ELECTRON-MOLECULE COLLISIONS AT VERY-LOW ELECTRON ENERGIES

Recent experimental advances that allow study of electron-molecule collisions at electron energies extending down to a few mu eV are reviewed. The new capabilities they afford are illustrated by considering collisions with molecules that attach low-energy electrons and with targets that have permanent dipole moments. Two different experimental approaches are discussed. The first makes use of free electrons produced by near-threshold photoionization. The electron energy can be controlled by varying the photon energy and, with the use of lasers, photoelectron energy resolutions as low as similar to 50 mu eV can be achieved. Low-energy electron scattering processes can also be investigated using atoms in high Rydberg states. For sufficiently large values of principal quantum number n, the excited electron and core ion can be considered as independent particles and the atom viewed, in essence, as a microscopic low energy electron trap with the trapping potential provided by the core ion. Measurements with very high n atoms, n greater than or similar to 1000, permit study of electron-molecule interactions at electron energies down to similar to 4 mu eV. The use of Rydberg atoms with low-to-intermediate values of n to probe the dynamics of dissociative electron attachment is also discussed with emphasis on determination of the lifetime of the excited intermediate and on how the excess energy of reaction is distributed between internal and translational motions of the products. The creation of weakly bound negative ions in Rydberg atom collisions is described focusing on low-energy electron attachment to van der Waals clusters and the formation of dipole-bound negative ions.