New ionic states of alkali halide molecules, which are isoelectronic to the neutral rare gas halide molecules, have been generated by direct photoionization using the soft x-ray flux from a laser-produced plasma. The fluorescence emission of three ionic excimer transitions has been observed at 185 nm for CsF, 208 nm for CsCl, and 130 nm for RbF. The emission wavelengths are in good agreement with those calculated using Rittner-type potentials. The vibrational structure in the emission spectrum of CsF has been calculated and excellent agreement with the experiment is found. A model for photoionization of these molecules has revealed that the vibrational population of the bound photoionized state is Boltzmann distributed if the neutral ground state has a thermal vibrational distribution. Molecular halogen transitions of Cl$\sb2$* at 258 nm and F$\sb2$* at 158 nm have been observed in CsCl and RbF, respectively, as a result of predissociation of the ionic states of these molecules. Rate constants for the molecular halogen formation and lifetimes for the precursor states have been experimentally evaluated. The predissociation probability of the ionic state by adiabatic curve crossing has been estimated to describe the observed spectral intensity. A new kinetic model has been developed to describe the photoionization of CsF vapor. The calculated number density of the Cs$\sp{2+}$F$\sp-$ ionic state of 6 $\times$ 10$\sp{13}$ cm$\sp{-3}$ is in good agreement with the measured one. Based on the kinetic model, it is concluded that a CsF ionic excimer laser is feasible when a short pulse excitation with a line focusing geometry is employed. The absorption spectra of CsF, CsCl, and RbF in the UV and VUV have been determined using a VUV plasma continuum probe light source. The observed spectral peaks have been identified as the transitions from the ground states to the neutral covalent states. The spectra have been analyzed using the reflection method, providing potential curves of the neutral covalent states. The absorption oscillator strengths of these transitions have been evaluated from the absorption spectra.

Thermodynamic properties of associating polyatomic fluids are studied using molecular simulation and theory. The overall goal of this study was to develop a theory to predict the properties of fluid mixtures with intermolecular and intramolecular association. Intermolecular association is well understood mainly due to a theory developed by Wertheim for spherical associating molecules. In this work we performed the first tests of Wertheim's theory and its extensions for a wide variety of model fluids. Predictions from Wertheim's theory for associating hard sphere (HS) and Lennard Jones (LJ) molecules with four bonding sites were tested against simulation results. We found that the first order perturbation form of Wertheim's theory is in good agreement with simulation results. The theory was also tested for associating diatomic, triatomic, and longer chain molecules. The approximations used to simplify the theory are explained and we show how a spherical reference can be used to predict the properties of an associating chain fluid. A hard chain equation of state was previously obtained by extending Wertheim's theory to mixtures. In this work, a new chain equation of state is derived which predicts properties in much better agreement with molecular simulation, especially for long chains ($>$8 segments). The chain equation of state was also extended to LJ chains and mixtures of LJ chains. The results from this extension are in good agreement with simulation results. Prior to this work, no theory could accurately predict the properties of intramolecularly associating fluids. Such a theory is essential to the prediction of the properties of associating polymer solutions. A theory for intramolecular association in the absence of intermolecular association was developed. This theory is in good agreement with simulation results for tetramers with association sites on the terminal segments which can only bond intramolecularly. To develop a theory for the competition between inter and intramolecular association we combined Wertheim's theory for intermolecular association with our theory for intramolecular association. This combined theory gives good agreement with simulation results for tetramers with association sites that can bond intermolecularly and intramolecularly. The theory also explains some of the anomalous behavior of these systems.

Macromolecular dynamics is important for both understanding biological processes and practical applications such as MR imaging data interpretation. We have used the QNS method to study the dynamics of trypsin chain segments. Trypsin powder and trypsin in D$\sb2$O were studied for temperatures from 100K to 300K. Energy spectra of the scattered neutrons were measured for various neutron momentum transfer. Diffusive type motion of chain segments are observed for trypsin solutions at temperatures above the freezing point, while powder and frozen samples display minimum chain motion. The motion of trypsin chain segments can be fitted by a general "jump-diffusion" model which describes the conformational changes of a macromolecule as transitions between its substates. The diffusion coefficient of the trypsin chain segment is 2.4 $\times$ 10$\sp{-6}$ cm$\sp2$/sec and the average residence time of trypsin in its substates is 1.3 $\times$ 10$\sp{-11}$ second when trypsin is in D$\sb2$O at 300K. We measured the average mean square thermal vibration amplitude of trypsin (0.65 A$\sp2$) which is slightly larger than the results from computer simulations and X-ray diffraction studies. We also have tested macromolecular dynamics theory on poly-acrylic acid. We measured the frequency dependence of proton T$\sb1$ relaxation rates of poly-acrylic acid in D$\sb2$O solutions with different pDs and salt concentrations. Frequency dispersion data analyzed using both a flexible chain model and a stiff chain model give a maximum correlation time of 1.5-2.3 $\times$ 10$\sp{-7}$ second depends on the model. No significant change due pD and salt concentration difference was found.

Ab initio self-consistent field Hartree-Fock calculations employing minimal and double-zeta basis sets have been carried out on the possible isolated-pentagon fullerene isomers of C$\sb{76}$ and C$\sb{78}$. Two possible isolated-pentagon fullerene isomers exist for C$\sb{76}$: a chiral $D\sb2$ structure with a closed-shell electronic configuration and a $T\sb{d}$ structure with an open-shell electronic configuration that symmetry lowers to a closed-shell $\sp1A\sb1$ state in $D\sb{2d}$ symmetry. The D$\sb2$ isomer is found to be 43 kcal/mol more stable than the $T\sb{d}$ $\to$ $D\sb{2d}$ isomer. Five isolated-pentagon isomers exist for C$\sb{78}$ ($C\sb{2v}$(I), $C\sb{2v}$(II), $D\sb3$, $D\sb{3h}$(I) and $D\sb{3h}$(II)). The predicted order of stability for the five structures is found to be $C\sb{2v}$,(I) $>$ $C\sb{2v}$(II) $>$ D$\sb3$ $>$ $D\sb{3h}$(I) $>$ $D\sb{3h}$(II). Thermodynamic predictions of C$\sb{76}$ and C$\sb{78}$ seem to straightforwardly correlate with experimental results.

The rare gas alkali ionic molecules are established as a new class of ionic excimers which emit in the vacuum ultraviolet (VUV) regime of the electromagnetic spectrum. Temporal and spectral characteristics of these species with (XeRb)$\sp{+}$ at 164 nm and (XeCs)$\sp{+}$ at 160 nm have been investigated by soft x-ray excitation in a laser-produced plasma and by high energy pulsed electron beam pumping in this work. Soft x-ray pumping of XeF(B $\to$ X) and (XeRb)$\sp{+}$ yielded the first observation of excimer molecules formed by reactive kinetics in a laser-produced plasma. The spectrum of (XeRb)$\sp{+}$ was observed. The spectral structure could be assigned to 3 dipole allowed transitions originating from the 0$\sp{+}$(II), 1(II), 1(I) upper states ending in the 0$\sp{+}$(I) ground state. A kinetic study of electron beam pumped mixtures of Ar/Xe/Rb and Ar/Xe/Cs was performed. The observed temporal decays of (XeRb)$\sp{+}$ and (XeCs)$\sp{+}$ were analyzed. The results suggest that electron deactivation is surprisingly not a dominant quenching process for rare gas alkali ions. The radiative lifetimes are 150 $\pm$ 50 ns for (XeCs)$\sp+$ and 250 $\pm$ 50 ns for (XeRb)$\sp+$. In addition several quenching rate constants were extracted from the experimental results. These rate constants and lifetimes were incorporated into a kinetic model for these species. This kinetic model reproduced the experimental observations well providing the electron quenching rate coefficient is kept to a maximum of 5 $\times$ 10$\sp{-9}$ cm$\sp3$s$\sp{-1}$. Estimates for the upper state densities were computed using this model to be $\sim$4 $\times$ 10$\sp{15}$ cm$\sp{-3}$ for (XeCs)$\sp+$ and 2 $\times$ 10$\sp{15}$ cm$\sp{-3}$ for (XeRb)$\sp+$. Including absorption due to the photoionization of the alkali atoms, the net gain coefficients are computed to be on the order of 10$\sp{-2}$ cm$\sp{-1}$. Therefore the rare gas alkali ionic excimers appear to be a promising class of candidates as storage media for VUV lasers.

We have obtained high-lying vibrational spectra for the $\rm 1\sp3\Sigma\sbsp{g}{+}$ and $\rm 1\sp1\Sigma\sbsp{u}{+}$ excited states of both $\rm \sp7Li\sb2$ and $\rm \sp6Li\sb2$ via photoassociation of colliding ultracold lithium atoms confined in a magneto-optical trap. Photoassociative spectroscopy of ultracold atoms is a powerful tool for probing molecular states near the dissociation energy that are inaccessible by other methods. For an ultracold lithium vapor, the distribution of unbound ground state energies is so small that precise free-bound spectroscopy is possible. Molecular hyperfine structure is resolved. We calculate accurate dissociation energies for both the $\rm 1\sp3\Sigma\sbsp{g}{+}$ and $\rm 1\sp1\Sigma\sbsp{u}{+}$ excited states of Li$\sb2$. A model potential is constructed from a combination of experimental and theoretical information, and used to assign vibrational quantum numbers. We observe rotational structure and assign rotational quantum numbers.

Negative ion formation via electron transfer in thermal energy collisions between K(nd) Rydberg atoms and simple polyatomic molecules is studied at low-to-intermediate values of principal quantum number, n (n $<$ 40). At these values of n, the Rydberg electron can no longer be considered simply as a free electron of equivalent energy, because effects associated with the reduced size of the Rydberg atom and the proximity of the atom's charged core become important. We have observed and investigated several novel phenomena, associated with both dissociative and non-dissociative electron transfer. For example, marked n-dependences have been observed in the measured rate constants for free ion production by Rydberg electron attachment. These are due to the rapidly decreasing size of the Rydberg atom which results in atomic opacity, and to the increasing post-attachment electrostatic attraction between the product positive and negative ions. In the case of dissociative Rydberg electron transfer to simple halogenated hydrocarbons, XY, K(nd) + XY $\to$ K$\sp+$ + (XY$\sp-)$* $\to$ K$\sp+$ + X$\sp-$ + Y where XY is CF$\sb3$I, CF$\sb2$Br$\sb2$, CF$\sb3$Br, CH$\sb2$Br$\sb2$, CCI$\sb4$, CFCl$\sb3$, or CHCl$\sb3$, angular asymmetries were discovered in the velocity distributions of the product negative ions. Analysis of these data provides valuable insight into the translational energy release that accompanies dissociation of the transient intermediate molecular negative ions, and their lifetimes. Measurements of the spatial distributions of the product K$\sp+$ ions provide additional information on the dissociative attachment process. A new reaction channel was discovered in the case of non-dissociative Rydberg electron transfer to CS$\sb2$. This channel, which results in the formation of long-lived CS$\sb2\sp-$ ions that undergo rapid electric-field-induced electron detachment in fields of only a few kilovolts per centimeter, is discussed together with a possible theoretical model of this surprising phenomenon.

Measurements of the trap loss rate in ultra-cold magneto-optically trapped $\sp6$Li and $\sp7$Li are presented and compared. Clear evidence is presented for two different trap loss mechanisms involving inelastic collisions between one ground state atom and one excited state atom. The fine-structure changing (FSC) mechanism, in which the excited state atom changes fine-structure level during a collision, is seen to dominate the rate at low trap laser intensities. However, as the intensity is increased, the trap becomes sufficiently deep to be able to contain the products of FSC collisions and this mechanism no longer contributes to trap loss. It is a unique aspect of Li that the energy imparted to an atom in a FSC collision is in a range that may be recaptured with experimentally attainable trap parameters. When the products of all FSC collisions remain trapped, only the radiative escape (RE) trap loss mechanism, due to the emission of a less energetic photon than the initial excitation photon, contributes to the trap loss rate. At small detunings, the rate of the RE trap loss mechanism is seen to be over two orders-of-magnitude smaller than the FSC rate. The FSC trap loss rate for $\sp6$Li and $\sp7$Li were found to be largest at smaller detunings and of comparable magnitude in $\sp6$Li and $\sp7$Li. The RE trap loss rate in $\sp6$Li was seen to be roughly four times the RE trap loss rate in $\sp7$Li. To understand better the dynamics of trap loss and magneto-optical traps in general, a sophisticated model of magneto-optical trap kinetics has been developed. This model has demonstrated that most critical factor determining the maximum velocity an atom may have and yet remained trapped is the initial atom's frame detuning, $\Delta$ - k v, where $\Delta$ is the trap laser detuning, k is the propagation vector of the trap beam most nearly anti-parallel to v, the atom's velocity. All else being equal, minimizing the magnitude of this quantity maximizes the velocity that a trap will retain. More detailed results of this model are also presented.

The quasi-elastic neutron scattering (QNS) method is a useful technique to study biomolecular dynamics. The versatility of the method makes possible motional studies of biomolecules in different forms: powder, crystalline, and solution; and at different temperatures. Thus, it allows investigation of biomolecular dynamics in different states of matter. We have used the QNS method to study the motion of the trypsin chain segments in powder and in D$\sb2$O solution at temperatures of 200K, 280K, and 300K. The scattering spectra S(Q,$\omega$) were measured in constant-Q mode. The S(Q,$\omega$) for trypsin protons in liquid solution exhibits a broadening due to diffusive motion which is absent in the powder and the frozen solution. This diffusive motion has the character of a jump diffusion. The high-frequency thermal motion obtained from the Debye-Waller factor $\langle$u$\sp2\rangle$/3 $\cong$ 0.33A$\sp2$ at T = 300K is consistent with earlier measurements. The DW factor at lower temperatures for trypsin solution shows deviation from theoretical predictions.

In this thesis, we study two fundamental problems in the theory and experiment of interfaces in simple and complex fluids. The first problem concerns the critical behavior of the thickness of interface of simple fluids in two dimensions of space. We study this problem using both theory and computer simulations. In our theoretical study of this problem, the equilibrium interface is assumed, in accord with current ideas, to consist of an intrinsic interface of the non-classical van der Waals type broadened by capillary wave fluctuations. It is shown that the interfacial thickness exhibits a crossover with change of critical exponent from capillary wave behavior at low temperatures to intrinsic structure behavior at temperatures extremely close to the critical temperature. The location of the crossover temperature is determined by a critical amplitude ratio. In our computer simulation study of this problem, the interfacial thickness between two two-dimensional Lennard-Jones fluid phases is determined by using the method of molecular dynamics. Within statistical uncertainty, the results for the interfacial thickness are found to be consistent with the prediction of the non-classical van der Waals theory of the intrinsic interface. The second problem studied in this thesis concerns the wetting behavior of three-component surfactant systems. In a three-phase equilibrium system, the middle phase either wets or does not wet the interface between the upper and lower phases. In this work, we perform a systematic experimental search for wetting transitions between wetting and nonwetting behaviors in three-component systems water/n-alkane/C$\sb{\rm i}$E$\sb{\rm j}$, where C$\sb{\rm i}$E$\sb{\rm j}$ denotes the surfactant polyoxyethylene alcohol C$\sb{\rm i}$H$\sb{\rm 2i+1}$(OC$\sb2$H$\sb4$)$\sb{\rm j}$OH. It is found that two systems, water/n-hexadecane/C$\sb6$E$\sb2$ and water/n-octadecane/C$\sb6$E$\sb2$, exhibit a wetting transition lying at least 10$\sp\circ$C below the upper critical temperature, and one system, water/n-tetradecane/C$\sb6$E$\sb2$, exhibits a wetting transition lying 4.3$\sp\circ$C below the upper critical temperature.