Photoelectron Spectroscopy on Magnesium Ensembles in Helium Nanodroplets

The light-induced collapse of a metastable magnesium atom foam inside helium nanodroplets is studied by imaging its electronic properties. Upon single-photon absorption near the atomic $3^1P_1\leftarrow3^1S_0$ transition, the fragile complex implodes and the dopant ensemble gains energy in excess of the photon energy. Subsequent photon absorption within the laser pulse leads to electron emission triggered on a ns timescale, which serves as an experimental probe of the resulting sample. The photoelectron spectra reveal population of atomic levels well beyond $3^1P_1$, evidencing an energy release process being relevant for a wide range of droplet sizes and doping conditions. From a statistical analysis the maximum number of Mg atoms which could be stabilized as a foam in droplets of a given size are deduced.