Ionization and fragmentation of C₆₀ with sub-50 fs laser pulses

Gas-phase C₆₀ photoionization and photofragmentation experiments were performed using a sub-50 fs Ti Sapphire laser system and reflectron time-of-flight (RETOF) mass spectrometer. The dependence of the C₆₀⁺ and C₆₀²⁺ signals on the laser intensity for the fundamental (795 nm) and second harmonic (ca. 400 nm) has been determined. For low laser intensities, before the onset of fragmentation, single ionization is a direct multiphoton process. Double ionization is a sequential process in which C₆₀²⁺ originates from already singly ionized fullerenes. At laser intensities beyond the onset of C₆₀²⁺ there is considerable metastable fragmentation indicating a strong coupling of electronic excitation energy into vibrational degrees of freedom that appears to be in competition with multiple ionization.