15min:

FLORIAN LACKNER, GÜNTER KROIS, MARKUS KOCH AND WOLFGANG E. ERNST, Institute of Experimental Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria.

Rubidium atoms on the surface of superfluid helium droplets have been excited into Rydberg states. The excitation spectrum of the Rb-He_N system has been recorded from the 5²D state manifold up to the ionization threshold by resonant three-photon-ionization time-of-flight spectroscopy. The observation of droplet size dependent shifts of excited states with respect to bare atom states is explained by a decreased quantum defect and a lowered ionization threshold. Within the scope of a Rydberg model we demonstrate that quantum defects and ionization thresholds are constant for each specific Rydberg series, which confirms the Rydberg character of excited Rubidium states on helium droplets. A set of six Rydberg series could be identified. Individual Rydberg states are observed with effective principle quantum numbers up to n^* ~ 19 and l \leq 3, for which the expectation value of the electron orbital radius is about ten times larger than the droplet radius.

% S.~Fei, X.~Zheng, and M.~C.~Heaven, Chem.~Phys.~Lett. 176, 373