The vibrational wave-packet dynamics of diatomic rubidium molecules (Rb2) in triplet states formed on the surface of superfiuid 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 1~3Σ_g~+ is found to be constant γ≈0.5 ns~(-1) for the lowest vibrational levels v approx < 15 and to increase sharply when exciting to higher energies.
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