XUV fluorescence as a probe of interatomic coulombic decay of resonantly excited He nanodroplets.

Superfluid He nanodroplets resonantly excited by extreme ultraviolet (XUV) pulses can relax via interatomic coulombic decay (ICD), generally considered an ultrafast process. Here, we introduce a novel approach to probe the dynamics of ICD in He nanodroplets over timescales ranging from femtoseconds to nanoseconds. Our method relies on detecting XUV fluorescence emitted from the nanodroplets as they are driven into a nanoplasma by subsequent intense infrared pulses. Nanoplasma ignition is facilitated by tunnel ionization of XUV-excited He[Formula: see text] atoms attached to the droplets; it thus serves as a sensitive probe of their relaxation dynamics. The observed nanosecond-scale decay is attributed to ICD between pairs of He[Formula: see text] atoms undergoing roaming motion on the droplet surface, as supported by quantum-mechanical and classical model calculations.