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 1 3 Σ + g is found to be constant γ ≈ 0.5 ns −1 for the lowest vibrational levels v < ∼ 15 and to increase sharply when exciting to higher energies.

No abstract

The quantum ratchet current is studied in the parameter space of the dissipative kicked rotor model coupled to a zero temperature quantum environment. We show that vacuum fluctuations blur the generic isoperiodic stable structures found in the classical case. Such structures tend to survive when a measure of statistical dependence between the quantum and classical currents are displayed in the parameter space. In addition, we show that quantum fluctuations can be used to overcome transport barriers in the phase space. Related quantum ratchet current activation regions are spotted in the parameter space. Results are discussed based on quantum, semiclassical and classical calculations. While the semiclassical dynamics involves vacuum fluctuations, the classical map is driven by thermal noise.

No abstract

We thoroughly investigate vibrational quantum dynamics of dimers attached to He droplets, motivated by recent measurements with K 2 (Claas et al 2006. 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. Gesellschaft provide a gentle, since weakly disturbing, host for embedded species, which can be studied with high-resolution spectroscopy [4]. However, spectra of immersed species are slightly broadened and shifted away from their gas-phase value due to their interaction with the surrounding He droplet. Inhomogeneous broadening in pure rotational spectra is ascribed to the motion of the purity inside the droplet [5,6] or to coupling to collective degrees of freedom of the droplet [7]. The overall spectral features, which are unseen in gas-phase measurements, have been used to study the weak interaction between dopant and helium droplet [5,[8][9][10][11][12]. Attached species allow us to probe the peculiar properties of the superfluid He droplet itself [13]. Further insight into the interaction and the quantum properties of the droplet can be gained from recent time-resolved studies [1,2,14,15].Helium nanodroplets, typically consisting of several thousands of 4 He atoms, are ideally suited for studying relaxation (cooling) of embedded species [16][17][18][19][20]. Whether dissipation plays a role depends on the involved energy scales, the coupling strengths or typical time scales in the system and 'bath' [21,22]. The group of Miller has studied relaxation of hydrogen fluoride (HF) inside He droplets [23]. They have reported on ineffective vibrational relaxation due to a mismatch in energy scales. However, rotational relaxation in the immersed HF system is fast and appears as Lorentzian line broadening in the rotational transition spectrum. (See also [7] and references therein.)Alkali metal atoms and molecules are known to reside in bubble-like structures on the surface of He droplets [24][25][26]. Attached Rb 2 dimers in the triplet state reveal the presence of vibrational relaxation on the measurement timescale [15,27]. For lithium dimers interacting...

No abstract