The Stark effect on the doubly excited states of helium below the $N=2$ threshold has been studied by vacuum ultraviolet fluorescence yield spectroscopy. Two new series of states are observed at moderate fields ($<10\mathrm{kV}/\mathrm{cm}$), and assigned to the previously unobserved even ${}^{1}P^e$ series, and a group of ${}^{1}D^e$ series. The ${}^{1}S^e$ states are observed indirectly via their mixing with nearby ${}^{1}P^o$ states. The observations at moderate field contradict theoretical predictions that field strengths about an order of magnitude greater are necessary to observe the Stark effect on He doubly excited states at low quantum numbers.

• Received 24 August 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.093001