High resolution studies of electron excitation II. The 3 3 D, 4 3 D and 4 1 D states of helium: features in the n = 4 excitation functions

We present excitation functions of the 4 $^{1}$D, 3 $^{3}$D and 4 $^{3}$D states of helium for energies within about 1 eV of threshold. We find a series of broad features and some sharp features and discuss their interpretation. The broad features appear to be shape resonances and to occur, at least in part, as a result of cascade from F states. We suggest that the prominent features in the 4S excitation functions are due to Feshbach resonances having configurations 1s 5s$^{2}$ and 1s 5p$^{2}$. We interpret a very small feature at the threshold of both 4D functions as a 1s 4d$^{2}$ Feshbach resonance.