A propensity rule indicated by the observed dominance of substate populations for double-atom excitation in He-He collisions

Abstract

A new interpretation is given to experiments on double-atom excitation in He-He collisions that were focused on the relative population and coherence of atomic substate combinations. It is shown that a proposed two-state propensity rule may describe basic characteristics of the measurements. More generally by choosing the quantization axis perpendicular to the beam axis it can be understood why the two-state propensity model is successful. It also turns out that the diagonal elements of the new density matrix are independent of the kinetic energy. From experimental excitation parameters the angular part of the atomic charge density for double-atom excitation is viewed by fixing the direction of one of the excited electrons and showing the angular charge distribution of the other. The shape of the atoms changes dramatically with the collision energy, especially if the direction of the first electron is 'frozen' in the direction of the initial relative velocity.