Photodissociation of oriented molecule beams and reactive scattering

The requirement of proper mutual orientation of reagents is second only to the energy requirement for chemical reaction to take place. Since the introduction of hexapole field focusing and orientation of symmetric-top molecule in 1965, considerable insight has been gained on the role of orientation in reactive scattering dynamics. To achieve a more quantitative understanding of the steric effect in reactive scattering, a pulsed supersonic molecular beam machine with a 3-meter long, electrostatic hexapole field has been constructed. Focusing and rotational state selection has been achieved for prolate symmetric-top molecules, such as CH{dollar}sb3{dollar}X (X = F, Cl, Br and I). Partial state selection has been achieved for oblate symmetric-top molecules. Various polar asymmetric top molecules have also been focused, as well as the pseudo-symmetric tops, (CH{dollar}sb3{dollar}){dollar}sb3{dollar}CI and (CH{dollar}sb3{dollar}){dollar}sb3{dollar}CBr.; These molecules have been studied in some detail. They have been focused and oriented with a homogeneous field. The degree of laboratory orientation has been measured for the first time with a newly developed polarized laser photofragmentation technique. It is found that the goodness of orientation achievable has the following sequence: (CH{dollar}sb3{dollar}){dollar}sb3{dollar}CBr {dollar}>{dollar} (CH{dollar}sb3{dollar}){dollar}sb3{dollar}CI {dollar}>{dollar} CH{dollar}sb3{dollar}I.; Various techniques of producing reactive atomic beams have been explored and tested, including a seeded supersonic nozzle metal beam source, a seeded laser evaporation metal beam source, a Cl atom source utilizing microwave discharge, and an effusive metal beam oven. The effusive oven of Sr produces very stable beams and has therefore been used in a study of the crossed beam reaction of Sr + CH{dollar}sb3{dollar}I.; In order to detect weak product signal (SrI) from the crossed-beam reaction, a sensitive detection technique has been developed utilizing single photon ionization of SrI with 193 nm excimer laser radiation and time-of-flight mass spectrometry. The SrI{dollar}sp{lcub}+{rcub}{dollar} signal is readily detected. The much stronger Sr{dollar}sp{lcub}+{rcub}{dollar} ion beam (from the photoionization of the Sr reagent) is deflected by a gated high voltage pulse. By changing the carrier gas of the seeded, pulsed CH{dollar}sb3{dollar}I beam to achieve different collision energies, the excitation function of Sr + CH{dollar}sb3{dollar}I reaction has been measured. The reaction threshold is estimated to be 0.07 {dollar}pm{dollar} 0.02 eV, in accord with expectation based on analogous reactions and semi-empirical theory.