On the excitation spectrum of crystalline anthracene in the vacuum ultraviolet

The excitation spectrum (fluorescence intensity as a function of the energy of the exciting photon) of crystalline anthracene is presented from the absorption limit to 107 eV. Up to about 10 eV the spectrum is distinctly structured, the structure being clearly related to the absorption spectrum. Beyond 10 eV the fluorescence intensity increases monotonically with the increasing energy of the exciting photon. This behaviour is interpreted in terms of secondary excitations by the photoelectrons which are released within the crystal. With increasing excess energy the probability of such secondary excitation increases. The efficiency for singlet excitation by the photoelectrons as a function of their kinetic energy is determined for excitation energies up to 23 eV. These efficiencies are derived from the excitation spectrum and the energy distribution of the photoelectrons, which is known from available photoelectron spectra. The calculation of the efficiencies and the implied limitations of the method represent the major part of this work. Finally, the energy expenditure for a fluorescence photon in the 100 eV range is compared with the scintillation energy yield of anthracene for gamma excitation. (author)