Studies of elementary collision processes under single collision conditions (1999)
AuthorsVallance, Claireshow all
Absolute electron impact ionization cross sections have been measured for CH₄ and the methyl halides CH₃F, CH₃Cl, CH₃Br and CH₃I. A theoretical model has been developed which predicts absolute maximum cross sections in good agreement with these measurements and also with results published in the literature for a range of small molecules. In addition, the model is able to qualitatively reproduce the recently observed orientation dependence of the electron impact ionization process. The dependence of the ionization cross section on molecular polarizability has been investigated and general expressions for the calculation of maximum electron impact ionization cross sections have been deduced. State specific cross sections have been measured for collisional attenuation of upper Stark state selected beams of CH₃F with a range of scattering gases inside a hexapole collision chamber. Cross sections are found to lie in the range between 246 and 654 ∓. This corresponds to a long range interaction involving the transfer of tiny amounts of energy, consistent with ΔM₁ = ∓1 rotational transitions. The parameters affecting orientation of upper Stark state selected beams have been investigated. The minimum uniform electric fields required to maintain spatial orientation for beams of CH₃F, CH₃Cl, CH₃Br and CH₃I have been measured and rationalised in terms of the hyperfine interaction between the rotational angular momentum and nuclear spin of the beam molecules. The time for which orientation is retained under field free conditions has been considered. A crossed beam ion imaging experiment has been designed and constructed for the investigation of collisions between electrons or ions and spatially oriented molecules. Software for analysing the data obtained from these experiments has been developed and tested on simulated images.