Excitation transfer in the n=4 levels of He following dye-laser pumping to the 41D level in a flowing afterglow

The 4 1 D level in a helium flowing afterglow was populated by two-stage excitation: resonance radiation trapping in a flowing afterglow populated the 2 1 P level which was then pumped with a tunable dye laser. Fluorescence decay of the other n=4 levels was observed, and this excitation transfer is ascribed to neutral collisions. The data is fitted by a simple model giving excitation transfer and associative ionization cross sections. This model suggests that excitation to the triplet levels only occurs via the F states and that the selection rule Delta L=+or-1 is preferred for all transitions.