In the first application of a new method for determining this information, rates of collisional vibrational energy removal have been measured for the T$\sb1$ state of pyrazine with vibrational energy contents ranging from 2000 to 5500 cm$\sp{-1}$. This method is the first to enable the measurement of energy removal from electronically excited states in the energy regime above that of well-separated vibrational states. It is also the first method applicable to excited triplet states in any energy regime. Energized T$\sb1$ pyrazine is formed through intersystem crossing from various laser-excited vibronic levels of the S$\sb1$ electronic state. Triplet-triplet transient absorption spectrometry is then used to monitor the T$\sb1$ decay kinetics under various collisional conditions. The new method of analyzing these kinetic data, based on the known variation of T$\sb1$ pyrazine's nonradiative decay rate with vibrational energy, allows the extraction of the rate of vibrational energy removal by the collision partners as a function of pyrazine vibrational energy. The key feature of the results is an approximately tenfold increase in the energy removed per collision within the $\sim$3500 cm$\sp{-1}$ range of energy contents studied. At $\langle$ ET$\sb1\rangle$ $\approx$ 2000 cm$\sp{-1}$, all buffers remove 5 to 10 cm$\sp{-1}$ per collision; at $\sim$5500 cm$\sp{-1}$, the deduced energy removal rates per collision are $\sim$55 cm$\sp{-1}$ for helium, 175 cm$\sp{-1}$ for argon, 160 cm$\sp{-1}$ for H$\sb2$, 530 cm$\sp{-1}$ for SF$\sb6$, and 360 cm$\sp{-1}$ for pyrazine. This increase, which is significantly steeper than found for vibrationally hot ground state molecules at higher energy contents, may be related to differences in the electronic characters of the molecules studied or in the regime of vibrational state densities investigated. The development of this new method for measuring energy removal rates and the intriguing results found with it should serve to stimulate further theoretical and experimental research into collisional relaxation of electronically excited states.

This thesis reports measurements of absolute differential cross sections for elastic scattering of atomic oxygen at 0.5- and 1.5-keV laboratory energies. Measurements for targets of He, Ne, Ar, Kr, Xe, H$\sb2$, N$\sb2$, O$\sb2$, CO, CO$\sb2$, H$\sb2$O, CH$\sb4$, CF$\sb4$, SF$\sb6$, SO$\sb2$, and NH$\sb3$ are performed over a laboratory angular range of 0.2 to 5.0 degrees. Using a partial wave analysis, the cross sections for several targets are inverted and estimates of model interaction potentials are made. These potentials are then used to predict the elastic scattering cross sections at a laboratory energy of 100 eV.

Direct laser vaporization of transition-metal/graphite composite rods produced single-walled carbon nanotubes (SWT) in the condensing vapor in a heated flow tube. A much higher yield of nanotubes was found, with little of the amorphous overcoating on those produced by the metal-catalyzed arc-discharge method. A number of parameters were varied to achieve the highest yield. A mixture of Co with Ni catalyzed about 50% of all the carbon vaporized to SWT. A model for SWT growth is presented for both the present case and the arc.

A tunable diode laser infrared high-resolution spectrometer was constructed to study the spectroscopy and kinetics of free radicals. A method for producing diode laser frequency scans which are several wavelengths long, linear in frequency, and readily and accurately calibrated from reference spectra has been devised. The laser is current scanned under computer control over short overlapping segments between each of which a temperature step is made. Each segment is then linearized and pieced together to provide a final spectral scan. High-resolution spectra of transient species were collected using either magnetic rotation spectroscopy or flash kinetic spectroscopy. The high-resolution infrared spectrum of the CH stretching fundamental of the formyl radical (HCO) was observed by means of infrared kinetic spectroscopy using 308 nm (XeCl) excimer laser flash photolysis of formaldehyde or acetaldehyde followed by diode or difference-frequency laser probing of the transient absorption. The spectra obtained were assigned and fitted with rotational, spin-rotational, and centrifugal distortion constants resulting in a v$\sb1$ band origin of 2434.48 cm$\sp{-1}$. New ground state constant are reported from a least-squares fit combining the v$\sb1$ infrared data with previous microwave and far-infrared laser magnetic resonance (FIR LMR) measurements. Fixed-frequency studies of infrared absorptions using the spectrometer allow the time-resolved investigation of chemical dynamics on submicrosecond time scales. A method of locking the diode laser output frequency to an external vacuum-spaced etalon by modulating the optical pathlength of the etalon was investigated. Using this technique, the rate constant of the reaction of HCO with O$\sb2$ was measured.

A tandem time-of-flight (TOF) apparatus was designed to study the structure and chemistry of cold transition metal cluster ions from supersonic beams. By means of a photodissociation laser fluence dependence technique, binding energies of Nb$\sb{\rm x}\sp{+}$ (x = 2 $-$ 20), Co$\sb{\rm x}\sp{+}$ (x = 4 $-$ 20) and etc. were found to generally increase with cluster size. The desorption energies of Nb$\sb{\rm x}$N$\sb2\sp{+}$ (x = 2 $-$ 17) and Nb$\sb{\rm x}$CO$\sp{+}$ (x = 2 $-$ 10) also increase with cluster size with some oscillations similar to the size dependent reactivities of these clusters. Photodetachment studies revealed that electron affinities of copper clusters increase with cluster size with a sharp even/odd alternation. Unlike other noble metals, Ag$\sb{\rm x}\sp{-}$ clusters display two competing processes: photodissociation and photodetachment. Relative reactivities of cluster ions of Nb, Co, Ag, and etc. have been measured using a fast flow cluster reactor, displaying a similar function of cluster size to that of the neutrals. In addition, preliminary photoelectron experiments have been performed on Cu$\sb{\rm x}\sp{-}$ and Nb$\sb{\rm x}\sp{-}$. A magnetic Time-of-flight ultraviolet photoelectron spectrometer (MTOFUPS) has been developed to study electronic structures of cold metal and semiconductor cluster anions prepared in supersonic beams. Application of this spectrometer to carbon clusters with a F$\sb2$ laser (7.9 eV) allowed their electron affinities and UPS patterns to be measured,demonstrating a remarkable structural evolution of these clusters: Chains (C$\sb2\sp{-}$-C$\sb9\sp{-}$) - Rings (C$\sb{10}\sp{-}$-C$\sb{29}\sp{-}$) - Cages (C$\sb{38}\sp{-}$-C$\sb{84}\sp{-}$). In particular, the UPS of C$\sb{60}\sp{-}$ is in excellent agreement with the CNDO/S calculation, providing a striking spectral evidence for the highly symmetric icosahedral soccer ball structure--Buckminsterfullerene. For comparison, the UPS of Si$\sb{\rm x}\sp{-}$ and Ge$\sb{\rm x}\sp{-}$ are presented. Unlike carbon clusters which prefer structures of low dimensionality, these clusters tend to take a network structure. In conjunction with calculations, some tentative structural assignments were possible.

Experiments to verify a semiclassical model of Rydberg atom collision processes revealed discrepancies between the model predictions and experimental results for the reaction$$\rm K\sp{**}(np)+SF\sb6\to K\sp{+}+SF\sbsp{6}{-*}$$at low to intermediate values of the principal quantum number n (n ${<}{\approx}$ 20). Initial measurements using non-velocity selected Rydberg atoms yielded ion angular distributions that did not agree with model calculations. To explore this further, experiments were initiated using velocity selected Rydberg atoms. These data show that the discrepancy between the model and experiment is due to transfer of energy from the SF$\sbsp{6}{-*}$ ion into translational energy of the product ion pair through a close collision. This process is not included in the semiclassical model. This conclusion is substantiated by study of the dissociative electron transfer reaction$$\rm K\sp{**}(np)+CH\sb3I\to K\sp{+}+CH\sb3I\sp{-*}\to K\sp{+}+I\sp-+CH\sb3$$for which similar energy transfer cannot occur. The data agree well with model calculations.

A detailed experimental and theoretical study of the interaction of very-low-energy electrons with the polar target HF is presented. This interaction is investigated experimentally by measuring rate constants for ionization and state-changing in collisions between K(np) Rydberg atoms with $90\sbsp{\sim}{<}n\sbsp{\sim}{<}400$ and HF. The data are found to be consistent with the results of rotational close-coupling calculations that include possible effects associated with dipole-supported real or virtual states. The value of this state is effectively the only free parameter in the theoretical model. Comparison with the data suggests that for low energy electron-HF scattering there is a virtual state which, for $J=0,$ has an energy of 1-1.5 meV. The present work points to the importance of dipole supported states in electron-polar molecule scattering.

Experiments using the laser vaporization technique for production of metal clusters have been performed. The reactions of neutral metal clusters with various gases have been studied using a fast flow reactor. Dramatic reactivity variations were observed which depended on cluster size, metal, and reactant. A laser vaporization disc source has been developed for the study of semiconductor clusters. Some preliminary studies on neutral germanium and silicon clusters were performed. Their ionization potentials have been bracketed and the clusters were found to fragment by a fissioning process and to have long lived (100 nanoseconds) excited electronic states. A detailed study has been undertaken into carbon clusters. Laser synthesis of astrophysically important polyyne molecules such as H-C-(C-C)$\sb{\rm 2n}$-N has been done. Chains containing up to 22 carbon atoms are formed in a vaporized carbon and reactant gas plasma. A photophysically stable and chemically inert cluster, C$\sb{60}$, has been discovered and hypothesized to have the structure of a truncated icosahedron. All even clusters in the 60 atom size range were found to be inert to highly reactive gases, while odd clusters readily reacted. The results are consistent with a whole series (30-90 atoms) of closed cage-like structures. Closure of even clusters only is possible via the inclusion of twelve pentagons into a hexagonal network. Odd clusters show neither the photophysical nor chemical stability of the even clusters. A mechanism for the formation of spherical soot particles has been developed. Stable organometallic complexes of the formula C$\sb{\rm 2n}$M (20 $<$ n $<$ 40 and M = La, Ba, Sr, Ca) have been laser synthesized. The dominant complex observed was C$\sb{60}$M$\sp+$. These species are photophysically stable, chemically inert, and no C$\sb{\rm 2n}$M$\sb2$ species were detected. The ultraviolet and visible absorption spectrum of C$\sb{60}$ has been measured. Because excited electronic states are not expected to live long in a molecule of this size, a unique spectroscopy independent of excited state lifetime was developed. Predissociation spectroscopy was performed on the cold van der Waals complexes C$\sb{60}$--CHCL$\sb2$ and C$\sb{60}$--benzene. A single sharp (50 cm$\sp{-1}$ wide) absorption band was found at 3860 A and 3863 A for the two complexes respectively. This peak is tentatively assigned to the 0-0 band of the lowest $\sp1$T$\sb{\rm 1u}$ $\gets$ $\sp1$A$\sb{\rm g}$ (LUMO+1 $\gets$ HOMO) transition of a truncated icosahedral carbon shell structure.

The incoherent quasi-elastic neutron scattering (IQNS) method is a useful technique to study biomolecular dynamics. The versatility of the method makes possible motional studies of biomolecules in different forms: powder, crystal, and solution; and at different temperatures. Thus, it allows for the investigation of biomolecular dynamics over a wide-range of physical conditions. We have used the IQNS method to study the motions of side chains in trypsin and myoglobin at various D$\sb2$O hydration levels. The scattering spectra S(Q,$\omega$) were measured in constant-Q mode. The protein in powder form exhibits vibrational high-frequency motions, while the protein in solution and in crystals are characterized by diffusive jumps, and high-frequency vibrations. At temperatures below 200K, the S(Q,$\omega$) for these proteins in solution is similar to an harmonic solid. As temperature increases, a transition is seen at 200K, above which the protein becomes more liquid-like with rapid transitions between conformational substates. The diffusion constant D for the side chains is on the order of 10$\sp{-6}$ cm$\sp2$/sec.

In this dissertation, we develop a method for molecular simulation based on principal component analysis (PCA) of a molecular dynamics trajectory and least squares approximation of a potential energy function. Molecular dynamics (MD) simulation is a computational tool used to study molecular systems as they evolve through time. With respect to protein dynamics, local motions, such as bond stretching, occur within femtoseconds, while rigid body and large-scale motions, occur within a range of nanoseconds to seconds. To capture motion at all levels, time steps on the order of a femtosecond are employed when solving the equations of motion and simulations must continue long enough to capture the desired large-scale motion. To date, simulations of solvated proteins on the order of nanoseconds have been reported. It is typically the case that simulations of a few nanoseconds do not provide adequate information for the study of large-scale motions. Thus, the development of techniques that allow longer simulation times can advance the study of protein function and dynamics. In this dissertation we use principal component analysis (PCA) to identify the dominant characteristics of an MD trajectory and to represent the coordinates with respect to these characteristics. We augment PCA with an updating scheme based on a reduced representation of a molecule and consider equations of motion with respect to the reduced representation. We apply our method to butane and BPTI and compare the results to standard MD simulations of these molecules. Our results indicate that the molecular activity with respect to our simulation method is analogous to that observed in the standard MD simulation with simulations on the order of picoseconds.