Picosecond study of electron ejection in aqueous phenol and phenolate solutions

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Abstract
The photoionization of aqueous phenol and phenolate solutions was investigated using picosecond absorption spectroscopy. Phenol and phenolate were excited in the first singlet excited state using a single picosecond pulse of 27 ps duration, at 265 nm (4th harmonic of a Nd–YAG laser). Hydrated electron formation was ascertained by following the kinetics of formation and disappearance at 630 nm with a picosecond pulse obtained by stimulated Raman scattering (SRS). Similarly to the ferrocyanide ion, the excited phenolate ion undergoes a very fast electron ejection process. The lifetime of this process is shorter than the time resolution of our experimental device. In the excited phenol molecule, hydrated electron formation is retarded, the delay between the half-rise times of the exciting pulse and the electron absorption signal being (12±1) ps. This delay is not reduced at low pH. These observations and the high yield of ̄eaq formation are best explained by a consecutive two photon process, the second photon being absorbed by the excited singlet state S1 and the deprotonation occuring after ejection of the electron. The kinetics of hydrated electron disappearance with NO3− as scavenger, are in excellent agreement with literature values. Using H3O+ and Cd++ to scavenge the hydrated electron, its initial formation yield remains constant, showing that Cd++ does not react with any hydrated electron precursor. This difference between our results and those obtained in picosecond pulse radiolysis shows that the precursor of the hydrated electron is not the same in the two techniques. The precursor obtained in pulse radiolysis appears to be more reactive with scavengers. A possible explanation is that after phenolate or phenol excitation, the resultant internal state of the aromatic molecule is capable of undergoing electron transfer to the adjacent solvent molecules, thus producing at least partially hydrated electrons directly.