Photofragmentation of C₆₀ in the extreme ultraviolet: Statistical analysis on the appearance energies of C_60-2n ^z+ (n ≥ 1, z = 1-3)

The ion yield curves for C_60-2n^z+ (n = 1-5, z = 1-3) produced by photoionization of C₆₀ are measured in the photon energy (hν) range of 25-150 eV. The appearance hν values are higher by 30-33 eV than the thermochemical thresholds for dissociative ionization of C₆₀ leading to C_60-2n^z+. Evaluation is made on the upper limits of the internal energies of the primary C₆₀^z+ above which C_60-2n+2^z+ fragments (n ≥ 1) cannot escape from further dissociating into C_60-2n^z+ C₂. These upper limits agree well with the theoretical internal energies of C ₆₀^z+ corresponding to the threshold for the formation of C_60-2n^z+. The photofragmentation of C₆₀ ^z+ is considered to be governed by the mechanism of internal conversion of the electronically excited states of C₆₀^z+, statistical redistribution of the excess energy among a number of vibrational modes, and sequential ejection of the C₂ units.