Unravelling the Full Relaxation Dynamics of Superexcited Helium Nanodroplets

TL;DR

This study thoroughly investigates the relaxation pathways of superexcited helium nanodroplets using advanced spectroscopy and theoretical modeling, revealing detailed dynamics, ionization processes, and state-specific relaxation times.

Contribution

It provides a comprehensive experimental and theoretical analysis of helium nanodroplet relaxation, identifying key ionization pathways and state-resolved dynamics.

Findings

Identification of droplet autoionization, pump-probe photoionization, and re-excitation autoionization pathways.

He$_2^+$ is the most common product of autoionization and photoionization.

Relaxation to metastable helium states occurs with a slightly faster process for triplet states.

Abstract

The relaxation dynamics of superexcited superfluid He nanodroplets is thoroughly investigated by means of extreme-ultraviolet (XUV) femtosecond electron and ion spectroscopy complemented by time-dependent density functional theory (TDDFT). Three main paths leading to the emission of electrons and ions are identified: Droplet autoionization, pump-probe photoionization, and autoionization induced by re-excitation of droplets relaxing into levels below the droplet ionization threshold. The most abundant product of both droplet autoionization and photoionization is He$_2^+$, whereas the delayed appearance of He$^+$ is indicative of the ejection of excited He atoms from the droplets. The state-resolved time-dependent photoelectron spectra reveal that intermediate excited states of the droplets are populated in the course of the relaxation, terminating in the lowest-lying metastable singlet and triplet He atomic states. The slightly faster relaxation of the triplet state compared to the singlet state is in agreement with the simulation showing faster formation of a bubble around a He atom in the triplet state.