Thesis (M.Sc.)--Memorial University of Newfoundland, 2005. / Bibliography: leaves 81-83.

>Magister Scientiae - MSc / The study of neutral atomic hydrogen (HI) plays a vital role in improving the knowledge about galaxy evolution since HI serves as the raw fuel for star formation. However, the current knowledge about it is limited to the nearby Universe due to its intrinsic faintness and the inadequate sensitivities of the current radio telescopes.

Novel two dimensional nanoscale materials like graphene and metal dichalcogenides (MX2) have attracted the attention of the scientific community, due to their rich physics and wide range of potential applications. It has been shown that novel graphene based transparent conductors and radiofrequency transistors are competitive with the existing technologies. Graphene’s properties are influenced sensitively by adsorbates and substrates. As such not surprisingly, physical properties of graphene are found to have a large variability, which cannot be controlled at the synthesis level, reducing the utility of graphene. As a part of my doctorate dissertation, I have developed atomic hydrogen as a novel technique to count the scatterers responsible for limiting the carrier mobility of graphene field effect transistors on silicon oxide (SiO2) and identified that charged impurities to be the most dominant scatterer. This result enables systematic reduction of the detrimental variability in device performance of graphene. Such sensitivity to substrates also gives an opportunity for engineering device properties of graphene using substrate interaction and atomic scale vacancies. Stacking graphene on hexagonal boron-nitride (h-BN) gives rise to nanoscale periodic potential, which influences its electronic graphene. Using state-of-the-art atomic-resolution scanning probe microscope, I correlated the observed transport properties to the substrate induced extrinsic potentials. Finally in efforts to exploit graphene’s sensitivity to discover new sensor technologies, I have explored noncovalent functionalization of graphene using peptides. Molybdenum disulfide (MoS2) exhibits thickness dependent bandgap. Transistors fabricated from single layer MoS2 have shown a high on/off ratio. It is expected that ad-atom engineering can be used to induce on demand a metal-semiconductor transition in MoS2. In this direction, I have iii explored controlled/reversible fluorination and hydrogenation of monolayer MoS2 to potentially derive a full range of integrated circuit technology. The in-depth characterization of the samples is carried out by Raman/photoluminescence spectroscopy and scanning tunneling microscopy

Although the collisions of electrons and atomic hydrogen has been studied for several decades, there is still neither a complete database of scattering data, nor a universal method that would let generate such data. For astronomical and other purposes the cross sections of electron-hydrogen collisions are necessary, in a broad range of energies - from tenths of electronvolt to millions of electronvolts. In this work the author concentrates on several established approaches to electron-atom scattering and confronts results of his own implementations of these methods against the published data and results of freely available computer codes. A special attention is given to the overlaps of different methods, so that in the end a database with easy user interface can be offered for common practical usage of scattering data in applied fields.

Since 1982, the Si (111)-(7x7) surface has been extensively studied both theoretically and experimentally with the modern powerful tools of STM and Scanning Tunneling Spectroscopy (STS). In this work, a simple atomic orbital model for the Si (111)-(7x7) surface is developed to simulate the experimental results of STM and STS. Based on Tersoff-Hamann’s theory for the tunneling current, simulations of clean Si (111)-(7x7) constant-current images are presented. The direct, real-space simulated topographic images of the surface are compared to experimental results qualitatively and quantitatively. The simulation of spectroscopic imaging and normalized conductance spectra are also included. The adsorption of atomic hydrogen atoms onto the Si (111)-(7x7) surface is also simulated. / xiv, 146 leaves ; 29 cm.

Thesis (Ph. D.)--Missouri University of Science and Technology, 2010. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed April 21, 2010) Includes bibliographical references (p. 82-87).

The reactions of atomic hydrogen with methanethiol and that of atomic hydrogen with carbon disulfide were studied experimentally using flash-photolysis resonance-fluorescence techniques. Rate constants were determined over a range of temperatures and pressures, and through analysis and comparison to theoretical work details of the reactions were ascertained.

Jakub Benda Astrophysically important processes in collisions of electrons with hydrogen atoms This thesis focuses on calculations of the cross sections and other scattering quantities that characterize the outcome of collisions of electrons with hydro- gen atoms. For the chosen energy range and atomic transitions the scattering process is solved within the non-relativistic quantum mechanics by discretiz- ation of the Schr¨odinger equation in the basis of B-splines, which transforms the equation into a linear-algebraic problem. The thesis discusses the boundary conditions, methods of solution of the linear system, preconditioning of the sys- tem and interpretation of results, including several original ideas that proved to be very beneficient for the calculations. The calculated data are provided by means of graphs at the end of the thesis. Also, a custom web-based scattering database containing the results has been set up, freely available to the expected audience of this project. 1

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.

En Astrophysique, la compréhension du processus de formation d'étoiles reste l'une des principales questions. Elle est directement reliée à l'évolution du gaz interstellaire dans les galaxies, et en particulier aux processus de refroidissement et de condensation pour lesquels la turbulence et l'instabilité thermique jouent un rôle dominant. Ce travail se concentre sur l'évolution du gaz atomique et diffus qui fournit les conditions initiales à la formation des nuages moléculaires et se base sur une comparaison étroite entre observations à 21 cm et simulations numériques hydrodynamiques. Pour comprendre les rôles de l'instabilité thermique et de la turbulence dans la transition du gaz chaud (WNM, T ~ 8000 K, n = 0.5 cm-³) vers le gaz froid (CNM, T ~ 80 K, n = 50 cm-³), j'ai produit 90 simulations à basse résolution qui ont permis d'étudier l'influence de la densité initiale du WNM et de la compressibilité du forçage de la turbulence sur l'efficacité de la production de CNM. Un résultat important permet de conclure que le gaz chaud, dans les conditions de turbulence caractéristiques de ce qui est observé, ne transite pas vers le gaz froid quelque soit l'amplitude de la turbulence. Ces simulations à basse résolution ont aussi permis de déterminer quelles conditions initiales permettent de reproduire les propriétés déduites des observations telles que le nombre de Mach, la quantité de CNM en masse ou la dispersion de vitesse turbulente. Un processus de compression, que l'on peut reproduire soit en augmentant la densité initiale du WNM (n ≥ 1.5 cm-³) soit en appliquant un champ de forçage compressif, est nécessaire. Ces conditions initiales ont ensuite été utilisées pour produire deux simulations à haute résolution (1024³) pour lesquelles j'ai montré que les propriétés de la turbulence et de l'instabilité du milieu atomique neutre sont bien reproduites. Les histogrammes de température portent en effet la trace d'un milieu biphasique et les distributions de pression sont semblables aux observations. D'autre part, les spectres de puissance de la densité sont caractéristiques d'un milieu fortement contrasté alors que ceux de la vitesse restent caractéristiques d'une turbulence subsonique. Finalement, les structures froides de ces deux simulations reproduisent les relations masse-échelle et dispersion de vitesse-échelle observées dans les nuages moléculaires, suggérant que la structure des nuages moléculaires pourrait être héritée de celle des nuages de HI à partir desquels ils se sont formés. Le dernier aspect de mon travail est relié à la difficulté rencontrée lors de l'interprétation des données qui n'est possible qu'à partir de grandeurs projetées en deux dimensions. J'ai donc comparé en détails les deux simulations à haute résolution à des observations de cirrus en créant des observations artificielles à 21 cm. Les spectres d'émission et les cartes de densité de colonne ainsi produits sont semblables aux observations. De plus, les simulations donnant accès à l'information en trois dimensions, j'ai étudié les effets de l'auto-absorption dans la création de cartes de densité de colonne à partir de spectres de température de brillance. J'ai conclu de cette étude que l'auto-absorption ne peut être négligée mais qu'elle ne concerne que les lignes de visée les plus brillantes et les plus denses et que la correction habituellement appliquée sur les observations est efficace. Finalement, j'ai appliqué une méthode de décomposition en gaussiennes sur les spectres synthétiques. Cette méthode a pour objectif d'étudier les propriétés de chacune des deux phases thermiques du HI. Les résultats montrent qu'elle est prometteuse pour l'analyse des données de spectro-imagerie à 21 cm, bien que nécessitant des améliorations. Elle permet en effet de bien séparer les phases chaude et froide du milieu atomique et d'en déduire la distribution massique de chacune d'elles. / One of the main current questions in Astrophysics is the understanding of the star formation process, directly related to the processes involved in the cooling and the condensation of the gas yielding to intricate filamentary structures of molecular clouds. Thermal instability and turbulence are playing dominant roles in this complex dynamics. The work presented here is focused on the evolution of the atomic and diffuse interstellar medium that provides the initial conditions to the formation of molecular clouds and is based on the comparison of hydrodynamical numerical simulations and observations. To understand the roles of thermal instability and turbulence in the WNM (warm neutral medium, T ~ 8000 K, n = 0.5 cm-³) to CNM (cold neutral medium, T ~ 80 K, n = 50 cm-³) transition, I produced 90 hydrodynamical numerical simulations of thermally bistable HI and used them to study the impact of the WNM initial density and the compressibility of the turbulent stirring on the efficiency of the CNM production. The main result here is that the warm gas in the observed turbulent conditions do not transit naturally to cold gas whatever the amplitude of turbulent motions. These small resolution simulations also allowed me to determine which initial conditions lead to the reproduction of the observed properties, as the Mach number, the amount of CNM or the amplitude of the turbulent motions. A compression is needed to trigger this transition either by increasing the initial density (n ≥ 1.5 cm-³) or by stirring with a compressive field. These initial conditions have been used to produce two high resolution simulations (1024³). I showed that these two simulations reproduce well the properties of the turbulence and the thermal instability. The temperature histograms present the evidences of a bistable gas and the pressure distributions are in agreement with the observations. On the other hand, the power spectra of the density are characteristic of a high contrasted medium while the power spectra of the velocity remain characteristic of subsonic turbulence. Finally the cold structures of these two simulations reproduce well the mass-size and velocity dispersion-size relations observed in molecular clouds. This suggests that the molecular cloud structure could be inherited from the clouds of atomic gas from which they are born. One of the main limitations in the analysis of observations comes from the fact that it can only be done on integrated quantities in two dimensions. In the last part of my work I compared the two high resolution simulations to observations by creating synthetic 21 cm observations. The emission spectra and column density maps produced in that way are similar to the ones observed. Besides, with the three dimensional informations, I was able to study the effect of the self-absorption in the creation of the column density maps from the brightness temperature spectra. I concluded from this study that the self-absorption cannot be neglected but that it only concerns the brightest and densest lines of sight and that the correction usually applied on observations is efficient. Finally I applied a method of gaussian decomposition on the synthetic spectra. This method has been build to study the properties of each thermal phase in the HI. The results show that it is a highly promising method for the analysis of 21 cm spectro-imaging data even if some improvements are needed. Indeed, it allows a good separation of the cold and warm phases of the atomic medium and a reasonable deduction of the massive distribution of each one.