The Role of Excited-State Proton Relays in the Photochemical Dynamics of Water Nanodroplets.

In this work, we applied nonadiabatic excited-state molecular dynamics in tandem with ab initio electronic structure theory to illustrate a complete mechanistic landscape underpinning the ultraviolet absorption-initiated photochemical dynamics in water nanodroplets. The goal is to understand the nonequilibrium excited-state molecular dynamics initiated by the relaxation of a solvated photoelectron and consequential photochemical processes. The lowest-lying excited state shows the proton dissociation for a single water molecule forming intermediate hydronium complexes through a proton relay. At approximately 100 fs, the proton relay process gives rise to the relaxation of the excited state accompanied by a rapid increase in the nonadiabatic coupling strength with the ground state, and the nanodroplet nonradiatively decays. The nonadiabatic transition to the ground state produces excited vibrational states that facilitate the recombination of the dissociated proton and hydroxyl group, eventually leading to the desorption of water molecules from the nanodroplet. Additionally, lifetimes of transient photochemical events are also resolved for the relaxation of a solvated electron, excited-state proton relay, and nonradiative transition.