Formation ofK*HeExciplexes on the Surface of Helium Nanodroplets Studied in Real Time

Superfluid helium nanodroplets are doped with potassium atoms to form complexes with the alkali atom residing on the surface of the droplets. Dispersed laser-induced fluorescence spectra of such systems already revealed the formation of $M^{*}\mathrm{He}$ ( $M=\mathrm{Na},\mathrm{K}$) 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 ${\mathrm{K}}^{*}{\mathrm{He}}_{n=1\mathrm{}}$ to be formed within 180 fs. Furthermore, the existence of exciplexes with $n>\mathrm{}1\mathrm{}$ is quantified suggesting that the first ring around the potassium atom contains four helium atoms.