Potassium atoms in selected high-lying Rydberg states (n $<$ 1200) are used as a tool to probe threshold behavior in electron-molecule collisions. Collisions with non-polar electron attaching molecules such as CCl$\sb4$ are dominated by electron capture in a binary interaction between the Rydberg electron and target molecule allowing the study of electron attachment at electron energies of only a few $\mu$eV. Analysis of the data shows the cross section for electron capture is consistent with the Wigner threshold law for an inelastic s-wave process. Collisions with polar molecules can lead to Rydberg atom destruction through transfer of molecular rotational energy to the Rydberg electron. Rydberg atom data obtained with polar targets are not consistent with scattering from a static dipole. However, the data are consistent with a threshold law that takes into account a rotationally-averaged induced dipole interaction that can possibly support bound or virtual states.

Many interesting physical phenomena have been observed in electron-stimulated desorption studies of the cyanide molecule on alkali halide and alkali metal surfaces. We have performed a theoretical investigation of the nature of the cyanide-surface bond and of the desorption process using an ab initio density functional theoretic method of calculating electronic structure in the local density and Born-Oppenheimer approximations. We compare our results with experiment, and offer an explanation for an anomalous non-Boltzmann, temperature-independent rotational distribution experimentally observed in cyanide desorbed from these surfaces. As a verification of the method, we have performed extensive calculations on different bare alkali halide and alkali metal clusters and compared them with experimental results.

Although it has been known for more than 60 years that azoalkanes (R-N=N-R) dissociate under the influence of light or heat, few definitive conclusions have been reached on the mechanism of their photodissociation. Recently, the issue of whether the photodissociation is concerted or stepwise has been settled through kinetic resolution of the process into two steps, implying a methyldiazenyl radical intermediate. Our ab initio CASSCF quantum chemical calculations confirm the stability of this methyldiazenyl radical, and the dissociative transition state on its ground state ($\sp2$A$\sp\prime$) surface has been located. A barrier height of 410 cm$\sp{-1}$ has been found for this transition state, which leads to the dissociation into methyl radical plus N$\sb2$. The lowest energy path for this decomposition has been calculated using the CASSCF method with a 6-31G* basis set including nine electrons in nine active orbitals. Methyldiazenyl's $\nu\sb5$ mode (651 cm$\sp{-1}$) was found to correspond closely to the dissociation coordinate. In addition, UHF calculations have revealed that the lowest-lying triplet state of azomethane has a perp configuration at its equilibrium geometry, suggesting that first-step photodissociation may occur from the T$\sb1$ surface. Energies and methyldiazenyl vibrational frequencies obtained from the quantum calculations have been combined with existing thermochemical data as input to an energy disposal analysis that includes both statistical and impulsive modelling of product state energy distributions measured with time-resolved coherent anti-Stokes Raman spectroscopy (CARS). It appears that the first dissociative step may have significant impulsive character. In the second step, although some findings fall outside the range spanned by the impulsive and statistical models, the experimental nitrogen vibrational distribution is in excellent agreement with the prediction of the separate statistical ensemble (SSE) model. Further experimental and theoretical research, particularly on the first step, will be required before an adequate understanding of azomethane photodissociation can be achieved.

A variety of quantum mechanical methods have been developed and applied to the study of highly energized "pre-reactive" species involved in chemical reactions. We have examined the character of transition states for both unimolecular systems and bimolecular collisions. For unimolecular reactions, we have studied vibrational predissociation of hydrogen peroxide. The nodal lines of the predissociative resonance states are found to bend toward the dissociative side. This character should be largely responsible for the dissociation of the molecule. To compute the dissociation rates, we have combined the complex coordinate method and the Lanczos algorithm. The complex Lanczos recursion method is found to be insufficient to produce well converged resonance widths for this large system due to round-off errors. For bimolecular collisions, we have computed the absorption spectra of the transition states of the reaction K + NaCl + hv $\to$ KCl + Na$\sp{\*}$. The absorption probabilities show a strong dependence on laser frequency. This dependence is well explained by Franck-Condon calculations. By contrast, a linear curve crossing model is quantitatively incorrect. After carefully examining the excited wave packet dynamics and the time evolution of the transition probabilities, we believe the excitation process is not localized to the crossings of the field-dressed potential curves. We have also studied the effects of overall molecular rotation on the vibrational dynamics and unimolecular reaction rates. For a simple collinear triatomic model, the dissociation rates are uniformly increased as a function of angular momentum J, generally in a manner close to $J\sp2$. The reaction rates could be changed by a factor of three for some predissociative states, while remain almost unchanged for some other ones. The differences in the J-dependency correlate well with the existence of Fermi resonance conditions. The rotational effects are further investigated using a more realistic three dimensional model for HCN/HNC isomerization. We have developed a parameter dependent basis set for the study of this particular system. The importance of overall molecular rotation is confirmed in this study. However, the overall rotation is found to have non-uniform effects for different initial states.

Jet-cooled radical spectroscopy has been developed for its potential application to high resolution infrared spectroscopic studies of large radicals. In general, radicals containing more than three atoms heavier than hydrogen can not be studied in a room temperature cell using high resolution techniques. For such large species, the infrared spectrum becomes congested and unresolvably complex because of the presence of overlapping rotational lines and vibrational hot bands. By cooling radicals in a supersonic expansion, excited rotational and vibrational levels are depopulated, giving simplified spectrum. In this technique, radicals are produced inside a slit supersonic nozzle by 193 or 248 nm excimer laser photolysis of a gas mixture consisting of 1% suitable precursor seeded into 1-11 atm carrier gas (typically helium). To reduce the vibrational temperature of the hot radicals produced upon photolysis, the radicals are thermalized by collisions with the room temperature helium inside the slit thermalization region before expansion. The radicals are then cooled rotationally in the subsequent expansion, and their transient absorption is probed downstream of the slit orifice by a tunable, computer-controlled color center laser. The jet-cooled infrared spectroscopy technique was first tested on small radicals with known high resolution spectra. Small radicals such as NH$\sb2$, OH, and CH$\sb3$ have been observed in the jet with excellent sensitivity and low rotational temperatures. Rotational temperatures ranging from 13-25K and signal-to-noise ratios of 30-150 have been obtained for these radicals. Additionally, rotational temperatures of 10K have been observed for trans-nitrous acid, a stable species produced upon photolysis of nitric acid in the jet. Jet-cooled infrared spectroscopic studies of larger radicals were initiated since the technique proved successful in the production, cooling, and detection of small radicals. Spectroscopic searches were made for CH$\sb2$OH, t-HOCO, CH$\sb3$NH, C$\sb2$H$\sb3$, C$\sb3$H$\sb5$, and OH-Ar (radical van der Waals complex). Although no new radical species were conclusively observed in the jet photolysis experiments, the initial results of searches for these radicals, including the observation of several stable molecules produced upon photolysis of various precursors, have been described.

The use of spin-labelling to study the dynamics of He (2 $\sp3$S) metastable atom ionization of atoms and simple molecules is described. Briefly, He (2 $\sp3$S) atoms created by a microwave discharge are spin-polarized in a flowing afterglow via optical pumping with circularly-polarized 1.08 $\mu$m (2 $\sp3$S) $\Rightarrow$ (2 $\sp3$P) radiation from an LNA laser. A target gas is injected into the flowtube, and electrons created in Penning ionization reactions diffuse through a differentially-pumped aperture and are energy- and spin-analyzed with a hemispherical energy analyzer in series with a retarding-potential Mott polarimeter. Data are reported for Ar, CO$\sb2$, CO, H$\sb2$O, O$\sb2$, NO, NO$\sb2$, SO$\sb2$, and Cl$\sb2$ target gases. The generally accepted model of Penning ionization, the so-called exchange model, suggests that a target electron of appropriate spin tunnels to fill the He 1s hole with simultaneous ejection of the He 2s electron, which, for polarized He (2 $\sp3$S) atoms, produces fully polarized electrons. It is found that fully polarized electrons are ejected in reactions with closed-shell, negative electron affinity targets such as Ar and CO$\sb2$, independent of the positive ion final state, in agreement with the exchange model. Substantially lower polarization is measured for open-shell targets, such as O$\sb2$ and NO, and for targets with large electron affinity, such as NO$\sb2$ and SO$\sb2$. A strongly attractive entrance channel potential is possible in these cases. The exact depolarization mechanism is unclear, but is probably related to the formation of the ionic quasi-molecule. For some targets (Cl$\sb2$, for instance), excitation transfer with subsequent autoionization of the core-excited Rydberg target state created occurs, and these reactions also produce electrons with reduced polarization.

A new method for the treatment of holonomic constraints in molecular dynamics simulation has been developed. In this method, constraint forces are solved explicitly. The method explores the special mathematical property (sparse and symmetric positive definite) of an matrix inherent to the molecule under study. The method is especially useful when some bond angles are constrained. A lattice model for proteins is proposed. The connection between implicit-solvent model and explicit-solvent model has been established. The structural features of globular proteins are studied by the model. It is found that the proteins are driven to compact conformations by strong water-water interactions. The constrained molecular dynamics simulations are applied to a short chain in solvent to infer the possible move set for Monte Carlo simulations. It is found that the end monomers should be more mobile than the inner ones and that one and two-monomer moves are the integral part of the set of basic moves. The moves involving four or more monomers can be modeled as the result of several basic moves.

Absolute differential cross sections for electron capture and loss by neutral hydrogen atoms incident on various gases are presented. The measurements cover a laboratory angular range of 0.02$\sp\circ$ to 1.77$\sp\circ$ and a laboratory energy range of 2.0 to 5.0 keV. The target gases include H$\sb2$, O$\sb2$, N$\sb2$, Ar and He. Integrated experimental cross sections are compared with total cross sections reported by other investigators.

A new FT-ICR apparatus (Fourier Transform - Ion Cyclotron Resonance) has been developed, where clusters produced in a laser vaporization supersonic cluster source were efficiently injected and trapped in an elongated cylindrical Penning type ion trap located in the center of a magnetic field of 6 Tesla. The successful injection of multiple cluster pulses into the ion trap makes the experiment largely independent of the intensity of the supersonic cluster source. Long trapping times of up to 30 minutes and a high mass resolution are additional features of this apparatus. In a first application, the apparatus was used to systematically investigate the chemisorption behavior of positively-charged group V and group IX transition metal cluster ions towards H$\sb2$. The absolute rate constant of the chemisorption process for the addition of the first hydrogen molecule as well as the saturation values for the complete hydrogenation of the clusters were measured. In the cases of Vanadium, Niobium and Cobalt, the reactivity of the positive cluster ions were then compared to the reactivity of the neutral cluster species. The patterns in reactivity for the positive as well as for the neutral clusters showed striking similarities. This led to the conclusion that the reactivity of these clusters is by and large independent of their charge state. Within the niobium and tantalum cluster series isomeric forms of a particular cluster species were found exhibiting a large difference in reactivity towards hydrogen. In the third series of cluster experiments performed on the FT-ICR apparatus, the chemisorption of CO on positively-charged group IX transition metal cluster ions was investigated. In the case of rhodium, the measured saturation values for these gas phase cluster carbonyls scaled parallel to the saturation values found for the rhodium carbonyl clusters in the liquid phase indicating similar structures for both gas phase as well as liquid phase clusters. In the presence of oxygen which is dissociatively chemisorbed on the clusters' surface, formation and desorption of CO$\sb2$ were found during the CO chemisorption process on all three group IX metals.

The photoexcitation spectra of transition region species formed in bimolecular reactions of K + NaX (X = Cl, Br, I) have been observed by measuring the intensity of sodium D line emission at 589.0 nm as a function of excitation wavelengths between 595 and 640 nm. The portion of the spectrum measured for the K + NaCl system is qualitatively similar to that previously observed by Magurie, et al (MAG86). The spectra obtained for the K + NaBr and NaI systems are significantly different than that of the K + NaCl system, and show a distinct feature centered at approximately 610 nm, and indicate that the reaction dynamics are quite different for the heavier sodium halide systems. The results of classical trajectory calculations performed using K + NaCl potential energy surfaces suggest that the structure in the K + NaBr and K + NaI spectra is not due to a mass effect; but rather from unique features of the potential energy surfaces for these reactions which are, as yet, unknown. Because of insufficient theoretical and experimental information, interpretation of these results in terms of the dynamical processes in these reactions is not yet possible. However, several different possible mechanisms of this structure are discussed which can be experimentally tested. Results of proposed future experiments should be able to provide the necessary information to understand the nature of these spectra.