15min:

MORITZ THEISEN, FLORIAN LACKNER AND WOLFGANG E. ERNST, Institute of Experimental Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria; FRANCESCO ANCILOTTO, Dipartimento di Fisica `G. Galilei', Università di Padova, via Marzolo 8, 35131 Padova, Italy; CARLO CALLEGARI, Sincrotrone Trieste, Strada Statale 14 - km 163.5, 34149 Basovizza, Trieste, Italy.

We present the first sequential excitation of atom-doped helium nanodroplets. Rubidium atoms on the surface of a helium nanodroplet are selectively excited to the 5²P_1/2 state so as not to desorb from the droplet. From there they are excited by a laser pulse to the 5²D state; a laser-induced fluorescence (LIF) spectrum is recorded by monitoring the 6²P 5²S_1/2 emission. We find some difference in the LIF spectrum as compared to that of the two-photon one-color direct excitation spectrum 5²D 5²S_1/2. This indicates that the system does relax vibrationally during the lifetime of the 5²P_1/2 state. To model the LIF spectra we calculate the energy levels of the Rb atom as a function of its distance R from the center of the droplet. The Franck-Condon factors of the resulting potential energy curves agree with the experimental findings.

A similar behavior has been found for cesium. New measurements predict that it also stays bound on the surface of the droplet in its 6²P_1/2 state. From there we further excited Cs monomers into their 6²D state, where also the LIF spectrum is recorded by watching the 7²P 6²S_1/2 emission.

In the future these states can be used as a springboard to reach high-lying ²S and ²D states, and possibly create an artificial super-atom.