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By use of an rf electric resonance technique, the fine structure of the \s4f multiplet of He I and its splitting in a static electric field have been investigated. Our results for the fine-structure intervals v u between the l F 3 level and the 3 Fj levels are v l2 =232.8(2.0) MHz, v u =490.8(2.0) MHz, and vi3=704.3(l.0) MHz. Furthermore, a singlet-triplet mixing ratio 0=0.757(3) and the exchange interaction 2^ = 158(3) MHz have been derived from the measurements.

Abstract. The magnetic depolarization of the impact radiation at 2(41D-2tP)=492nm and 2(31D-2~P) = 668 nm as well as the splitting of the magnetic depolarisation signals in electric fields were investigated for t0-25 keV H~-(D~-)-He collisions (only 492 nm line) and for 5-22.5 keV He-He collisions (both lines). The results are compared with analogous measurements for He + -He and Ne + -He collisions and the dynamics of the excitation process are qualitatively discussed. In particular, we emphasize the importance of inertia for the evolution of the electron cloud during the final phase of the collision process.