TRANSITIONS BETWEEN RYDBERG LEVELS OF HELIUM INDUCED BY ELECTRON AND NEUTRAL COLLISIONS

An experimental study of electron- and neutral-induced collisional transfer between Rydberg levels of helium was made for intermediate values (8 to 17) of the principal quantum number. The techniques of time-resolved, laser induced fluorescence spectroscopy were used to follow the collisional depopulation of the laser pumped level as well as transfers between levels of different principal quantum number in a high purity, well diagnosed helium afterglow. Transfers between singlet and triplet subsystems of atomic helium were found to be negligible. Electron-induced collisional transfer follows closely the functional variation of the Mansbach and Keck hydrogenic theory, but experimental values are on the average smaller by a factor of 0. 64 than predicted. Transfers induced by collisions with **3He and **4He atoms are in marked disagreement with Flannery's classical binary-encounter theory for the smaller values of the principal quantum number, although disagreement is less pronounced for higher principal quantum numbers. This fact, and the absence of isotopic effect on rate coefficients, indicate that a simple binary-encounter theory is not suitable for intermediate Rydberg levels, and that interatomic potentials should be included in a two-center, molecular model with an excited electron.