Stark manifolds and electric-field-induced avoided level crossings in helium Rydberg states

The linear Stark effect in 1snp ${}_{}{}^1{}_{}{}^{}$,3P Rydberg states (n≊40) of the fundamental two-electron atom helium was studied with the resolution of cw laser spectroscopy. The evolution of the angular-momentum manifolds was followed up to the regime where Stark states originating from different n values interact. Narrow avoided level crossings were detected with high precision. Stark manifolds were also calculated by diagonalization of the complete energy matrix in the presence of an electric field. In these calculations, even at moderate values of the field up to five n values have to be included to accurately reproduce the experimental data.