Formation Times of RbHe Exciplexes on the Surface of Superfluid versus Normal Fluid Helium Nanodroplets

Nanodroplets of either superfluid ⁴He or normal fluid ³He are doped with Rb atoms that are bound to the surface of the droplets. The formation of RbHe exciplexes upon 5P3/2 excitation is monitored in real time by femtosecond pump-probe techniques. We find formation times of 8.5 and 11.6 ps for Rb ⁴He and Rb ³He, respectively. A comparison to calculations based on a tunneling model introduced for these systems by Reho et al. [J. Chem. Phys. 113, 9694 (2000)] shows that the proposed mechanism cannot account for our findings. Apparently, a different relaxation dynamics of the superfluid opposed to the normal fluid surface is responsible for the observed formation times.