Stark effect of atomic helium second triplet series in electric fields up to 1600 kV cm^-1

We present experimental and theoretical investigations of the spectral series 2 ³P-n ³Q (n=3-10, Q=S, P, D, ..., n-1) in electric fields up to 1600 kV cm^-1. Such fields cause - for n>6 - shifts of the upper levels of the observed transitions which are larger than the separation between levels with different principal quantum numbers. The patterns belonging to a certain principal quantum number become similar to hydrogen patterns; they are nearly symmetric and show a nearly linear Stark shift in higher electric fields. The applied fields were high enough that patterns belonging to neighboring principal quantum numbers begin to overlap, which leads to interesting level-anticrossing effects. The experimental results are compared with numerical calculations taking into account mixing between states of different principal quantum numbers and also between singlet and triplet states. The agreement between experimental and theoretical line shifts is quite good.