REACTIONS OF EXCITED ATOMS AND MOLECULES WITH ATOMS AND MOLECULES .2. ENERGY ANALYSIS OF PENNING ELECTRONS

Energy distributions of electrons ejected in thermal collisions of He(21S) and He(23S) metastables with Ar, Kr, Xe, and Hg are measured using a retarding electric field in combination with an electrostatic lens. The distributions are broadened and shifted with respect to the energy difference (E*-IP) between excitation energy of the metastable (E*) and the ionization energy of the target atom (IP), due to the mutual interaction. The shapes and shifts of the distributions are very different for He(21S)- and He(23S) metastables and show a marked change with the relative energy of the collision. The shifts ΔE=Eel-(E*-IP) vary from +45 mV for Ar to -50 mV for Hg as the size of the target atom is increased. The ratio of associative- to Penning ionization, approximately predictable from the shifts, is in agreement with the mass spectrometrically determined ratio. The molecular ions are formed in highly vibrationally and rotationally excited states with binding energies varying from zero to thermal energies. Details of the distributions are discussed in terms of potential curves. It is shown, that the known experimental facts on Penning ionization are best interpreted, if it is assumed that the process takes place by electron exchange between metastable- and target atom. Based on this assumption an approximate expression for the Penning cross section is derived. © 1969 Springer-Verlag.