Photoionization in dielectric fluids: Variation of alkyl composition at the donor site

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Abstract
The yield of mobile charge carriers obtained by photoionization of solutions containing N,N,N′,N′ tetramethyl‐, tetraethyl‐, or tetrabutyl‐paraphenylenediamine dissolved in tetramethylsilane, 2,2‐dimethylbutane, cyclopentane, n‐pentane, or n‐hexane at room temperature was determined under comparable experimental conditions. The photoionization yield from the ethyl and butyl derivative was found to be as much as 100 times larger than the yield from the methyl derivative in each of the five fluids. The enhanced yield obtained with normal alkyl chains located at the donor site (attached environment) is opposite to the depression of yield obtained with normal alkyl hydrocarbons as solvent (unattached environment). The relative enhancement of photoionization was largest at lower excitation energy and decreased at higher excitation energy. The photoionization yield was found to exponentially increase with increasing photon energy along the lower energy rising edge of ionization up to about 5.5 eV in each fluid. At 5.5 to 5.6 eV, there was an abrupt decline in ionization yield for each donor independent of fluid. The enhanced photoionization from the ethyl and butyl substances enables levels of equal ionization to be attained at as much as 0.25 eV lower photon energy than for the methylated donor in each fluid. The density of electronic states of the fluid which are accessible from excited molecular states of donors have not contributed sufficient structure to the photoionization yield curves to permit identification of band tail states. The shape and breadth of the photoionization yield curves was consistent with an interpretation in which the charge transfer probability was largely determined by electron transfer matrix elements.