In the present work very-high-n Rydberg-atoms $(n \sim 100 - n \sim 400)$ are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low $\rm({\sim}85\mu eV - {\sim}1.4meV),$ analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\sim$1$\sp\circ$K, which are far below those accessible using any alternate approach. Rate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions $$\rm K({\it np\/}) + XY \to K\sp+ + (XY)\sp{-*}\ or\ \to K\sp+ + X\sp- + Y\eqno(1)$$and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization $$\rm K({\it np\/}) + XY({\it J\/}) \to K\sp+ + {\it e}\sp- + XY({\it J\/}-1)\eqno(2)$$ The n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process. The essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\eqalignno{&e\sp- + \rm XY \to (XY)\sp{-*}\ or\ \to X\sp- + Y&(1\sp\prime)\cr &e\sp- + \rm XY({\it J\/}) \to {\it e}\sp- + XY({\it J\/}-1)&(2\sp\prime)\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.

In the first use of the Raman effect to detect sulfur atoms, transient CARS spectroscopy has revealed novel relaxation effects in sulfur formed through multiple photon excitation of carbon disulfide vapor. Ground term $\sp3P$ sulfur was monitored through its 396 cm$\sp{-1}$ and 573 cm$\sp{-1}$ fine structure transitions ($\sp3P\sb1\leftrightarrow\ \sp3P\sb2$ and $\sp3P\sb0\leftrightarrow\ \sp3P\sb2$) at various delays after the photolysis laser pulse. It was found that quenching of $\sp1D\sb2$ sulfur to the $\sp3P$ term by collision with the rare gases argon, krypton, and xenon gives clear kinetic and spectral evidence of a population inversion between the $\sp3P\sb0$ and $\sp3P\sb2$ fine-structure levels. Kinetic data indicate no such inversion between the $\sp3P\sb1$ and $\sp3P\sb2$ levels. Extensive modeling of the kinetic data taken at the $\sp3P\sb0\leftrightarrow\ \sp3P\sb2$ transition was performed to obtain the branching ratio into the $\sp3P\sb0$ level for the three rate gases studied. Although kinetic models including all possible processes in this system contain too many unknown parameters to be useful, a simple three parameter model gives reasonable fits to the data. This model yields branching ratios of 0.83, 0.75, and 0.73 for the fractional formation of sulfur's $\sp3P\sb0$ level through quenching from $\sp1D\sb2$ by Ar, Kr, and Xe, respectively. The results of MCSCF-CI calculations of the relevant low-lying Ar-S potential curves suggest that quenching proceeds through a single energetically accessible intersection between molecular terms, which, when correlation and coupling rules are considered, leads adiabatically to the $\sp3P\sb0$ product level that is experimentally observed to dominate. Although calculated Xe-S and Kr-S potential energy curves are not available, comparison with the similar rare gas oxide system suggests that a general explanation for the selective quenching mechanism may involve differences in the spin-orbit coupling strengths between molecular terms at the two crossings that adiabatically correlate with the $\sp3P\sb0$ and $\sp3P\sb2$ levels.

This thesis presents a description of a novel technique for the accurate experimental determination of absolute partial electron impact ionization cross sections of atoms and molecules by a calibrated time of flight mass spectrometer. The simple design of the present apparatus insures the total collection of the energetic fragment ions. Measurements for argon and nitrogen targets are presented over a range from threshold to 1 keV. Previously published electron impact data on argon, nitrogen, and oxygen are reviewed. Computer programs for the modeling of the data and fragment ions energy analysis are also presented.

Thermal energy collisions of K(nd) Rydberg atoms with electron attaching molecules which result in reactions of the following types K(nd) + SF$\sb6$ $\to$ K$\sp+$ + SF$\sb6\sp-$ K(nd) + CF$\sb3$I $\to$ K$\sp+$ + I$\sp-$ + CF$\sb3$ have been investigated at intermediate n. Using the field ionization technique, both rate constants for Rydberg atom destruction and for formation of free negative ions are directly measured for n as low as 12. The data show that, at intermediate n, the rate constants for Rydberg atom destruction start to decrease with decreasing n and to deviate from those predicted on the basis of the simple free electron model. A semi-classical model is presented to explain this decrease. It reveals explicitly the relationship between the rate constant for Rydberg atom destruction and the rate constant for free electron attachment. The data also show that, at intermediate n, only a fraction of the Rydberg electron attachment events leads to formation of free negative ions. Post-attachment interaction between the positive and negative ions formed is investigated. The kinetic energy distribution of the ion pairs in their center-of-mass frame and their separation probability are calculated. The angular dependence of the kinetic energy distribution and the separation probability are also studied. The present experiments, in conjunction with the theoretical calculations, advance understanding of the collisions substantially. Investigation of Zeeman-effect quantum beats observed in the selective field ionization spectra of D state potassium Rydberg atoms in also presented.

The formation of long-lived ($\tau$ $\gg$ 10 $\mu$sec) product ions through associative ionization reactions of the type: K(nd) + BC $\to$ (KBC)$\sp+$ + e$\sp-$ has been investigated for 8 $\leq$ n $\leq$ 18, using a variety of polyatomic target molecules (denoted BC). Although such reactions clearly involve the Rydberg core ion, the presence of the Rydberg electron is also essential to stabilize the collision product. Rate constants for these reactions have been measured with the mixed-gas technique and are compared with those predicted by ion-molecule reaction theories. Studies with a number of different target molecules have been undertaken, so as to investigate the factors that influence the rate constants for associative ionization. In particular, it is observed that the rate constants increase with both the dipole moment and complexity of the target, and they also depend strongly on the principal quantum number, n, of the Rydberg state involved.

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.

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 theoretical discussion of laser cooling of multilevel atomic systems using intense, blue detuned laser beams is given. A method based on matrix continued fractions is presented, which enables the efficient calculation of the semi-classical force on a multilevel system. The method is applied to a three-level system driven by one standing wave and one travelling wave. The force curves obtained exhibit strong cooling features. Cooling of the three-level system is simulated using the fully quantum mechanical Monte Carlo wave function technique. The simulation predicts efficient cooling to sub-Doppler temperatures. The three-level model is related to a multilevel cooling experiment performed on $\sp7$Li. The experimental results are found to be in reasonable agreement with the three-level model. However, a comparison of the simulation and experiment for a two-level atomic system reveals significant discrepancies, which raises questions about the model and experiment.

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.

Dissociative and non-dissociative electron attachment processes are investigated via electron transfer in collisions between velocity selected Rydberg atoms and target molecules. Measurements of the velocity distribution of the reaction products and of the decay of product negative ions during passage to the detector provide information about the energetics of the interactions and about the lifetime of the intermediate negative ion formed. Collisions with CBrCl 3 result in a dissociative process producing Br- and Cl- as products. Br- production occurs through two reaction channels. The intermediate negative ion dissociates immediately in one channel, but the second channel involves a long-lived intermediate having a lifetime of &sim;5 ps. A single reaction channel produces Cl -, but it is uncertain if the process is immediate or if it occurs after a few vibrational periods of the intermediate ion. Non-dissociative electron attachment occurs with C7F14 and with C 6F6. Collisions with C7F14 lead to the production of free electrons and of long-lived (tau > 100 mus) C 7F14-- ions. The free electrons, which result from autodetachment, are all created within a very short time period (tau &lsim; 15 ps) following initial electron capture. Collisions with C6F6 result in the formation of C6F 6-* ions that have a broad range of lifetimes against autodetachment. Data analysis and model calculations are discussed in addition to possible theoretical models explaining the results. Future experimental studies are also proposed.