Fate of metastable He(1s2s 1S) atoms bound to helium nanodroplet surfaces

Determine the subsequent evolution and dominant relaxation pathways of helium atoms excited to the 1s2s 1S state that remain bound to the surface of helium nanodroplets following resonant XUV excitation, including whether they predominantly desorb, persist as surface-bound species roaming the droplet surface, associate into He2* excimers, or undergo interatomic Coulombic decay, and establish the relevant timescales under typical experimental conditions.

Background

Helium nanodroplets, when resonantly excited, relax through complex pathways that include bubble formation around localized He* excitations and migration to the droplet surface. While excimer formation (He2*) is known in bulk liquid helium and a small fraction may form directly in droplets, prior time-resolved experiments and electron spectroscopy suggest only minor excimer contributions in nanodroplets. Other channels such as desorption, long-lived surface binding, and interatomic Coulombic decay (ICD) are relevant, but their relative importance at the droplet surface remains uncertain.

The paper introduces an XUV–NIR pump–probe fluorescence scheme that probes the presence and evolution of He* on the droplet surface over femtosecond–nanosecond delays. Even with these measurements and supporting simulations, the overarching question of the ultimate fate and branching of 1s2s 1S He* atoms that remain surface-bound is stated to be not exactly known, motivating a precise determination of pathways and timescales.