ELECTRON-MOLECULE COLLISIONS AT VERY-LOW ELECTRON ENERGIES

Recent experimental advances that allow study of electron-molecule col lisions at electron energies extending down to a few mu eV are reviewe d. The new capabilities they afford are illustrated by considering col lisions with molecules that attach low-energy electrons and with targe ts that have permanent dipole moments. Two different experimental appr oaches are discussed. The first makes use of free electrons produced b y near-threshold photoionization. The electron energy can be controlle d by varying the photon energy and, with the use of lasers, photoelect ron energy resolutions as low as similar to 50 mu eV can be achieved. Low-energy electron scattering processes can also be investigated usin g atoms in high Rydberg states. For sufficiently large values of princ ipal quantum number n, the excited electron and core ion can be consid ered as independent particles and the atom viewed, in essence, as a mi croscopic low energy electron trap with the trapping potential provide d 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 atom s with low-to-intermediate values of n to probe the dynamics of dissoc iative electron attachment is also discussed with emphasis on determin ation of the lifetime of the excited intermediate and on how the exces s energy of reaction is distributed between internal and translational motions of the products. The creation of weakly bound negative ions i n Rydberg atom collisions is described focusing on low-energy electron attachment to van der Waals clusters and the formation of dipole-boun d negative ions.