode, the bubble is<br /> hard to break up and will bounce back and eventually attach<br />to the rigid wall. In the second mode, the bubble may break up after<br />it collides with the rigid wall, which is determined by the film<br />drainage. In the third mode, which prevails at high Bo, the bubble<br />breaks up due to the bottom surface catches up the top surface<br />during the interaction.<br /><br />Secondly, we simulate the interaction between a rigid particle and a<br />free surface. In order to isolate the effects of viscous drag and<br />particle inertia, the gravitational force is neglected and the<br />particle gains its impact velocity by an external accelerating<br />force. The process of a rigid particle impacting a free surface and<br />then rebounding is simulated. Simplified theoretical models are<br />provided to illustrate the relationship between the particle<br />velocity and the time variation of film thickness between the<br />particle and free surface. Two film thicknesses are defined. The<br />first is the thickness achieved when the particle reaches its<br />highest position. The second is the thickness when the particle<br />falls to its lowest position. The smaller of these two thicknesses<br />is termed the minimum film thickness and its variation with the<br />impact velocity has been determined. We find that the interactions<br />between the free surface and rigid particle can be divided into<br />three regimes according to the trend of the first film thickness.<br />The three regimes are viscous regime, inertial regime and jetting<br />regime. In viscous regime, the first film thickness decreases as the<br />impact velocity increases. Then it rises slightly in the inertial<br />regime because the effect of liquid inertia becomes larger as the<br />impact velocity increases. Finally, the film thickness decreases<br />again due to Plateau-Rayleigh instability in the jetting regime.<br />We also find that the minimum film thickness corresponds to an<br />impact velocity on the demarcation point between the viscous and<br />inertial regimes. This fact is caused by the balance of viscous<br />drag, surface deformation and liquid inertia.<br /><br />Thirdly, we consider the interaction between a rigid particle and a<br />deformable bubble. Two typical cases are simulated: (1) Collision of<br />a rigid particle with a gas bubble in water in the absence of<br />gravity, and (2) Collision of a buoyancy-driven rising bubble with a<br />falling particle in highly viscous liquids. We also compare our<br />simulation results with available experimental data. Good agreement<br />is obtained for the force on the particle and the shape of the<br />bubble.<br /><br />Finally, we investigated the collisions of groups of bubbles and<br />particles in two dimensions. A preliminary example of the oblique<br />collision between a single particle and a single bubble is conducted<br />by giving the particle a constant acceleration. Then, to investigate<br />the possibility of particles attaching to bubbles, the interactions<br />between a group of 22 particles and rising bubbles are studied. Due<br />to the fluid motion, the particles involved in central collisions<br />with bubbles have higher possibilities to attach to the bubble. / Ph. D.

No description available.

This thesis presents a theoretical and experimental investigation of the interaction of electronically excited Rydberg hydrogen atoms with metal surfaces and the associated charge-transfer process. As a Rydberg atom approaches a metal surface, the energies of the Rydberg states are perturbed by the surface potential generated by the image charges of the Rydberg electron and core. At small atom-surface separations, the Rydberg atom may be ionised by resonant charge transfer of the Rydberg electron to the continuum of delocalised unoccupied metal states, with which the Rydberg electron is degenerate in energy. Typically, this ‘surface ionisation’ can be measured by extracting the remaining positively charged ion-cores with externally applied electric fields. By applying various levels of theory, from classical to fully time-dependent quantum calculations, this thesis explores various experimentally relevant effects on the charge-transfer process, such as the magnitude and direction of the externally applied electric field, the atom collisional velocity, the presence of local surface stray fields and electronically structured surfaces. The theoretical results give insight into the previous experimental work carried out for the xenon atom and hydrogen molecule, and point out some of the fundamental differences from the hydrogen atom system. Experiments involving Rydberg hydrogen atoms incident on an atomically flat gold surface, a rough machined aluminium surface and a single crystal copper (100) surface are presented, providing for the first time the opportunity to make a quantitative comparison of theory and experiments. The ability to control the critical distance at which charge-transfer occurs is demonstrated by using Rydberg states of varying dimensions and collisional velocities. By changing the collisional angle of the incident Rydberg beam, the effect of Rydberg trajectory is also investigated. By manipulating the polarisation of the Rydberg electron with electric fields, genuine control over the orientation of the electron density distribution in the charge-transfer process is demonstrated. This property was predicted by the theory and should be unique to the hydrogen atom due to its intrinsic symmetry. By reversing the direction of the electric field with respect to the metal surface, electrons rather than positive ions are detected, with ionisation dynamics that appear to be very different, as predicted by quantum calculations. Experiments involving the single crystal Cu(100) surface also suggests possible resonance effects from image states embedded in the projected bandgap which are shown from quantum calculations to play an important role in the surface charge transfer of electronically structured metal substrates. The experimental technique developed in this work provides some exciting future applications to study quantum confinement effects with thin films, nanoparticles and other bandgap surfaces. The ability to control the Rydberg orbital size, electronic energy, collisional velocity and orientation in the charge-transfer process will provide novel ways of probing the surface’s electronic and physical structure, as well as being a valuable feature in offering new opportunities for controlling reactive processes at metallic surfaces.

abstract: Protein-surface interactions, no matter structured or unstructured, are important in both biological and man-made systems. Unstructured interactions are more difficult to study with conventional techniques due to the lack of a specific binding structure. In this dissertation, a novel approach is employed to study the unstructured interactions between proteins and heterogonous surfaces, by looking at a large number of different binding partners at surfaces and using the binding information to understand the chemistry of binding. In this regard, surface-bound peptide arrays are used as a model for the study. Specifically, in Chapter 2, the effects of charge, hydrophobicity and length of surface-bound peptides on binding affinity for specific globular proteins (&beta-galactosidase and &alpha1-antitrypsin) and relative binding of different proteins were examined with LC Sciences peptide array platform. While the general charge and hydrophobicity of the peptides are certainly important, more surprising is that &beta-galactosidase affinity for the surface does not simply increase with the length of the peptide. Another interesting observation that leads to the next part of the study is that even very short surface-bound peptides can have both strong and selective interactions with proteins. Hence, in Chapter 3, selected tetrapeptide sequences with known binding characteristics to &beta-galactosidase are used as building blocks to create longer sequences to see if the binding function can be added together. The conclusion is that while adding two component sequences together can either greatly increase or decrease overall binding and specificity, the contribution to the binding affinity and specificity of the individual binding components is strongly dependent on their position in the peptide. Finally, in Chapter 4, another array platform is utilized to overcome the limitations associated with LC Sciences. It is found that effects of peptide sequence properties on IgG binding with HealthTell array are quiet similar to what was observed with &beta-galactosidase on LC Science array surface. In summary, the approach presented in this dissertation can provide binding information for both structured and unstructured interactions taking place at complex surfaces and has the potential to help develop surfaces covered with specific short peptide sequences with relatively specific protein interaction profiles. / Dissertation/Thesis / Doctoral Dissertation Biochemistry 2014

When approaching a metal surface, the electronic structure of Rydberg atoms or molecules is perturbed by the surface potential and at close enough distances resonant ionisation of the Rydberg electron into the conduction band of the surface can occur. It is possible to interfere in this process and steer the ionisation distance by making use of the polarisability of the Rydberg orbital in the presence of electric fields. The resulting ions from the surface can extracted via electric fields and subsequently detected via well established ion detection schemes. The question of how this charge-transfer process is affected by different properties of the surface (both electronic and structural) represents the main aspect of the work presented in this thesis. At first, the charge transfer of atomic hydrogen Rydberg atoms with a flat gold metal surface is investigated. While such a surface might appear homogeneous, stray fields are present in its vicinity due to local variations in the surface work function. The surface ionisation process as a function of applied electric field is therefore measured experimentally and the results are compared with classical Monte-Carlo simulations (which include stray field effects). This way the possibility to utilize Rydberg states as a probe of the magnitude of such stray fields is demonstrated. To investigate the effect the surface structure can have on the ionisation process, the interaction of Rydberg atoms with surfaces covered by nanoparticles is investigated. Surface ionisation is measured at a 5 nm nanoparticle monolayer surface and it is shown that population transfer between surface- and vacuum-oriented Rydberg states occurs. In addition, results are presented, which suggest a dependence of the ionisation process on the relative size of Rydberg orbital and nanoparticle. Furthermore, charge transfer between a Rydberg state and discrete electronic states at the surface vacuum interface are investigated by performing experiments with a Cu(100) band-gap semiconductor surface. By analysing surface ionisation as a function of collisional velocity ionisation rates can be determined and are subsequently compared with theoretical predictions. The potential of identifying resonant ionisation is thereby demonstrated. Last, a new method to produce 2s atomic hydrogen via mixing of the 2s and 2p state in an electric field is proposed and first experimental results are presented, thus demonstrating viability of the idea. The experiments presented in this thesis represent the most in depth analysis of the charge-transfer process between atomic hydrogen Rydberg states and a range of different surfaces to date. As such, they demonstrate the potential of utilizing the unique properties of Rydberg states and their applicability as surface probes. In addition, these results pave the way for further experiments involving thin films or the phenomenon of quantum reflectivity.