As semiconductor technology continues to miniaturize, surface phenomenon become prevalent. Clusters can be considered as model compounds of the surface in free space. By performing experiments on clusters, it is equivalent to focussing on local sites of the bulk surface, and building it one atom at a time. The development of supersonic beams of clusters has proved to be one of the most powerful methods in preparing the "nano" state, the intermediate range between atomic and the macroscopic description of matter. Fourier Transform - Ion Cylcotron Resonance (FT-ICR), a mass spectrometry technique, coupled with these cluster beams was used to study gallium arsenide, a well-utilized semiconductor. The complexity of isomers and the stoichiometric distribution within a given cluster size has been simplified using mathematical methods for signal deconvolution. By studying reaction parameters with ammonia, gallium arsenide clusters proved to be an adequate comparative model for real surfaces. Furthermore, local density theoretical calculations, which are feasible on these small clusters, provided a corroborative and predictive proving ground for this system.

Field emission from carbon nanotubes has been measured. Typical values are 100-1000 nA at a nanotube bias of $-$80V. The amount of field emission is found to depend strongly on the state of the nanotube tip. This should serve in future work as a good indicator of tip condition. It is believed that open tip nanotubes unravel at their ends while field emitting to form a carbon chain. An atomic chain being the ultimate geometry for a field emitter, nanotube tips may have applications for monoenergetic electron sources, electron microscope field emitters, and electron holography.

Negative ions formed in the reaction A** + BC (--->) A('+) + BC('-) where A** is a highly-excited Rydberg atom and BC is an electron-attaching molecule exhibit very long lifetimes against autodetachment. This has led to speculation that these negative ions may be stabilized by a post-attachment interaction with the Rydberg atom ion core. In such a stabilization process the Rydberg ion core would carry away some of the vibrational energy of the negative ion. To investigate this possibility the kinematics of Xe(nf) - SF(,6) collisions have been studied and the data compared to the results of a Monte Carlo calculation. These comparisons indicate that few electron transfer reactions lead to significant energy transfer to the Rydberg ion core, implying that the observed long lifetime of the SF(,6)('-) ion is a consequence of the very low initial translational energy of the Rydberg electron, rather than a post-attachment interaction.

Low energy collisions between Rydberg atoms and neutral molecules have been investigated over a wide range of principal quantum numbers n, and for several different neutral targets. The results have been used to validate the free-electron, independent particle model of Rydberg atom collisions. Comparison between theory and experiment show that at large values of n, ionization of Rb(nS,nD) Rydberg atoms in the reaction: (UNFORMATTED TABLE FOLLOWS) Rb(nS,nD) + SF(,6) (--->) Rb('+) = SF(,6)('-) (1)(TABLE ENDS) proceeds by electron transfer from the Rydberg atom to the SF(,6) molecule. The rate constants measured for this reaction are much the same as for the attachment of free, low-energy electrons to SF(,6). Thus, Rydberg collision studies can provide information about low-energy free electron interactions. Studies of the rate constants for free ion production in the reaction: (UNFORMATTED TABLE FOLLOWS) K(nD) + SF(,6) (--->) K('+) + SF(,6)('-) (2)(TABLE ENDS) showed these to decrease sharply at smaller n, falling far below the value expected on the basis of Rydberg electron attachment to SF(,6). This behavior is attributed not to breakdown of the free-electron model, but to post-attachment electrostatic interactions between the product ions, which are formed closer to each other at lower n. Model calculations that take this electrostatic interaction into account confirm this prediction. Other Rydberg atom collision processes, such as: (UNFORMATTED TABLE FOLLOWS) K(nD) + O(,2) (--->) K('+) + O(,2)('-) (3) K(nD) + H(,2)O (--->) KH(,2)O('+) + e('-) (4)(TABLE ENDS) have been studied, as they require both the Rydberg ion core and electron to participate in the collision. Since O(,2)('-) ions formed by free electron attachment have short lifetimes against autodetachment, the observation of long-lived O(,2)('-) reaction product suggests that the K('+) core ion plays a role in stabilizing the excited O(,2)('-) ions formed by Rydberg electron attachment. Stable KH(,2)O('+) ions cannot be formed in two-body K('+) - H(,2)O interactions. The detection of long-lived KH(,2)O('+) ions thus demonstrates that the Rydberg electron can play an important role in collision processes involving the core ion by serving as a third body to carry off excess energy.

We have used quasi-elastic neutron scattering techniques to study the dynamics of solvent water protons in H$\sb 2$O solutions of deuterated poly(ethylene oxide) (Polyox) at room temperature. Using a slightly modified technique, we also have obtained values for the mean-square vibrational amplitude of polymer protons in powder and D$\sb 2$O solutions of Polyox and trypsin at temperatures between 300K and 75K. To study the dynamics of solvent water in deuterated Polyox (dPolyox) solutions, we used a high energy-resolution spectrometer ($\Gamma\sb{\rm res}\approx$ 100$\mu$eV) so that we could measure the width of the quasi-elastic line for various values of the momentum transfer, Q. The Q-dependence of the quasi-elastic linewidth shows that the diffusive properties of water in these Polyox/water solutions is slightly affected at low (5 polymer wt%) and intermediate (10-20 wt%) polymer concentrations. At high polymer concentration (38 wt%), the water clearly has a reduced diffusion coefficient and an increased residence time compared to bulk water and this may be attributed to the nature of the interaction of the water molecules with the polymer. We were motivated to study the mean-square vibrational amplitude of protons in powder and D$\sb 2$O solutions of Polyox and trypsin because of recent computer simulations on biological macromolecules. We measured the vibrational mean-square amplitude by using a low energy-resolution spectrometer ($\Gamma\sb{\rm res}\approx$ 700$\mu$eV) to measure the intensity of the quasi-elastic peak in a relatively short time compared to standard high energy-resolution techniques. Our results on these 20 wt% solutions at 300K are consistent with conventional quasi-elastic neutron scattering results on the Polyox and with computer simulations of trypsin in solution. The vibrational amplitude was proportional to temperature throughout the temperature range studied, and the proportionality constant is interpreted in terms of an effective spring constant for the motion of the hydrogens. These spring constants are smaller than those for some common chemical bonds indicating that there are significant contributions to the motion of the hydrogens besides vibrational motion.

Two experimental approaches to dissociative resonance Raman spectroscopy, Raman emission spectra and Raman excitation profiles, have been applied to investigate the photodissociation dynamics of ozone in the Hartley and Huggins absorption bands. Resonance Raman spectra of ozone at 266 and 270 nm excitation wavelengths have been measured up to 11000 cm$\sp{-1}$ with improved accuracy. Several bands beyond the dissociation limit have been observed. All prominent bands observed are well fit by an analytical Darling-Dennison model. A new analytical two-dimensional potential energy surface for the electronic ground state of ozone has been constructed based on the new experimental data. Resonance Raman spectra with excitation in the Huggins band were also measured. The symmetry issue of the electronic state responsible for Huggins band is addressed. A novel continuously-scanned Raman excitation profile technique has been applied to the red wing (276-297 nm) of the Hartley band. Several Raman excitation profiles for the lowest three vibrational bands have been determined. These spectra show characteristic oscillatory structures. We also report the discovery of the chemiluminescence of O$\sb2$ and OH following the photodissociation of ozone.

A nanofluidic slit embedded with a lattice of square pits was used to entropically trap polymers. DNA in the system was confined to two dimensions and underwent self-assembly into discrete conformational states based on the number of occupied pits. The molecules diffused by undergoing transitions to higher or lower pit-occupancy states and relaxing to their equilibrium state. A statistical mechanical model was used to predict the mean occupancy state as a function of various geometric parameters. Experiments confirm many of the predictions of the model. Regions of parameter space over which a single state dominates were observed, indicating that entropic trapping can be used to create stable self-assembled single polymer conformations. Measurements of diffusion showed it to be geometry dependent, allowing a fine-tuning of diffusivity. The diffusion showed non-monotonic behaviour: local minima corresponding the stable equilibrium states were observed. This demonstrates that the diffusion can be fine-tuned to a local resonance using entropic trapping. The results show that polymers can self-assemble into entropically stable structures, with implications for nanotechnology and biophysics. / Nous avons piégé des polymères avec un trellis de fosses nanofluidiques de forme carrée. Nous avons confiné l'ADN en deux dimensions. Il s'est auto-assemblé en états discrets et qui determine du nombre du fosses. Les molecules one sauté et sont tombe es dans un nouvel état, et diffusaient. Nous avons utilisé un modèle mécanique statistique pour une prédire le nombre moyen de fosses. Les expériences confirment le modèle. Nous avons vu des plateaux avec un unique état dominant. Cela signifie que nous pouvons utiliser le piège entropique pour la nanotechnologie : car les structures sont stables. Nous avons mesuré la diffusion. Elle dépend de la géométrie et est non monotonique. Elle a un minimum local vers la region des plateaux. Nous pouvons utiliser le piège entropique pour contrôler la diffusion. Les résultats sont utiles pour la nanotechnologie et biophysique.

In this thesis, we report theoretical investigations ofnonlinear and nonequilibrium quantum electronic transport propertiesof molecular transport junctions from atomistic first principles.The aim is to seek not only qualitative but alsoquantitative understanding of the correspondingexperimental data. At present, the challenges to quantitativetheoretical work in molecular electronics include two most importantquestions: (i) what is the proper atomic model for the experimentaldevices? (ii) how to accurately determine quantum transportproperties without any phenomenological parameters? Our research iscentered on these questions. We have systematically calculatedatomic structures of the molecular transport junctions by performingtotal energy structural relaxation using density functional theory(DFT). Our quantum transport calculations were carried out byimplementing DFT within the framework of Keldysh non-equilibriumGreen's functions (NEGF). The calculated data are directly comparedwith the corresponding experimental measurements. Our generalconclusion is that quantitative comparison with experimental datacan be made if the device contacts are correctly determined.We calculated properties of nonequilibrium spin injection from Nicontacts to octane-thiolate films which form a molecular spintronicsystem. The first principles results allow us to establish a clearphysical picture of how spins are injected from the Ni contactsthrough the Ni-molecule linkage to the molecule, why tunnelmagnetoresistance is rapidly reduced by the applied bias in anasymmetric manner, and to what extent ab initio transporttheory can make quantitative comparisons to the correspondingexperimental data. We found that extremely careful sampling of thetwo-dimensional Brillouin zone of the Ni surface is crucial foraccurate results in such a spintronic system.We investigated the role of contact formation and its resultingstructures to quantum transport in several molecular wires and showthat interface contacts critically control charge conduction. It wasfound, for Au/BDT/Au junctions, the H atom in -SH groupsenergetically prefers to be non-dissociative after the contactformation, which was supported by comparison between computed andmeasured break-down forces and bonding energies. TheH-non-dissociated (HND) junctions give equilibrium conductances from0.054G0 (equilibrium structure) to 0.020G0 (stretchedstructure) which is within a factor of 2-5 of the measureddata. On the other hand, for all H-dissociated contact structures - whichwere the assumed structures in the literature, the conductance is atleast more than an order of magnitude larger that the experimentalvalue. The HND-model significantly narrows down thetheory/experiment discrepancy. Finally, a by-product of this work isa comprehensive pseudopotential and atomic orbital basis setdatabase that has been carefully calibrated and can be used by theDFT community at large. / Cette thèse présentera nos recherches théoriquessur les propriétés quantiques de transport électroniquedes jonctions de transport moléculaire. Cette analyse a été effectuéeà l'aide de méthodes ab initio atomiques qui sont validesdans les régimes non-linéaire et hors-équilibre.L'objectif est de rechercher non seulement une compréhensionqualitative des données expérimentales mais aussi quantitative. Les deux questions les plus importantesquant au travail théorique en électronique moléculaire sont:(i) quel est le bon modèle atomique pour simuler les dispositifsexpérimentaux? (ii) comment déterminer avec précisionles propriétés de transport quantique sans l'utilisation deparamètres phénoménologiques?Nos recherches sont centrées sur ces questions. Nous avonssystématiquement calculéles structures atomiques de jonctions moléculaires en effectuantla relaxation structurelle dans le cadre de la théorie de la fonctionnelle de la densité (DFT).Les calculs de transport quantique ont été reàlises encombinant la DFT avec les fonctions de Green hors-équilibre deKeldysh (NEGF).Les calculs sont directement comparésaux données expérimentales correspondantes. Notre conclusiongénérale est qu'un accord quantitatif entre les valeurs théoriqueset empiriques est possible si la structure atomique du contact estcorrectement déterminée.Nous avons calculé les propriétés hors-équilibred'injection de spin à travers un film d'octane-thiole en contactavec des électrodes en Ni, formant ainsi un systèmespintronique moléculaire.Les résultats obtenus par premiers principes nous fournissentune compréhension claire sur la façon dont les spins sontinjectés à partir des électrodes en Ni à la molécule par la liaisonNi-molécule. De plus, nous expliquonspourquoi la magnéto-résistance à effet tunnel décroîtrapidement avecune augmentation du potentiel électrique, et ce, de manière asymétrique.Finalement, nous démontrons que la théorie ab initiodu transport électronique est en mesure d'effectuer des comparaisonsquantitatives avec les données expérimentales.Nous avons constaté qu'un échantillonnage minutieux de la zonede Brillouin 2D de la surface du Ni est crucial afin d'obtenir desrésultats précis dans un tel système spintronique.Nous avons étudié le rôle de la formation du contact,ainsi que la structure atomique associée sur l'influence dutransport quantique dans le cas de plusieurs jonctions moléculaires.Nous démontrons que l'interface reliant les électrodes aux moléculescontrôle très sensiblement la conduction de charge.Il a été trouvé, pour les jonctions Au/BDT/Au, quel'atome d'hydrogène dans les groupes -SHpréfère énergétiquement la non-dissociation après la formationdu contact. En effet, ceci a été corroboré par la comparaison entreles donnéees calculées et mesurées des forces de rupture et desénergies de liaison.Les jonctions avec l'hydrogène non-dissocié (HND) donnentdes valeurs de conductances à l'équilibre de 0.054G$_0$ (structure d'équilibre) à0.020G$_0$ (structure étirée). Ces valeurs sontà l'intérieur d'un facteur de 2-5 aux données expérimentales actuelles.D'autre part, toutes les structures de contact H-dissociées --- quiont été les structures supposées dans la littérature --- résultenten des valeurs de conductancescalculées au moins un ordre de grandeur plus élevé que lesvaleurs empiriques.Le modèle HND réduit de manière significative l'écart entrela théorie et les expériences. Pour terminer, une conséquence de ce travail estle regroupement d'une base de données complète incluant des pseudo-potentielset des orbitales atomiques. Celle-ci a été soigneusement calibrée et est disponibleà toute la communauté DFT.

Experiments involving DNA or other charged polymers have shown that the presence of counter-ions induces an attraction between the like-charged chains. Previous simulations to study this like-charged attraction modeled the polymers as rigid cylindrical surfaces with uniformly distributed charge. We performed Brownian Dynamics simulations to examine the potential of mean force between two infinite like-charged chains of discrete particles when immersed in solution with ions of opposite charge. / We found that the attraction was reproduced, and that its general features are very similar to those found using the line-charges. However, our force curves also show several small discrete jumps, which may be due to a competition between the counter-ions's desired positions and the ones imposed by the monomer spacing. / We also varied the counter-ion/polymer charge ratio, the counter-ion sizes, as well as the temperature of the simulations. The attractive force disappears above a threshold temperature of kBTc ≈ 400K. / The strength of the attractive force decreases linearly with temperature, demonstrating that the like-charged attraction is due to the ground-state. Simulated annealing revealed several low-energy configurations close to the ground state. Analytical calculations using the sample configurations yielded attractive forces. / Finally, we studied the stability of the chains against sinusoidal perturbations. We obtained the growth rate as a function of the distance R between the chains. This was done for several perturbation wavelengths, and the results were compiled into a buckling diagram, which predicts several regions of (λ,R)-space where the polymers will buckle. / Des expériences avec l'ADN et d'autres polymères chargés ont prouvé que la présence des contre-ions induit une attraction entre les chaînes de même charge. Des simulations précédentes étudiant cette attraction de même charge ont modelé les polymères comme surfaces cylindrique rigides ayant une charge uniformément distribuée. Nous avons donc decidé d'effectuér des simulations Brownian pour examiner le potentiel de force moyenne entre deux chaînes comme-chargées infinies du particules discrètes, une fois immergées en solution avec des ions de charge opposée. / Nous avons constaté que l'attraction a été reproduite, et que ses caractéristiques generales sont très semblables à ceux trouvés en utilisant les charges en ligne. Cependant, nos courbes de force montrent également plusieurs petits sauts discrets, qui peuvent être dus à un opposition entre les endroits désiré par les contre-ions et celles imposées par l'espacement du monomère. / Nous avons également varié la proportion de charge contre-ion/polymère, les tailles de contre-ion, aussi bien que la température des simulations. La force d'attarction disparaît au-dessus d'une température critique de kBTc ≈ 400K. / La puissance de la force d'attraction diminue linéairement avec la température, démontrant que l'attraction des charges identiques est due à l'état fondamental. Le recuit simulé a montré plusieurs configurations à énergie réduite près de l'état fondamental. Les calculs analytiques utilisant les configurations d'échantillonnées ont rapporté les forces d'attraction. / Pour finir, nous avons étudié la stabilité des chaînes contre des perturbations sinusodales. Nous avons obtenu le taux de croissance en fonction de la distance R entre les chaînes. Ceci a été fait pour plusieurs longueurs d'onde de perturbation, et les résultats ont été compilés dans un diagramme de boucle, qui prévoit plusieurs régions de (λ,R)-space o les polymères plieront.

The term polymer brush refers to the class of structures formed by polymers when attached to a surface at sufficiently high surface density so as to cause the polymers to stretch away from the surface to avoid each other. Considering specifically brushes formed by flexible, linear homopolymers attached by a single end to an impenetrable planar surface, this thesis investigates the stability of the laterally homogeneous phase of the brush in response to fluctuations of polymer density. The controlling factors studied are: the energetic interactions of the polymers with the bath of solvent particles in which the brush is immersed; the excluded volume of the polymers, which results from the mutual repulsion that exists between any two polymer segments; and the polymeric content in the brush, controlled by the grafting density and the length of the polymers. It is found that the stability of the laterally homogeneous phase of the brush is enhanced by increasing the polymer excluded volume, the polymeric content in the brush, or both. When incompressibility is modeled at the mean field level, it is found that there exists a region of parameter space in which the laterally homogeneous phase is stable for all values of the polymer-solvent interaction.