PHOTOIONIZATION OF A SINGLET EXCITON IN AN ANTHRACENE SINGLE-CRYSTAL THROUGH 2-COLOR, 2-STEP EXCITATION

Photoionization of a singlet exciton has been studied in an anthracene single crystal. The photoionization spectrum and optical-absorption spectrum have been measured with two-color, two-step excitation. By comparing the two spectra, the ionization efficiency (PHI) of higher excited states could be obtained as a function of the excitation energy. It was found that PHI = 2 X 10(-4) between 5.1 and 5.5 eV, independent of excitation energy. We interpret this result to indicate that autoionization occurs after a rapid relaxation from an initially populated higher excited state. Above 5.5 eV, PHI increases sharply. This is interpreted as the onset of another ionization channel, possibly the direct ionization. Polarization study of the photocarrier generation indicates that the intermediate state before the photoionization is the relaxed lowest singlet exciton state.