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

Abstract

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.

Low-lying, Rydberg states of polycyclic aromatic hydrocarbons (PAHs) and cyclic alkanes

“…Evidence was also found for the existence of excited fullerene SAMO states in fs laser photoelectron spectroscopy experiments. [2][3][4][5] Theoretical studies have confirmed the unusual nature of these lowest-lying members of Rydberg series in fullerenes, showing a deviation in the binding energies extrapolated from higher-lying, more conventional Rydberg states due to the proximity to the molecular core and also having a significant component of the electron density inside the hollow cage. 6 SAMO states have also been predicted in theoretical calculations of fullerene anions.…”

Diffractive Imaging of C60 Structural Deformations Induced by Intense Femtosecond Midinfrared Laser Fields

“…Since its discovery, the soccer ball shaped C 60 -the archetypical fullerene -remains the focus of most ongoing scientific studies aimed at unveiling its properties (see [3] for an in-depth review). The nonlinear, nonresonant interaction of C 60 with strong femtosecond lasers has been the object of numerous investigations [4][5][6][7][8][9][10][11][12][13]. These studies revealed that at photon energies comparable with the separation between the first electronic excited state and the ground state (∼1.6 eV [14]), an efficient transfer of laser energy into C 60 's internal degrees of freedom occurs via electronic and nuclear (vibrational) couplings.…”

Fullerene Dynamics with X-Ray Free-Electron Lasers

“…The laser photoionization mechanisms of fullerenes have been found to be wavelength and pulse duration-dependent [9,10,18]. For IR pulses (800 nm) of about 30 fs duration and intensities below 5 × 10 13 W/cm 2 , it was found that multiphoton processes dominate when ionizing C 60 , while tunneling, and/or over-the-barrier ionization and ionization due to induced electron re-collision [8] have a low probability to occur under these conditions.…”