Photoelectron Emission in Solutions: Its Discovery, Kinetics, and Energetics

Abstract

At a certain stage of development of photoemission studies, the key question was how to prove the existence of photoemission in solutions, i.e., unambiguously distinguish this type of photoeffect from others enumerated in part in Section 0.1. The problem is less simple than it appears. The first direct experimental evidence of electron photoemission at the metal-vacuum interface was reported in the classic papers of Lenard (1900) and Thomson (1899) who showed that illuminated metals emit in vacuum the same particles as those previously found in cathode rays, i.e., electrons. Similar, direct evidence of electron photoemission in solution would, obviously, require a demonstration of the existence of solvated electrons in the vicinity of the electrode. However, it should be remembered that as opposed to vacuum where free electrons can exist for infinite periods of time, solvated electrons in electrolytes disappear relatively fast, being captured either by acceptors or by the electrode surface. Therefore, their stationary concentration at electrodes illuminated with usual intensities of light does not exceed 10¹⁰ to 10¹² electrons per cubic centimeter. This is much below the sensitivity of modern physical methods for detection of hydrated electrons (absorption spectra, EPR, etc.).