Composition effects on optical absorption spectra of solvated electrons in alcohol/water mixed solvents

Abstract

Addition of a few percent of water to an alcohol has a relatively large effect on the shape of the optical absorption spectrum of solvated electrons in the liquid. This occurs whether the optical absorption energy in the pure alcohol is greater or smaller than that in water. Addition of up to 10 mol% of water causes EAmax in methanol and primary alcohols to decrease, while it increases in secondary and tertiary alcohols. At around 10 mol% water in primary alcohols EAmax passes through a minimum and increases again at higher water concentrations, reaching a plateau at about 30 mol% and remaining constant up to about 95 mol% water; over the last part of the composition range to pure water EAmax decreases slightly. The behavior in secondary and tertiary alcohols containing > 30 mol% water is similar to that in primary alcohols. The width of the band at half height W1/2 is divided at EAmax into "red side" and "blue side" portions Wr, and Wb, respectively. In methanol and in primary and secondary alcohols, addition of up to 30 mol% of water greatly reduces Wb but has relatively little effect on Wr. At > 30 mol% water Wb and Wr are similar to those in pure water. In tertiary butyl alcohol the band width is similar to that in pure water, so addition of water to the alcohol makes little change in the band width. The water/alcohol composition effects on the es− absorption band parameters are attributed to changes in solvent structure. This is especially evidenced by the minimum in EAmax at 10 mol% water in a primary alcohol. The changes in band asymmetry Wb/Wr indicate that the types of electronic transition on the low and high energy sides of the band are different.