# Ultrafast relaxation of photoexcited superfluid He nanodroplets

## TL;DR

This study demonstrates that superfluid helium nanodroplets undergo ultrafast relaxation after photoexcitation, involving bubble formation and collapse within picoseconds, revealing detailed photodynamics via XUV pulses and simulations.

## Contribution

It provides the first detailed experimental and theoretical insight into the ultrafast relaxation pathways of photoexcited helium nanodroplets.

## Key findings

- Ultrafast interband transition occurs within 1 ps.
- A nanometer-sized bubble forms around excited helium within 1 ps.
- Metastable helium is released at the droplet surface after bubble collapse.

## Abstract

The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, helium nanodroplets are resonantly excited by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free-electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental photoelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He*) within 1 ps. Subsequently, the bubble collapses and releases metastable He* at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses.