The ejection distribution of solvated electrons generated by the one-photon photodetachment of aqueous I− and two-photon ionization of the solvent

Abstract

The ultrafast dynamics following one-photon UV photodetachment of ${\mathrm{I}}^{\mathrm{-}}$ ions in aqueous solution are compared with those following two-photon ionization of the solvent. Ultrafast pump–probe experiments employing 50 fs ultraviolet pulses reveal similar and very rapid time scales for electron ejection. However, the electron ejection process from water pumped into the conduction band and from iodide ions detached at threshold are readily distinguishable. The observed picosecond timescale geminate recombination and electron escape dynamics are reconstructed using two different models, a diffusion-limited return of the electron from $\text{∼15\hspace{0.05em}Å}$ to its parent and a competing kinetics model governed by the reverse electron transfer rate. We conclude that the “ejected” electron in the halide detachment is merely separated from the halogen atom within the same solvent shell. The assignment of detachment into a contact pair is based on the recombination profile rather than by the postulate of any new spectral absorption due to an electron in a contact pair. The contact pair is surprisingly long-lived and the nonadiabatic recombination is rather slow considering the proximity of the partners. Experiments in mixed solvents confirm our assignment of the two distinct ejection mechanisms. The detachment mechanism is therefore fundamentally different in the resonant (one photon) charge-transfer-to-solvent (CTTS) process from the multiphoton detachment of aqueous iodide ions, which bears more similarity to the direct solvent ionization.