Double Excitations of Helium in Weak Static Electric Fields

A dramatic electric field dependence has been observed in the photofluorescence yield spectrum of the doubly excited states in helium, where a rich phenomenology is encountered below the $N=2$ threshold. Fluorescence yields of certain states can be tuned to zero, while other dipole-forbidden states are significantly enhanced, for fields much weaker than 1 kV/cm. Using an R-matrix multichannel quantum defect theory, spherical-to-parabolic frame transformation method, we are able to reproduce the main features of the observed spectrum, and we explain the qualitative behavior in terms of weak electric field mixing.