Two-step excitation of Rb atoms on He nanodroplets

We present the first sequential laser excitation of atom-doped helium nanodroplets. Rubidium atoms on the surface of helium nanodroplets are selectively excited with a continuous wave laser to the 5(2)P(1/2) state so as not to desorb from the nanodroplets. From there they are excited by a laser pulse to the 5 2 D state; a laser-induced fluorescence (LIF) spectrum is recorded by monitoring the 6(2)P -> 5(2)S(1/2) emission. The LIF spectrum differs from that of the two-photon one-color direct excitation spectrum 5(2)D <- 5(2)S(1/2), indicating that the system does relax vibrationally during the lifetime of the 5(2)P(1/2) state. To model the LIF spectra we use calculated energy levels of the Rb atom as a function of its distance R from the center of the helium nanodroplet. The Franck-Condon factors of the resulting potential energy curves agree with the experimental spectra. In the future the 5(2)P(1/2) state can be used as a springboard to reach high-lying S-2 and D-2 states, and possibly create an artificial super-atom.