Transition from SAMO to Rydberg State Ionization in C60 in Femtosecond Laser Fields
journal contributionposted on 02.11.2016, 00:00 by H. Li, B. Mignolet, Z. Wang, K. J. Betsch, K. D. Carnes, I. Ben-Itzhak, C. L. Cocke, F. Remacle, M. F. Kling
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.