Femtosecond Electron Solvation at the Ionic Liquid/Metal Electrode Interface

Electron solvation is examined at the interface of a room temperature ionic liquid (RTIL) and an Ag(111) electrode. Femtosecond two-photon photoemission spectroscopy is used to inject an electron into an ultrathin film of RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyr](+)[NTf2](-)). While much of current literature highlights slower nanosecond solvation mechanisms in bulk ionic liquids, we observe only a femtosecond response, supporting morphology dependent and interface specific electron solvation mechanisms. The injected excess electron is found to reside in an electron affinity level residing near the metal surface. Population of this state decays back to the metal with a time constant of 400 +/- 150 fs. Electron solvation is measured as a dynamic decrease in the energy with a time constant of 350 +/- 150 fs. We observe two distinct temperature regimes, with a critical temperature near 250 K. The low temperature regime is characterized by a higher work function of 4.41 eV, while the high temperature regime is characterized by a lower work function of 4.19 eV. The total reorganizational energy of solvation changes above and below the critical temperature. In the high temperature regime, the electron affinity level solvates by 540 meV at 350 K, and below the critical temperature, solvation decreases to 200 meV at 130 K. This study will provide valuable insight to interface specific solvation of room temperature ionic liquids.