Detailed model study of dissipative quantum dynamics of K2 attached to helium nanodroplets

We thoroughly investigate vibrational quantum dynamics of dimers attached to He droplets, motivated by recent measurements with K-2 (Claas et al 2006 J. Phys. B: At. Mol. Opt. Phys. 39 S1151). For those femtosecond pump-probe experiments, the crucial observed features are not reproduced by gas-phase calculations, but agreement is found using a description based on dissipative quantum dynamics, as briefly shown in the work by Schlesinger et al (2010 Chem. Phys. Lett. 490 245-8). Here we present a detailed study of the influence of possible effects induced by the droplet. The helium droplet causes electronic decoherence, shifts of potential surfaces and relaxation of wave packets in attached dimers. Moreover, a realistic description of (stochastic) desorption of dimers off the droplet needs to be taken into account. Step by step, we include and study the importance of these effects in our full quantum calculation of the effective dimer dynamics. This approach allows us to reproduce and explain all major experimental findings. We find that desorption is fast and occurs within 2-10 ps after electronic excitation. A further finding is that slow vibrational motion in the ground state can be considered frictionless.