Femtosecond pump–probe spectroscopy has been used to study vibrational dynamics of potassium dimers attached to superfluid helium nanodroplets. Comparing the measured data with theoretical results based on dissipative quantum dynamics we propose that the most important effect of the helium environment is a general damping of the vibrational dynamics. The calculations allow us to explain crucial experimental findings that are unobserved in gas phase measurements. Remarkably, wave packet velocities of such alkali dimer vibrations are just about in the range of the Landau critical velocity of superfluid helium. We therefore also discuss a possible influence of superfluidity on the induced wave packet dynamics.