Photoionization Yield vs Energy in H₂O and D₂O

A simple conductivity jump method was used to measure the escaped solvated electron yield following two-photon excitation of water with Raman-shifted light from an amplified mode-locked Nd:YAG laser. Between 7.8 and 9.3 eV, the quantum efficiency for the escape yields changes from 1.9% to 22%, with an almost exponential dependence on the excitation energy. Quantum efficiency in D 2 O is smaller and resembles the H 2 O behavior at 0.35 eV lower energy. The quantum yield measured for one-photon excitation near the water absorption edge at 6.4 eV is a surprisingly large 1.3%. We propose that the mechanism for low energy photoionizaton of water is best described as a dissociative proton-coupled electron transfer to a preexisting trap.