Ring polymer molecular dynamics beyond the linear response regime: Excess electron injection and trapping in liquids

Ring polymer molecular dynamics (RPMD) is used to directly simulate the injection and relaxation of excess electrons into supercritical helium fluid and ambient liquid water. A method for modulating the initial energy of the excess electron in the RPMD model is presented and used to study both low-energy (cold) and high-energy (hot) electron injections. For cold injection into both solvents, the RPMD model recovers electronically adiabatic dynamics with the excess electron in its ground state, whereas for hot electron injection, the model predicts slower relaxation dynamics associated with electronic transitions between solvent cavities. The analysis of solvent dynamics during electron localization reveals the formation of an outgoing solvent compression wave in helium that travels for over 2 nm and the delayed formation of water solvation shells on the timescale of 300 fs. Various system-size effects that are intrinsic to the simulation of excess electron injection are discussed. Comparison of the RPMD simulations with previous mixed quantum-classical dynamics simulations finds general agreement for both the mechanisms and timescales for electron localization, although the electron localization dynamics in the RPMD model is essentially completed within 400 fs in helium and 150 fs in water.