Direct Measurement of a Biphotonic Photo-ionization in Liquid Solution

Abstract

THE history of low energy photo-ionization in organic solutions began with the work of G. N. Lewis et al.¹. Certain rigid organic solutions at 77° K were found to be photo-ionized by light quanta the energies of which were as little as half those required for gas phase ionization. More recently it has been demonstrated² that very similar systems also undergo low energy photo-ionization in their fluid state at room temperature. To determine how the energy requirements are met, several workers^3–5 have suggested that the mechanism of photo-ionization may involve two photons with a triplet state as a possible intermediate. The involvement of two photons in the photo-ionization of several benzene derivatives has now been clearly demonstrated in a rigid saturated hydrocarbon at 77° K by means of an analysis of the initial rise in the photoconductivity⁵. The results of these studies do not, however, suggest a triplet state as the intermediate state. For the molecule N, N, N′,N′-tetramethyl-p-phenylenediamine (TMPD) in 3-methyl-pentane (3-MP), an additional spectrophotometric study at 77° K revealed that the cation product, Wurster's blue, was also produced by a reaction involving two photons, and a triplet was identified as the intermediate state⁶. We have now completed a flash photoconductivity study of the same system in liquid solution at room temperature and find that the mechanism once again involves two photons. The nature of the intermediate state (or states) has not yet been established. In fact, energy considerations make it clear that inert solvents with low dielectric constant cannot radically alter the energy required for complete charge separation over that for the gaseous state. Thus low energy photo-ionizations which are well below (2–3 eV) the gaseous threshold must be expected to be no less than biphotonic whatever the details of the mechanism. The present extension of the earlier observations to a liquid would seem to confirm this point.