A new system has been developed for in-flight capture of continuous high energy mass separated radioactive ion beams of the ISOLDE type. To test the system, a precision high voltage ion gun based on surface ionization of cesium has also been developed. This gun delivers stable currents to 300 nA at energies to 65 keV with emittances at 50 keV of 1.3 $ pi$-mm-mrad. / The ion collection system is a high-field dc decelerator leading into a very large Paul trap at an elevated potential and employing buffer gas ion cooling. The ring electrode is of internal diameter 120 mm, the end electrodes are separated by 106 mm, and rf amplitudes of up to 10 kV can be applied on the ring. Tests showed that 50% of a 60 keV beam can be injected through the 8 mm hole into a trap at energies of a few tens of eV. / Trap performance was analyzed from time-of-flight observations of extracted beam pulses. Extraction included a pulse-down cavity capable of lowering the potential of the extracted ion cloud from 60 kV to zero. Beam pulses of less than 1 $ mu s$ were delivered at energies less than 5 keV to a detector 2 m from the trap with an overall efficiency of about 0.4% of the beam from the ion gun. / A thermodynamic model of the ions in the trap, with ion temperatures of 500 K for about $10 sp4$ ions and of 1000 K for about $10 sp7$ ions, almost fully accounts for the experimental results, the minor discrepancies between the modeled shape and the observed shape of the detector pulses being attributed to space charge effects in the collected ion clouds. The use of this model to simulate the extracted ion pulses showed the pulses to have a longitudinal emittance of about 5 $ pi$-eV-$ mu s$ and normalized transverse emittances of about 1 $ pi$-eV$ mu s.$ The model also indicates that the extraction system pulled only about 16% of the collected ions out of the trap. Methods are suggested for improving this extraction efficiency and further studies are suggested for improving the incoming ion collection efficiency.

The production, properties and purification of the novel carbon nanotubes have been systematically studied. A fully automatic electric arc carbon nanotube generator is designed and constructed. The state of the electric arc is monitored and adjusted by a computer to maintain a stable growth condition. The optimum conditions for the high yield and quality of carbon nanotubes were found in this apparatus. Nanotubes grown in the arc are found to be highly defective. These defects are ascribed to the tube-tube sintering due to the excessive heat in the arc. This sintering effect can be reduced through the use of a better water-cooled cathode but can't be eliminated. A method of purification of arc grown nanotubes was developed and was found to be highly effective. A growth model is also proposed based on properties of carbon nanotubes and the electric arc in which they were grown.

We have used the density functional ab initio method to conduct investigations on the effects of an applied electric field on the chemisorption bonds of adsorbate-surface systems, and on the reactivity of a gas phase semiconductor cluster. In STM current-induced excitation of adsorbates, lateral energy transfer among adsorbates tend to delocalize the excitation, and reduce resolution. We show that the strength of chemical bonds can be increased or decreased depending on the strength and direction of the applied electric field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from the site can be inhibited, and thereby lead to adsorbate excitation localization. The details of the field-induced shifts of an adsorbate-surface bonding features are shown to depend on their dipolar polarization. In the case of the reactivity of GaAs clusters with ammonia, recent experiments indicate that NH$\sb3$ adsorption rate depends strongly on cluster size, Ga/As composition ratio, and cluster charge state. We characterize the reactivity of NH$\sb3$ at various sites of a $\rm Ga\sb5As\sb5$ cluster in terms of the adsorbate binding energy and charge transfer, showing a strong correlation between the two. The dependence of the cluster reactivity on its charge state is deduced.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 68-02, Section: B, page: 1035. Adviser: Munir H. Nayfeh. Includes bibliographical references (leaves 100-104) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3065. Adviser: Taekjip Ha. Includes bibliographical references (leaves 101-104) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6455. Adviser: Munir Nayfeh. Includes bibliographical references (leaves 103-109) Available on microfilm from Pro Quest Information and Learning.

Many chemical applications, from spectroscopy to quantum chemistry, involve measuring or computing a large amount of data, and then compressing this data to retain the most chemically-relevant information. In contrast, compressed sensing is an emergent technique that makes it possible to measure or compute an amount of data that is roughly proportional to its information content. In particular, compressed sensing enables the recovery of a sparse quantity of information from significantly undersampled data by solving an l1-optimization problem. This thesis represents the application of compressed sensing to problems in chemistry. The first half of this thesis is about spectroscopy. Compressed sensing is used to accelerate the computation of vibrational and electronic spectra from real-time time-dependent density functional theory simulations. Using compressed sensing as a drop-in replacement for the discrete Fourier transform, well-resolved frequency spectra are obtained at one-fifth the typical simulation time and computational cost. The technique is generalized to multiple dimensions and applied to two-dimensional absorption spectroscopy using experimental data collected on atomic rubidium vapor. Finally, a related technique known as super-resolution is applied to open quantum systems to obtain realistic models of a protein environment, in the form of atomistic spectral densities, at lower computational cost. The second half of this thesis deals with matrices in quantum chemistry. It presents a new use of compressed sensing for more efficient matrix recovery whenever the calculation of individual matrix elements is the computational bottleneck. The technique is applied to the computation of the second-derivative Hessian matrices in electronic structure calculations to obtain the vibrational modes and frequencies of molecules. When applied to anthracene, this technique results in a threefold speed-up, with greater speed-ups possible for larger molecules. The implementation of the method in the Q-Chem commercial software package is described. Moreover, the method provides a general framework for bootstrapping cheap low-accuracy calculations in order to reduce the required number of expensive high-accuracy calculations. / Chemical Physics

Using the bond fluctuation algorithm of Carmesin and Kremer (Carmesin and Kremer 1988), we investigate the static and dynamic properties of self-avoiding linear polymers embedded in static, two-dimensional (d=2), quasi-periodic arrays of obstacles with entropic traps. The phenomenon of polymer collapse, the closely related enrichment and depletion of polymer configurations, the conformational relaxation, and the diffusive behaviour are all investigated within the framework of the lattice Monte Carlo method. Several distinct dynamical regimes are encountered: the (obstacle-free) Rouse-like regime (obstacle sub-array concentration c=0), the reptation regime for chains in perfectly periodic obstacle sub-arrays (c=1), and, in the presence of disorder and entropic traps (0<c<1), the anomalous regimes where the scaling properties differ from those predicted by the Rouse and reptation theories. Prior to the onset of normal diffusion, even systems characterized by very slight disorder (i.e., the existence of random isolated void spaces) are shown to lead to long, transient, subdiffusive regimes where the mean square displacement of the centre of mass scales as RCM 2∼D*tbeta where 0.5<beta<1 is the anomalous diffusion exponent and D* is the anomalous diffusion coefficient. In such disordered systems, conformational relaxation is shown to be coupled with centre of mass subdiffusion, resulting in long, time-stretched, exponential relaxation of the Rouse coordinates, viz. exp.[-(t/tau) alpha]. The stretching exponents 0.5<alpha<1 are shown to be closely related to the anomalous diffusion exponents beta and where the alpha, for a given chain, are shown to decrease with increasing mode number and with strong disorder. The molecular size-dependence of the steady-state diffusion coefficient, as well as that of the conformational relaxation time, is shown to be greatest when the concentration of obstacles is large and when that of the voids is non-vanishing (c ≲ 1). Thus, the dynamical scaling in entropic trapping systems is non-monotonic with respect to the concentration of obstacles. Polymer reptation dynamics thus appears to be intrinsically unstable with respect to static disordered systems of obstacles. Having demonstrated the coupling of centre of mass subdiffusion and conformational relaxation, we introduce a new relaxation length scale, lambda=(2dD*t alpha)1/2, that is more appropriate for characterizing disordered systems than is the ubiquitous radius of gyration used in both the Rouse and reptation theories. However, lambda could not be distinguished from the radius of gyration in terms of the molecular size scaling given the uncertainty in our data. Finally, having proposed a theoretical dynamic model of entropic trapping for dilute polymer solutions in embedded mesoscopic voids, we investigate the effect of polymer solution concentration on the dynamics for both monodisperse and polydisperse polymer solutions. New, unexplored dynamical behaviours are manifest as the conformational and translational entropies compete to minimize the system free energy.

Beginning in the early 1990's, the study of binary mixtures containing simple molecules under high pressure led to the discovery of a solid van der Waals compound: a weakly bound molecular solid whose cohesion is primarily due to van der Waals forces. The formation of this type of compound, such as He(N2)11 discovered in 1992 in the helium-nitrogen system by Vos et al. [Vos, 1992], is predicted in systems such as those containing nitrogen and methane. The nitrogen-methane binary system is studied at room temperature under high pressure in order to construct the pressure-concentration phase diagram up to 16 GPa. Over 20 mixtures of varying concentration have been studied within a diamond anvil cell through Raman spectroscopy and powder X-ray diffraction using synchrotron radiation. Within the phase diagram, phases exist that resemble the pure species at similar thermodynamic conditions, and additionally, two novel van der Waals compounds are observed.

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