Combined effect of Stark and singlet-triplet mixing on photon-yield spectra of singly excited helium

We present a theoretical framework for the calculation of photon-yield spectra from radiative-cascade decays of photoexcited helium atoms in a static electric field for incident photon energies below the first (N = 1) ionization threshold. The present theory takes full account of the singlet-triplet mixing among the singly excited states with principal quantum numbers n less than or similar to 10. The model predicts the enhancement of transition probabilities to triplet final states due to the field-induced mixing. In particular, for a field strength of 9.17 kV/cm, polarization of incident photons perpendicular to the field, and a 4 meV broad excitation function tuned to the 1s6p P-1 states, the 1s6l -> 1s2p P-3 and 1s5l -> 1s2p P-3 emitted photon yields are predicted to be about 70 times and 20 times lower, respectively, than the photon yields for transitions to the corresponding singlet final states.