Rydberg atoms on helium nanodroplets

This project is devoted to the understanding and characterisation of interactions between a cold helium nanodroplet and an atom on its surface from the short-range to the long-range regime. The atom-He surface interaction depends on the quantum state of the atoms. When the atom is excited to low lying states, the atom interacts strongly with the helium atoms and with the surface excitation modes (ripplons). This situation corresponds to a strongly correlated regime. As the atom is promoted to higher excited states, the size of the electronic orbital increases. The probability to find the excited electron close to the helium nanodroplet decreases. The correlations with the droplet, where the ion core is located are reduced but finer details of the couplings with the droplet will be observable. This situation converges towards the weakly correlated regime. Promoting the atom through the series of excited states up to the ionisation threshold therefore allows tuning the coupling strength between the atom and the droplet surface. Spectroscopic studies will be performed applying laser induced fluorescence (LIF) and beam depletion techniques on alkali atoms, which are known to reside on the surface of helium nanodroplets. We will investigate how the properties of highly excited states are affected by the droplet surface and to which extend they still follow scaling rules as a function of the principal quantum number n, which are characteristic of Rydberg atoms. Particularly interesting are the very high lying states just below the ionisation threshold. Systems consisting of an ion-core, which is solvated at the centre of the droplet and an electron orbiting around the droplet have been theoretically predicted and described. We hope to succeed with creating such a system and observe its electronic states, provided that the ionic core migrates from the surface to the centre of the droplet as it is expected. The extent of helium solvation of an atom prior to excitation plays an important role and will be investigated. For this purpose, we will choose atoms of group IIa, whose solvation state varies from fully solvated to fully surface as we move down the periodic table from Mg to Ba. Another longer-term goal is to study the long-range interactions between a Rydberg atom and a helium nanodroplet in states where the mean distance between the Rydberg atom and the droplet is larger than the electronic radii of the Rydberg atom. This type of pair system should allow studying the perturbations caused by a nano-object to the quantum states of an atom. The atoms that travel along with the helium nanodroplet beam without having been captured by a specific droplet can be excited to a Rydberg state. The presence of the nanodroplet should lead to a modification of the spectra of the Rydberg transitions and should give insight into the long-range pair interactions. Possibly, photoassociation of an atom and a helium nanodroplet could be induced through Rydberg excitation.