Molecular spectroscopy is used to probe a quantum degenerate gas of 7Li atoms. Several effects are investigated using both single-color and two-color (Raman) photoassociative spectroscopy. An unusual spectral shift with laser intensity is observed for single-color photoassociation and the intensity dependence agrees with theoretical predictions. Additionally, the prospects for altering the interactions between ground-state atoms is investigated for both the single-color and Raman schemes. The single-color scheme is found to be impractical experimentally (although feasible theoretically) while the Raman scheme may be experimentally feasible as well. The Raman spectra display an interesting lineshape which is found to reflect the rate for inelastic collisions involving ground-state molecules (produced via the Raman process) as well as the attractive mean-field interactions between atoms in the condensate. Under certain circumstances, a dispersive Raman lineshape is observed which may be due to coherence properties of both the condensate and the thermal gas. Additionally, the Raman lineshape appears to reflect the kinetic time scale for thermal atoms to enter the condensate via elastic collisions. The Raman technique is applied to studies of the condensate growth and collapse dynamics caused by attractive interactions between condensate atoms, and enables the first direct observations of this phenomenon.

The properties of fluids near interfaces, in particular, the fluid-solid interfaces on which this work is concentrated, are important in many processes, such as: wettability as related to oil recovery and environmental cleanup, biochemical separation, bio-compatibility of materials, membrane separation, adsorption in porous solids and micro- or nanomanufacturing of thin films. However, little of the past simulation and theoretical work in the field has considered associating fluids. In this work we perform Metropolis Monte Carlo computer simulations for one-sited (dimerizing), two-sited (linear chain forming), and four-sited (cluster forming) hard spheres against hard, smooth walls. Reported are results for density and fraction of monomers (which determines the change in Helmholtz free energy due to association according to Wertheim's theory) versus distance from the surface. Also computed are adsorption and for the four-site fluid, orientation, cluster size, and fraction of sites bonded as functions of distance from the surfaces. We also consider binary mixtures and an associating fluid near active surfaces. Except for orientation and cluster size, results are compared (favorably, in general) against a new density functional theory, which combines elements of the Tarazona density functional for hard spheres and Wertheim's theory of association. This dissertation concludes with ideas for further work in the area.

Low-energy electron transfer in collisions between a Rydberg atom and a target molecule can result in the formation of negative ions. Depending as to the properties of the target molecule the electron may be bound in a localized molecular orbital (valence-bound) or bound by its dipole potential in a diffuse orbital located at the positive end of the dipole (dipole-bound). Here we examine the properties of valence-bound and dipole-bound negative ions created through electron transfer in collision with Rydberg atoms, in particular, their lifetimes, collisional properties and electric-field-induced detachment characteristics. Intercomparison of the various data also leads to new insights into electron attachment to molecules that can support both valence-bound and dipole-bound negative ion states.

Understanding the effects of interactions on the ground-state of a paired degenerate Fermi gas throughout the BEC-BCS crossover has been a long standing physical quest for which countless papers have been written. The crossover describes the smooth transition of the physics of a BEC of tightly bound dimers to that of a paired BCS superfluid. A Feshbach resonance is used to tune the interactions necessary to study the crossover. Right around resonance the interactions are expected to be parameterized by a single universal parameter beta [1--3]. This thesis describes a measurement of the axial size of paired, 6Li clouds in the BEC-BCS crossover and provides a comparison with theory. In the BEC regime, absolute measurements of the molecular scattering length are compared with the atomic scattering length and the ratio is compared with theory. Finally, a measurement of beta is made and compared with theory.

A 461 nm laser was constructed for the purposes of studying ultracold neutral strontium. The dipole-allowed 1 S0 → 1P1 transition at 460.862 nm can be used in laser cooling and trapping; optical imaging, Zeeman slowing, and photoassociative spectroscopy. We produce light at this wavelength by converting infrared light at 922nm from various IR sources, notably a Ti:Sapphire laser, via second harmonic generation in a frequency doubling cavity using a potassium niobate crystal. This thesis will discuss the motivation, optical resonator, locking electronics, and characterization of a 461 nm laser.

Ab initio calculations of the electronic structure of bulk materials and large clusters are not possible on today's computers using current techniques. The storage and diagonalization of the Hamiltonian matrix are the limiting factors in both memory and execution time. The scaling of both quantities with problem size can be reduced by using approximate diagonalization or direct minimization of the total energy with respect to the density matrix in conjunction with a localized basis. Wavelet basis members are much more localized than conventional bases such as Gaussians or numerical atomic orbitals. This localization leads to sparse matrices of the operators that arise in SCF multi-electron calculations. We have investigated the construction of the one-electron Hamiltonian, and also the effective one-electron Hamiltonians that appear in density-functional and Hartree-Fock theories. We develop efficient methods for the generation of the kinetic energy and potential matrices, the Hartree and exchange potentials, and the local exchange-correlation potential of the LDA. Test calculations are performed on one-electron problems with a variety of potentials in one and three dimensions.

Triplet state pre-equilibration by reversible energy transfer has been observed by transient-absorption spectroscopy in mixed toluene solutions of C70 and C60 and of C70 and C60(CH 3)2. The equilibrium constants governing the asymptotic partitioning of triplet energy in these mixtures were determined as a function of temperature. The enthalpies of these excited states were found from van't Hoff plots of the equilibrium constant data to be -0.1 +/- 0.2 and -3.4 +/- 0.3 kJ mol-1 for C60 and C60 (CH3)2 respectively relative to a C70 triplet energy exchange partner. The corresponding relative entropies are 5.8 +/- 0.5 and -4.0 +/- 1.0 kJ mol-1 K-1 respectively. Transient spectra from high temperature C70/C60(CH3)2 mixed samples revealed evidence of a third, unidentified transient absorber that exhibited different kinetics from the pre-equilibrated triplet pool. Triplet state transient difference spectra and intrinsic decay kinetics were measured and compared for C60 and several derivatives of C 60. These derivatives were C60H2, C60(CH 3)2, ortho-xylyl-C60, N,N'-dimethyl-1,2-ethylenediamine-C 60, C60C(COOCH2CH3)2, and C60O. The spectral locations of the main triplet-triplet absorption peak for these compounds correlates linearly with the observed intrinsic intersystem crossing rate constant. The triplet state persistence of C60 was measured in toluene solution as a function of both ground state concentration and solution temperature. The unimolecular intersystem crossing deactivation channel shows very little thermal activation, whereas the observed bimolecular self-quenching decay channel is found to be highly activated. At room temperature, the deduced exponential lifetime of the solvent-caged encounter complex between triplet and ground state molecules is three orders of magnitude shorter than that of the isolated monomer triplet state. This suggests that the self-quenching process is not a simple perturbation of an isolated molecule's intersystem crossing, but rather occurs through a qualitatively different mechanism. The kinetic data are examined in light of the proposed excimer mechanism for triplet self-quenching. Finally, the self-quenching process is interpreted as a low-probability "mis-step" during energy transfer.

Dissociative electron attachment to CFCl$\sb3$, CF$\sb2$Br$\sb2$, and 1,1,1- and 1,1,2-C$\sb2$Cl$\sb3$F$\sb3$ is studied by measuring the velocity and angular distributions of negative ions produced through electron transfer in collisions with velocity selected K(np) Rydberg atoms. Data analysis is done with the aid of Monte Carlo simulations. Measurements indicate that the mean lifetimes of the transient ions CFCl$\sb3\sp{-*}$ and CF$\sb2$Br$\sb2\sp{-*}$ are $\sim$3.5 ps and $\sim$4.5 ps respectively and that only a small fraction of the excess energy of reaction appears in translation. The 1,1,1-C$\sb2$Cl$\sb3$F$\sb3$ data show one reaction channel resulting in immediate dissociation with an ion lifetime of $\sim$0 ps, and a second channel associated with a longer lived intermediate, $\tau\ \sim$ 0.1 to 1.0 ps. Only limited energy transfer occurs for the 1,1,2 isomer but the operative reaction channel(s) could not be identified. Possible reaction mechanisms are discussed with the aid of ab-initio calculations.

The screened Coulomb hybrid density functional theory developed in this work extends the applicability of hybrid functionals to large molecules and solids. Traditional hybrid functionals have been applied to medium sized molecules and some insulating solids with excellent results. However, the fundamentally long range of the Hartree-Fock (HF) exchange interaction limits their use for biomolecules and semi-conducting or metallic solids. The large spatial extent of the HF exchange can be reduced by utilizing a screened Coulomb potential. Such potentials have been widely used in solid state physics. Efforts of employing them in quantum chemistry were not as successful due to the unsatisfactory accuracy of the resulting methods. This work takes a new approach by combining a screened HF potential with both short and long range screened density functionals. The Heyd-Scuseria-Ernzerhof (HSE) screened Coulomb hybrid density functional is designed to produce exchange energies comparable to traditional hybrids while only using the short range, screened HF exchange. The accuracy of the HSE functional is assessed on a wide range of molecules and solids. Enthalpies of formation, geometries and vibrational frequencies of the G3 set of 223 molecules are predicted with performance equivalent to the best traditional hybrids. A set of 21 insulating, semi-conducting and metallic solids was used to determine the accuracy of lattice constants and bulk moduli. Both properties show significantly improved accuracy, compared to pure density functionals. HSE also accurately predicts band gaps of semi-conductors whereas pure density functionals severely underestimate the band gaps. Benchmarking of HSE in several semi-conducting and metallic solids shows a drastic decrease in computational cost, compared to established hybrid functionals. HSE achieves linear scaling for medium size systems (greater than 15 A) whereas regular hybrids scale as O (N2.5) for systems up to 100 A and scale linearly only beyond that. This late cross-over makes it computationally prohibitive to treat complex solids with traditional hybrid functionals. HSE circumvents this bottleneck and produces superior results while increasing computational cost by a factor of only two to four, compared to pure density functional theory.

I present a computational study of the reactivity between NH$\sb3$ and the $\rm Ga\sb5As\sb5$ cluster. A single NH$\sb3$ molecule approaches several sites on the cluster and the system is relaxed to its local lowest energy configuration. In this study, gallium sites are found to lead to lower total energies, due to greater contributions to densities of states near the Fermi level. Two ammonia molecule are then allowed to react with the cluster in sequence, each approaching different sites. Interaction at one site is seen to reduce the reactivity at other sites. These results are discussed in light of experimental studies performed on these clusters.