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

Gas-phase ${\mathrm{C}}_{\mathrm{60}}$ 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 ${\mathrm{C}}_{\mathrm{60}}^{\mathrm{+}}$ and ${\mathrm{C}}_{\mathrm{60}}^{\mathrm{2+}}$ 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 ${\mathrm{C}}_{\mathrm{60}}^{\mathrm{2+}}$ originates from already singly ionized fullerenes. At laser intensities beyond the onset of ${\mathrm{C}}_{\mathrm{60}}^{\mathrm{2+}}$ 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.