Electron solvation and solvation-induced crystallization of an ammonia film on Ag(111) studied by 2-photon photoemission.

Intermolecular interaction plays a crucial role in electron solvation in the condensed phase. Here, we present a femtosecond time-resolved and angle-resolved 2-photon photoemission (2PPE) study on the dynamics of electron solvation in a 2-dimensional ammonia film on a metal substrate. While the weakly chemisorbed first monolayer (ML) supports delocalized image-potential (IP) states that resemble those of the bare Ag(111) substrate, an additional monolayer localizes the IP state with a larger binding energy obtained through a pre-solvation process. Structural disorder in the metastable ammonia films (>2 ML) leads to a prominent photoelectron peak that is attributed to the long-lived trapped electron state (e(T)) located at 1.5 eV above the Fermi level. Photoinduced crystallization of the metastable phase, verified by the recovery of a delocalized IP state, is suggested to result from inelastic scattering between interfacial electrons and disordered ammonia molecules.