Evidence is presented of an unusual Rydberg system consisting of a helium nanodroplet containing a positively charged sodium ion and an orbiting electron. Rydberg states of this system with principal quantum number $n<20$ are found to be unstable on a nanosecond time scale. In contrast, Rydberg states with $n\gtrsim 100$ are found to have a lifetime of $1.1\mu \mathrm{s}$. In addition, it is found that the ionization threshold of sodium-doped helium is broadened and redshifted with respect to that of the free atom. These observations are successfully reproduced using a pseudodiatomic description of the system in which the interactions of the sodium and its ion with the helium are calculated as the sum of pair potentials.

• Received 10 December 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.083401