Rb-He exciplex formation on helium nanodroplets

Rubidium atoms have been deposited on helium nanodroplets and optical excitation and emission spectra of the 5 P-2-5 S-2 transition have been measured and interpreted. After laser excitation of the 5 P-2(3/2) state, fluorescence from a (RbHe)-He-* exciplex is observed while pumping the D-1 (5 P-2(1/2)-5 S-2) line yields only emission from free rubidium atoms. This observation is in agreement with the predictions from a recent model by Reho [J. Chem. Phys. 113, 9694 (2000)] about the extraction of an alkali-helium exciplex from the doped helium nanodroplet surface. A high barrier along the He-n-He-Rb-* axis of the 1 (2)Pi (1/2) potential prevents desorption of (RbHe)-He-* within the excited state lifetime, whereas the 1 (2)Pi (3/2) potential permits the exciplex extraction. The excitation spectrum, on the other hand, reflects the structure of the excited states 1 (2)Pi (1/2), 1 (2)Pi (3/2), and 2 (2)Sigma (1/2) of the HenRb complex whose potential surfaces will depend strongly on the alkali-He interaction dynamics near the droplet surface. For a heavy surface dopant like Rb or Cs the droplet surface will be strongly distorted upon vibrational excitation of the dopant. Some of the consequences for the potentials are discussed for the example of the 1 (2)Sigma (1/2) ground state. (C) 2001 American Institute of Physics.