  Ultrafast non-equilibrium dynamics offer a route to study the microscopic
interactions that govern macroscopic behavior. In particular, photo-induced
phase transitions (PIPTs) in solids provide a test case for how forces, and the
resulting atomic motion along a reaction coordinate, originate from a
non-equilibrium population of excited electronic states. Utilizing femtosecond
photoemission we obtain access to the transient electronic structure during an
ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface.
We uncover a detailed reaction pathway, allowing a direct comparison with the
dynamics predicted by ab initio simulations. This further reveals the crucial
role played by localized photo-holes in shaping the potential energy landscape,
and enables a combined momentum and real space description of PIPTs, including
the ultrafast formation of chemical bonds.
