Abstract
Abstract The external photoelectric effect of the molecular crystals in the vacuum ultraviolet region was observed. The ionization potential of tetrathiotetracene, indanthrone and perylene, as estimated by the threshold values of their spectral responses, were 4.55, 5.10 and 5.40 eV respectively. The band-gap energies, calculated on a band model, were 0.62, 0.68 and 1.72 eV respectively. These values were in fairly good agreement with those obtained from the temperature dependency measurements of their electrical resistivities. By analyzing the photocurrent-voltage relation for tetracene and pentacene, the kinetic energy distribution curves were obtained. From an analysis of those curves, it turned out that the one-electron process plays a dominant role in the emission processes and that illumination with a short-wavelength light (1200 Å—1500 Å) can excite electrons from the deeper levels of the valence band to a vacuum. The observed positions of the filled π-electron levels were almost consistent with the results obtained using the LCAO method; for tetracene the depths of lower levels in order from the uppermost one, were 1.52, 2.48, 2.96, 3.46 and 3.92 eV from the observations and 1.60, 2.34, 2.98, 3.32 and 3.89 eV from the calculations.
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