  The vibrational wave-packet dynamics of diatomic rubidium molecules (Rb2) in
triplet states formed on the surface of superfluid helium nanodroplets is
investigated both experimentally and theoretically. Detailed comparison of
experimental femtosecond pump-probe spectra with dissipative quantum dynamics
simulations reveals that vibrational relaxation is the main source of
dephasing. The rate constant for vibrational relaxation in the first excited
triplet state is found to be constant ~0.5ns^-1 for the lowest vibrational
levels v< 15 and to increase sharply when exciting to higher energies.
