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Helium nanodroplets doped with alkali atoms and molecules are intriguing systems at the border between gas-phase and condensed matter systems. Upon femtosecond laser excitation, alkali molecules exhibit vibrational wave packet oscillations which are only weakly affected by the helium environment. Subsequent desorption of the molecules off the helium droplets then leads to undamped wave packet motions. This allows for high-resolution Fourier spectra of vibrational levels in the electronic ground and excited states, as demonstrated e. g. for Rb2 and Rb3. The absence of vibrational damping in the triplet ground state of K2 in contrast to damped wave packet motion in electronically excited states is discussed in the context of Landau's critical velocity and frictionless motion in superfluid helium on the nano scale. |
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