The photoionization yield and electron thermalization range spectra of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in tetramethylsilane

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Abstract
The fractional change in the fluorescence yield ψF of TMPD in tetramethylsilane, due to imposition of an electric field of strength E=45.6 kV/cm, has been determined at 44 discrete excitation energies, ε, above the photoionization threshold from ε=4.77 to 7.52 eV. Two maxima are clearly resolved in the field quenching ratio, (ΔψF/ψF0)E, one at 5.71 eV and the other at 6.79 eV. Beyond 7.0 eV, (ΔψF/ψF0)E is again observed to slowly increase. For each of 12 of the previous excitation energies, (ΔψF/ψF0)E was obtained as a function of E from 18.2 to 45.6 kV/cm. Using an exponential radial distribution function of geminate pair separation distances, f(r)=β3e−βr/2, the field dependence was fitted to the Onsager theory and the average electron thermalization range, 〈r〉=3/β extracted as a function ε. With this, the yield of thermalized electrons, φ± was also obtained as a function of ε. The range spectrum is found to increase with increasing ε with an inflection at 5.9 eV, followed by a rapid rise to a maximum of 230 Å at 6.7 eV and then to decline continuously to 167 Å at 7.5 eV. The photoionization yield spectrum shows two maxima, one at 5.7 eV with φ±=0.44 and a second at 6.9 eV with φ±=0.52. Beyond 7.0 eV, φ± increases to 0.57 at ε=7.5 eV. It is concluded on the basis of an analysis of these spectra that photoionization of TMPD in tetramethylsilane from 5.9 to 7.5 eV generates in relatively high yield at least two electronically excited states of the positive ion.