Absorption spectrum of C-60 in the gas phase: Autoionization via core-excited Rydberg states

The absolute absorption cross section of C-60 in the gas phase (830-870 K) was measured as a function of the photon energy (3.5-11.4 eV) (absorption spectrum). Absorption peaks at 7.87, 8.12, 8.29, 9.2 eV and a dip at 8.45 eV observed are assigned as Feshbach resonances in the photoexcitation involving superexcited states. The superexcited states responsible for the 7.87, 8.12, and 9.2 eV peaks are assigned to be core-excited Rydberg states converging to the second, the third and the fourth ionization limits of C-60 (8.89, 9.12, 10.82-11.59 eV), respectively. The 8.29 eV peak is considered to originate from vibrational excitation of a totally symmetric pentagonal pinch mode of the superexcited state responsible for the 8.12 eV peak. Further, a relative photoionization quantum yield was estimated from the absorption cross section measured and the relative photoionization cross section reported. The yield increases particularly in the vicinity of 8 eV in accordance with a high efficiency of autoionization of the superexcited states. Ionization efficiency is not high in the vicinity of the first ionization energy, probably because of rapid energy dissipation into its vibrational modes. The spectrum below the ionization energy resemble the absorption spectra of C-60 in its solutions. (C) 1996 American Institute of Physics.