Part I. Crossed molecular beam methods have been used to measure the differential elastic scattering of molecular hydrogen and deuterium with a number of diatomic and polyatomic secondary molecules. In particular, H2 + O2, SF6, NH3, H2O, CO and CH4 and D2 + O2, SF6, NH3, and H2O were all studied using thermal energy beams. The H2 + NH3 and H2 + SF6 systems were further studied using an H2 beam cooled to liquid nitrogen temperature. In addition, the H2 + SF6 system was remeasured using a beam of cooled para-hydrogen in place of the normal-hydrogen. These studies cover a wide range of anisotropy, size and initial relative collision energy of the scattering partners, as well as the corresponding de Broglie wavelengths. Each system studied yielded rapid quantum oscillations in the differential cross section which were used to determine central-field intermolecular potentials. These potentials were found to be independent of the energy and the hydrogen isotope used as well as their assumed mathematical form. As a result, the effects of anisotropy on the differential elastic scattering of these H2 and D2 systems do not seem important. Part II. Variable angle electron impact spectroscopy has been applied to the systematic study of the electronic structure of the fluoroethylenes. Excitation spectra were obtained at 40 eV and 20 eV or 25 eV impact energies, and scattering angles from 0° to 80°. Each of the molecules shows an absorption maximum between 4.2 eV and 4.7 eV, corresponding to the singlet → triplet, π → π* transition similar to the N → T transition seen in ethylene. A weak absorption at 6.45 eV observed only in monofluoroethylene is assigned to the second singlet → triplet transition. Also observed in each spectrum is the strong singlet → singlet N → V transition, as well as a number of Rydberg features. Beyond the first ionization potential, a number of broad absorption features are observed in each molecule, corresponding to superexcited state transitions. Using a method based on term values, a number of these transitions have been assigned to Rydberg series converging to higher ionization potentials. The implications of these results for the photochemistry of the fluoroethylenes is also discussed. Chlorotrifluoroethylene has also been studied by the electron impact method, and the results are similar to those found for trifluoroethylene.