Molecular beam studies of excitation and electron transfer reactions

Abstract

Two studies were performed using crossed molecular beams. The first system studied was the reaction $\rm Na\sp* + KBr \to NaBr + K\sp*,$ determining how fine structure is transmitted through a reactive collision. Each fine structure state of Na$\sp*(3\sp2$P) is separately laser excited, and the fluorescence from the two fine structure states of K$\sp*(4\sp2$P) are separately monitored. The observed K* fine-structure state distributions were not simply statistical. While the product K* fine-structure states were statistically populated for excitation to Na$\rm\sp*(P\sb{1/2}),$ they were not for excitation to Na$\rm\sp*(P\sb{3/2}).$ These distributions were interpreted in terms of nonadiabatic interaction along different regions of the KBrNa molecular potential energy surfaces. These nonadiabatic interactions were also used to help explain the differing fine-structure state populations produced in the previous NaBr + K transition state spectra. A hyperthermal seeded supersonic alkali atom source was designed and constructed for use in collisional ionization experiments. The intensity of the new source was found to be ${\approx}10\sp5$ greater than the previous charge exchange source in the energy range of interest. This source was then used to determine preliminary appearance thresholds for collisional ionization between potassium and rubidium atoms and some molecules. From the thresholds, electron affinities for SF$\sb6$ and CF$\sb3$Br and the bond dissociation energy for the CH$\sb3$Br bond could be obtained. These values were in good agreement with the literature values, although the electron affinity for CF$\sb3$Br was slightly higher (1.06 $\pm$ 0.10 eV) than the previous result in the literature (0.91 $\pm$ 0.20 eV). The effect of electronic excitation of the alkali atom on collisional ionization was also explored. The cross section for the reaction of excited state $\rm Rb\sp* + SF\sb6$ appears to be less than that for the ground state reaction. This reduction in cross section suggests that the excited state crossing can be considered to be nearly completely nonadiabatic for the experimental conditions. This result was reproduced in Landau-Zener calculations of the nonadiabatic probabilities and the ratio of the cross sections.