THRESHOLD PHOTOIONIZATION SPECTRA OF EXCITED SOLUTE ELECTRONIC STATES IN NONPOLAR LIQUIDS

Photoconductivity spectra are reported for the first excited singlet (S1) and triplet (T1) states of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in nonpolar organic solvents at room temperature. The measured ionization potentials, corrected for the excited-state energies, agree well with those derived from previously published TMPD ground-state (S0) photoionization studies. The excited-state spectra and the S0 spectrum both increase in magnitude with increasing excitation frequency, first exponentially and then as a power law in excess energy above the ionization threshold. This behavior is similar to that of the absorption spectra of amorphous semiconductors. In each case, direct excitation of the photodonor into the conduction band of the solvent is indicated (instead of autoionization from a superexcited state of TMPD). In one instance unambiguous evidence for this path is encountered. © 1990 American Chemical Society.