Dynamics of solvation and desolvation of rubidium attached to He nanodroplets.

The real-time dynamics of photoexcited and photoionized rubidium (Rb) atoms attached to helium (He) nanodroplets is studied by femtosecond pump-probe mass spectrometry. While excited Rb atoms in the perturbed 6p-state (Rb*) desorb off the He droplets, Rb(+) photoions tend to sink into the droplet interior when created near the droplet surface. The transition from Rb(+) solvation to full Rb* desorption is found to occur at a delay time τ ∼ 600 fs for Rb* in the 6pΣ-state and τ ∼ 1200 fs for the 6pΠ-state. Rb(+)He ions are found to be created by directly exciting bound Rb*He exciplex states as well as by populating bound Rb(+)He-states in a photoassociative ionization process.