Revisiting Laser-Intensity-Dependent Ionization and Fragmentation of C_60

We revisit the laser-intensity-dependent ionization and fragmentation yields of C_60 molecules irradiated by 25-fs, 798-nm laser pulses based on the approach in which photoions are measured via a velocity map imaging spectrometer working in a time-sliced mode. This approach dramatically improves the signal-to-background ratio compared to those using a simple (traditional) time-of-flight mode (spectrometer), and thus allows us to measure the laser-intensity dependences down to a previously untouched region, which is expected to provide new insights into the intense-field ionization and fragmentation of C_60. Indeed, we find that the saturation intensities for C_60 ionizations and the onset intensity for C_60 fragmentation are much lower than those reported in previous experiments. Furthermore, the derived saturation-intensity dependence on charge distribution demonstrates the validity of the over-the-barrier ionization using a conducting sphere model.