Formation of K*He exciplexes on the surface of helium nanodroplets studiedin real time - art. no. 153401

Superfluid helium nanodroplets are doped with potassium atoms to form compl exes with the alkali atom residing on the surface of the droplets. Disperse d laser-induced fluorescence spectra of such systems already revealed the f ormation of M*He (M = Na, K) exciplexes upon electronic excitation [Reho et al., Faraday Discuss. 108, 161 (1997)]. By means of femtosecond pump-probe spectroscopy, this formation process now is followed in real time. We find K*He-n=1 to be formed within 180 fs. Furthermore, the existence of exciple xes with n>1 is quantified suggesting that the first ring around the potass ium atom contains four helium atoms.