Real-time tracking the energy flow in cluster formation
While photodissociation of molecular systems has been extensively studied, the photoinduced formation of chemical bonds remains largely unexplored. Especially for larger aggregates, the electronic and nuclear dynamics involved in the cluster formation process remain elusive. This limitation is rooted in difficulties to prepare reactants at well-defined initial conditions. Here, we overcome this hurdle by exploiting the exceptional solvation properties of helium nanodroplets. We load the droplets with Mg atoms and investigate the dynamical response of the formed Mg$_n$ aggregates to photoexcitation with time-resolved photoelectron spectroscopy. Beside the response expected for conventional Mg$_n$ clusters, consisting of a prompt signal rise and a decay characteristic for van der Waals bonds, the transient spectra also show a delayed photoelectron band peaking at 1.2 ps. This delayed signal rise is characteristic for nuclear dynamics and represents the transition of Mg$_n$ aggregates from a metastable, foam-like configuration, where Mg atoms are stabilized with a previously predicted interatomic spacing of 9.5 A, to a compact cluster. With global fitting analysis and ion-electron coincidence detection, the concerted electronic and nuclear dynamics can be tracked on a fs timescale. We find that cluster formation, proceeding with a ($450\pm180$) fs time constant, is accompanied by the population of highly-excited atomic states. We propose an energy pooling reaction in collisions of two or more excited Mg atoms during cluster formation as the mechanism leading to population of these high-lying Mg states. Additionally, conversion to kinetic energy through electronic relaxation leads to fragmentation and ejection of ionic cluster fragments from the He droplet. These results underline the potential of He droplets for time-resolved studies of bond formation and to uncover involved processes.