Dissociative electron attachment reactions of the type K(nd) + AB $\to$ K$\sp+$ + AB$\sp{-*}$ $\to$ K$\sp+$ + A$\sp-$ + B have been investigated for polyatomic molecules BrCN, CF$\sb2$Br$\sb2$, CF$\sb3$Br and Fe(CO)$\sb5$. Rate constants for the formation of various negative ions were measured for a range of principal quantum numbers n. At high n (n $>$ 40), the Rydberg electron can be treated as 'essentially' free electron and the present data are compared with the free electron data obtained by the TPSA technique. At lower n, the free electron model must be modified to take into account post-attachment electrostatic interactions between the product ions and effects associated with the reduced size of the Rydberg atom. The translational energy release in such reactions was measured using the TOF-PSD technique. Comparison with the excess energy of reaction for BrCN and CF$\sb2$Br$\sb2$ shows that the energy released by electron capture is well distributed among the internal modes of the intermediate negative ion prior to dissociation. In contrast, data for CF$\sb3$Br indicate that the electron capture takes place into a repulsive state resulting in the immediate dissociation of the intermediate negative ion before significant redistribution of excess energy can occur.

$C\sb{60}\sp-$ ion formation has been observed in collisions between K(np) Rydberg atoms with principal quantum numbers $\rm35\leq n\leq125,$ which corresponds to electrons with mean energies in the range $\sim$2 meV to $\sim$170 $\mu$eV, and $C\sb{60}$ molecules. This results from Rydberg electron transfer and has permitted study of the low energy electron attachment properties of Buckminsterfullerene, which is known to capture free electrons with energies up to $\sim$12 eV. Present data agree with a previous Rydberg atom study but disagree with theoretical predictions and also with previous free electron-$C\sb{60}$ studies. Results from both theory and the free electron experiments suggest that a potential barrier of $\sim$240 meV exists for electron capture by $C\sb{60},$ preventing electron capture at very low energies. The present data are compared with previous experiments and with theory, and possible explanations for the observed discrepancy, including evidence for an image-charge-bound state of $C\sb{60},$ are discussed.

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.

A new Ultraviolet Magnetic Time-of-Flight Photoelectron Spectrometer (MTOFPES) has been developed for the study of the electronic structure of clusters produced in a pulsed supersonic molecular beam. This is the first technique which has been successful in probing the valence electronic states of metal clusters. The ultraviolet photoelectron spectra of negative cluster ions of the noble metals have been taken at several different photon energies. These are presented along with the electron affinity and HOMO-LUMO gap measurements for Cu$\sb6\sp-$ to Cu$\sb{41}\sp-$, using 4.66 eV and 6.42 eV detachment energies; Ag$\sb3\sp-$ to Ag$\sb{21}\sp-$, using 6.42 eV detachment energy; and Au$\sb3\sp-$ to Au$\sb{21}\sp-$, using 6.42 eV and 7.89 eV detachment energies. The spectra provide the first detailed probes of the s valence electrons of the noble metal clusters. In addition, the 6.42 eV and 7.89 eV spectra probe the first one to two electron volts of the molecular orbitals of the d valence electrons of copper and gold clusters. The electron affinity and HOMO-LUMO gap measurements of the noble metal clusters agree with the predictions of the ellipsoidal shell model for mono-valent metal clusters. In particular, cluster numbers 8, 20, and 40--which correspond to the spherical shell closings of this model--have low electron affinities and large HOMO-LUMO gaps. The spectra of the gold cluster ions indicate that the molecular orbital energies of the cluster valence electrons are more widely spaced for gold than for copper or silver. This is to be expected for the heavy atom clusters when relativistic effects are taken into account.

Translational energy release in dissociative electron attachment has been investigated at subthermal electron energies through study of electron transfer reactions of the type K($n$d) + RX $\to$ K$\sp +$ + RX$\sp{-\*} \to$ K$\sp +$ + X$\sp -$ + R. The translational energy release is measured using a novel time-of-flight technique developed in the present work that incorporates position sensitive detection of the product ions. Data for CCl$\sb4$ and CFCl$\sb3$ show that the energy released by electron capture is well-distributed among the internal modes of the intermediate negative ion prior to dissociation. In this case, the kinetic energy release distributions are in good agreement with the predictions of quasi-equilibrium theories of unimolecular decay. In contrast, data for CF$\sb3$I and CF$\sb3$Br point to electron capture directly into an antibonding orbital localized on the I or Br atom followed by immediate dissociation before significant redistribution of excess energy can occur. The present results raise questions about the reliability of certain techniques for measurement of low energy electron attachment rate constants.

The ability to map valence electronic structure is the result of a recent advance in photoelectron spectroscopy; its union with cluster molecular beam technology. The task of interpreting the spectra is hampered by a serious lack of understanding of cluster electronic structure in general. Recently progress has been made in finding models for single s valence electron systems. Alkali and noble metal clusters can be treated as free electron systems and simple interatomic potentials can be used with rare gas clusters. Neither a smeared jellium background nor a simple interatomic potential is adequate to describe covalent bonding, however. The isoelectronic Group IV members have a valence configuration of ns$\sp2$np$\sp2$. All readily form clusters, and the elements differ in both their atomic and bulk properties; thus the series provides an ideal system for studying electronic structure. The mass selected cluster ion beam is crossed with a beam (6.42 or 7.9eV) and the resulting photodetached electrons collected with the aid of judiciously arranged magnetic fields. The spectra are found to be unique for each size cluster. Some spectra show a significant gap between the two lowest binding energy features, indicating that the neutral cluster is a closed shell species. The clusters with such gaps are minima in a plot of EA as a function of cluster size. The UPS also vary with the cluster composition. Carbon is unique; an even -odd alternation in electron affinities switches from odd minima for clusters containing less than ten atoms to odd maxima for larger clusters. This corresponds with an alternation in singlet and triplet ground states and a switch from chain to ring structures previously predicted by theory (K. S. Pitzer, E. Clementi, J. Amer. Chem. Soc. 81 4477 (1958) and R. Hoffmann, Tetrahedron 22 521 (1965)). The spectra of the remaining group IV members are remarkably similar to each other for clusters of up to ten atoms, as is the trend in the electron affinities as a function of cluster size. This similarity in electronic structure may imply similarity in geometries. At ten, Si and Ge behavior diverges from that of Sn and Pb.

Studies of intramolecular dynamics during unimolecular reactions are important in characterizing the energy transfer into the reaction coordinate responsible for the reactive event. We investigated the dynamics and mode-mode energy transfer immediately prior to the dissociation of hydrazoic acid and hydrogen peroxide. Hydrazoic acid decomposes into NH and N$\sb2$ when energy is "deposited" in the N-H stretch of the ground electronic state molecule by a pump laser. A specific vibrational motion generates an impulsive energy transfer, which leads to a singlet-triplet transition and subsequent central N-N bond fragmentation. Hydrogen peroxide decomposes into two OH fragments following overtone excitation of an O-H stretch via a specific bending motion capable of producing an impulse of energy flow into the reaction coordinate at the time of the O-O bond dissociation. Modeling the dynamics of overtone-excited HN$\sb3$, and using an existing model for the dynamics of overtone-excited H$\sb2$O$\sb2$, respectively, we first characterize the specificity of the vibrational dynamics preceding the reaction, then establish that these dynamics are stable, that is non-chaotic, and unique in the reaction.

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.