Transition from SAMO to Rydberg State Ionization in C₆₀ in Femtosecond Laser Fields

The transition between two distinct ionization mechanisms in femtosecond laser fields at 785 nm is observed for C₆₀ molecules. The transition occurs in the investigated intensity range from 3 to 20 TW/cm² and is visualized in electron kinetic energy spectra below the one-photon energy (1.5 eV) obtained via velocity map imaging. Assignment of several observed broad spectral peaks to ionization from superatom molecular orbitals (SAMOs) and Rydberg states is based on time-dependent density functional theory simulations. We find that ionization from SAMOs dominates the spectra for intensities below 5 TW/cm². As the intensity increases, Rydberg state ionization exceeds the prominence of SAMOs. Using short laser pulses (20 fs) allowed uncovering of distinct six-lobe photoelectron angular distributions with kinetic energies just above the threshold (below 0.2 eV), which we interpret as over-the-barrier ionization of shallow f-Rydberg states in C₆₀.