Relative partial cross sections for single, double, and triple photoionization of C₆₀ and C₇₀

Partial cross sections for the photoion formation from C₆₀ and C₇₀ were determined from the yields of singly, doubly, and triply charged ions which were measured by mass spectrometry combined with tunable synchrotron radiation at hv = 25-120 eV. The dependence of the detection efficiencies on the mass-to-charge ratio was evaluated by using the formula proposed by Twerenbold et al. Corrections of the detection efficiency were found to be critical for obtaining accurate partial cross sections for photoionization of fullerenes. Revisions were made of the partial cross-section curves for single and double photoionization of C₆₀ and C ₇₀. The curve for triple photoionization of C₇₀ was newly proposed. The ratios between the cross sections for double and single photoionization increase with hv and reach saturated values of 0.78 at 85 eV for C₆₀ and ∼1.3 at 100 eV for C₇₀. In contrast, the ratios at 120 eV between the cross sections for triple and single photoionization of C₆₀ and C₇₀ amount to 0.14 and ∼0.38, respectively. The formation mechanism of multiply charged fullerene ions was discussed in terms of valence-electron excitation to antibonding unoccupied orbitais and/or spherical standing waves inside the cavity of a fullerene. This excitation could be followed by Spectator Auger processes and transmission of the excess electronic energy among numerous vibrational degrees of freedom.