Electron Mobilities and Ranges in Liquid Ethers: Ion-like and Conduction Band Mobilities

Abstract

The free ion yields in X irradiated ethers are larger than those in alkanes because the dielectric constants of the former liquids are greater than those of the latter. The relative increase of the free ion yield with temperature is smaller in ethers than in alkanes because the dielectric constants decrease more rapidly with increasing temperature in the former. The density normalized penetration range (thermalization length) bGPd of the secondary electrons in dimethyl ether (DME) is 3.5 × 10−7 g/cm2. As the length of the n-alkyl groups on the ether is increased bGPd increases towards the value obtained for long chain n-alkanes, 4.5 × 10−7 g/cm2. Electron mobilities ue showed two types of behavior: (i) at low temperatures ue approaches a value of about 2u−, where u− is the mobility of the anions formed in the irradiated liquid; (ii) at higher temperatures the ratio ue/u− increases with temperature, and equals 21 in di-n-butyl ether (DBE) at 375 K. The activation energy of electron migration at low temperatures (ion-like mechanism) is similar to that of ion migration, 2–3 kcal/mol, while at high temperatures it increases to ∼6 kcal/mol. The larger activation energy is attributed to thermal excitation of electrons from the solvated state into a conduction band, and is equal to one-half of the optical excitation energy of the solvated electrons. Electrons in water, alcohols, and ammonia at 300 K migrate by the ion-like mechanism. Electrons in alkanes migrate almost exclusively by the conduction band mechanism. A plot of the Arrhenius temperature coefficient of electron mobility against mobility in different liquids at a given temperature displays a maximum which is temperature dependent.