Direct observation of transition from delayed ionization to direct ionization for freeC60andC70: Thermionic emission?

Transition from direct ionization to delayed ionization for free ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ is observed below the ionization potential (IP=7.5 eV). It depends on the excitation wavelength as well as the pulse energy. The appearance threshold for direct ionization is found at ∼5.8 eV. These can hardly be explained by thermionic emission, therefore, we propose that triplet states at ∼1.7 eV (=7.5-5.8 eV) should play an important role in the multiphonon ionization of free ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ and the intramolecular interaction of many triplet states gives rise to delayed ionization. To confirm our proposition, we have carried out further experiments using two laser ionization.