Cross sections for simultaneous 21P excitation of two colliding He atoms to various substate combinations

The simultaneous excitation of both collision partners to the 21P state in He-He collisions at 1250 eV has been studied by measuring (in a coincidence experiment) the angular correlations between the two UV photons emitted by the two simultaneously excited atoms in their decay to the ground state. A theoretical expression for the angular correlation function in terms of the various collision parameters is presented and is used to extract the available information from the experimental data. The results are not in agreement with conclusions which can be drawn from a simple reasoning based on the correlation diagram for the He-He system. (author)