Recent experimental work has demonstrated production of quasi free-standing graphene by methane intercalation. The intercalation weakens the coupling of adjacent graphene layers and yields Dirac fermion behaviour of monolayer graphene. We have investigated the electronic characteristics of methane intercepted graphene bilayer under a perpendicularly applied electric field. Evolution of the band structure of intercalated graphene as a function of the bias is studied by means of density-functional theory including interlayer van der Waals interactions. The implications of controllable band gap opening in methane-intercalated graphene for future device applications are discussed. Noncovalent functionalization provides an effective way to modulate the electronic properties of graphene. Recent experimental work has demonstrated that hybrids of dipolar phototransductive molecules tethered to graphene are reversibly tunable in doping. We have studied the electronic structure characteristics of chromophore/graphene hybrids using dispersion-corrected density functional theory. The Dirac point of noncovalently functionalized graphene shifts upward via cis-trans isomerism, which is attributed to a change in the chromophore's dipole moment. Our calculation results reveal that the experimentally observed reversible doping of graphene is attributed to the change in charge transfer between the light-switchable chromophore and graphene via isomerization. Furthermore, we show that by varying the electric field perpendicular to the supramolecular functionalized graphene, additional tailoring of graphene doping can be accomplished.

Alzheimer’s disease (AD), the most prevalent neurodegenerative disease, is characterized in part by disruptions in axonal transport. Axonal transport is a process by which motor proteins carry organelles and other cargo made in the neuronal cell body along microtubule tracks to distal regions of the axon. The microtubule-associated protein (MAP) Tau plays a crucial role in regulating axonal transport, and is implicated in the development of AD and other types of dementia collectively known as Tauopathies. Tau is a neuronal-specific MAP that has six isoforms alternatively spliced from a single gene. These isoforms differ by the presence of zero, one, or two N-terminal acidic inserts and three or four C-terminal microtubule binding repeats. Tau is also known to be an intrinsically disordered protein that undergoes a dynamic equilibrium between static and diffusive states on the microtubule surface. The dynamics of Tau are important in the regulation of motor protein mediated axonal transport in neurons. Isoform-specific differences in the dynamic behavior of Tau on the microtubule surface, however, are not yet fully understood. Diffusive Tau is thought to be stabilized by electrostatic interactions between its N- and C-termini while static Tau is proposed to be extended with its C-terminal repeats contacting the microtubule and the N-terminus projected away from the microtubule surface. Thus, the N-terminal inserts may help regulate Tau’s dynamic behavior and function during axonal transport. In this study, the dynamics of two different isoforms of Tau, both with three-microtubule binding repeats but a different number of N-terminal acidic inserts, were assessed using single molecule imaging techniques and novel data analysis methods.

The absolute probabilities for low energy ion bombardment induced secondary emission of electrons and anions have been measured as a function of adsorbate coverage of the surface. The primary ion beams were incident at less than 500 eV on metallic, semiconducting and insulating surfaces. The adsorbate used was chiefly oxygen, and the coverage range studied was zero to about one monolayer. The presence of an adsorbate was observed to significantly enhance secondary emission of electrons and anions in the case of O - and Na+ impacting metallic (W, Al) and semiconducting (Si) substrates; the effect of the adsorbate was little to minimal in the case of N2+, Ar+, Ne+ and He+ impacting these substrates, however. No appreciable adsorbate-induced changes in the secondary emission probability were measured for any of the probe beams incident on the insulating (MgO) substrate.;Secondary electron and anion kinetic distributions were also measured, as functions of projectile impact energy and of adsorbate exposure. The most probable energy of the secondary products was in the 1--3 eV region; the form of the distributions had little to no dependence on the impact energy or adsorbate exposure, but varied with different projectile and substrate species. The identities of the secondary anions were determined through mass spectroscopic techniques; atomic ion forms of the adsorbate and simple adsorbate-substrate molecular ions are the predominantly emitted species.;The data are discussed in terms of a model in which a molecular anion residing on the surface is collisionally excited, its subsequent decay giving rise to both electron and negative ion emission into the vacuum. The results of N2+, Ar+, Ne+ and He+ bombardment, in which secondary emission does not appear to be adsorbate-mediated, suggest that there exists a condition of excitation energy resonance which projectiles having high ionization potentials do not satisfy; experimental evidence shows that incident O- and Na+ satisfy this condition to a greater degree than do the above projectiles. The concepts of this excitation model can be represented mathematically and made to fit the observations with careful parameter choice; the parameters can be shown to reflect properties of the interaction.

We examine the energy spectrum of hydrogen in weak near-perpendicular electric and magnetic fields using quantum computations and semiclassical analysis. The structure of the quantum spectrum is displayed in a lattice constructed by plotting the difference between total energy and first order energy versus first order energy, for all states of a given principal quantum number n. For some field parameters, the lattice structure is not regular, but has a lattice defect structure which may be characterized by the transport of lattice vectors. We find that in near-perpendicular fields the structure of the spectrum is divided into six distinct parameter regions, which we characterize by the presence and type of lattice defect. to explain this structure we examine a corresponding classical system which we have derived by classical perturbation theory. Starting from Kepler action and angle variables, we give a derivation of a classical Hamiltonian to second order in perturbation theory; the derivation is different from, but the final result agrees with previous work. We focus especially on the topological structure of the reduced phase space and on the resulting topological structure of the trajectories. We show that construction of action variables by the obvious methods leads to variables that have discontinuous derivatives. Smooth continuation of these "primitive" action variables leads to action variables that are multivalued. We show how these multivalued actions lead to lattice defects in the quantum spectrum. Finally we present a few correlation diagrams which show how quantum eigenvalues evolve from one region of near-perpendicular parameter space to another and show how the structure of the quantum correlations is related to structures in the classical phase space.

We report laser desorption from a Room Temperature Ionic Liquid (RTIL) as a novel source for time of flight mass spectrometry. We use the 2nd harmonic of an Nd:YAG laser to deposit intensities of 1-50 MW/cm2 via backside illumination onto our RTIL desorption sample. A microstructured metal grid situated on top of a glass microscope slide coated with RTIL serves as our desorption sample. The RTIL we use, 1-Butyl, 3-Methylimidazolium Hexafluorophosphate, remains liquid at pressures below 10-8 torr. The use of liquid desorption sample allows for improved surface conditions, homogeneity and sample life as compared to Matrix Assisted Laser Desorption Ionization (MALDI) techniques. Our desorption technique is also unique as it allows the study of both multiphoton and acoustic desorption processes within the same time of flight spectra. Our technique yields intrinsically high resolution, low noise data. We observe differences between ion species in their preference for desorption by a particular desorption method. Specifically, we observe desorption solely by acoustic means of an entire RTIL molecule adducted with an RTIL cation. Finally, we report the applicability of this technique for the desorption of biomolecules.

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No description available.

In this thesis a theory of associative detachment (A('-) + B (--->) AB + e) is presented. The theory is based on the close-coupling theory of Wang and Delos, but is more general in that final states of the nuclei (AB) are treated quantum mechanically. This is necessary since the molecule may be in any one of several vibrational states. The Schroedinger equation is reduced to an infinite set of coupled equations using carefully chosen assumptions. The coupled equations are uncoupled and the resulting equation for the wave function of the negative ion is solved to zero and first order. The first order solution is then used to find the wave function for the final states of the molecule. Two systems were examined: H (D) + Cl('-) and H (D) + F('-). In both cases the survival probability of the negative ion showed a striking isotope effect, with the survival probabilities found for D + Cl('-) and D + F('-) much smaller than those found for H + Cl('-) and H + F('-). Experimental rate constants were reproduced for H + Cl('-) and H + F('-).

I have calculated the energy spectrum of a highly excited atom which lies in parallel, static electric and magnetic fields. In parallel fields the Coulomb quantum numbers n and m are still "good" quantum numbers but 1 is not: the calculation is for n = 30, m = 1 atoms.;The eigenvalues were obtained by semi-classical quantisation of first-order classical perturbation theory and have been calculated for a large range of electric and magnetic field strengths. The results are in good agreement with those found from first-order degenerate quantum perturbation theory.;The semi-classical analysis provides a correlation diagram connecting the Stark effect states to those of the diamagnetic effect.

A simple analytic formula for the photodetachment cross section of H$\sp{-}$ in perpendicular electric and magnetic fields is obtained. Oscillations in the spectrum are predicted by the formula, and these oscillations are correlated with closed classical orbits. We point out that the quantum mechanical derivation, using a stationary phase approximation, is in complete agreement with the three-dimensional semiclassical solution to the problem.