Theoretical study of the photoexcitation of dopants in superfluid helium nanodroplets

Work package:
The goal of this project is to study the dynamics induced by photoexcitation or ionization of a dopant in or on a superfluid helium nanodroplet. Helium nanodroplets are large helium clusters (typically thousands to several hundred billions of atoms) exhibiting remarkable properties : very low temperature (0.4 K), superfluidity, weak interaction with the dopant, very high thermal conductivity and very fast relaxation dynamics. This makes them an exceptional environment for dynamics studies [1]. During the past few years, several dynamics experiments have been conducted on these systems using highly sophisticated techniques : real time femtosecond pump-probe experiments, velocity map imaging, ... From a theoretical point of view, it is a serious challenge to study the real time dynamics of these systems because of the strong quantum effects due to the light mass of helium. Helium density functional theory (He-DFT) and its time-dependent version (He-TDDFT), which deal with the helium density rather than the many-body helium wave function, have emerged as an optimal compromise between accuracy and the ability to work with large numbers of helium atoms [2]. We will apply them to one of the following cases: dynamics initated by photoexcitation of a dopant atom or molecule, or solvation dynamics upon sudden ionization of a dopant, or Coulomb explosion of a two-dopant system used as a probe signal. This training will be conducted in collaboration with the team of M. Barranco and M. Pi from the University of Barcelona, who are experts in the He-(TD)DFT methods, and with the experimental team of Kristensen (Aarhus, Denmark).