C₆₀ can be ionized in an indirect, quasi-thermal boil-off process after absorption of a single photon [1]. The process involves a large number of incoherently excited valence electrons and yields electron spectra with a Boltzmann distribution with temperatures exceeding 10⁴ K [2].

Experimentally, electron spectra were measured at the gas phase beamline at the synchrotron ring Elettra with a velocity map imaging (VMI) detector and in coincidence with the produced ion for a range of photon energies, from 13.5 to 65 eV. The thermal electron effect was deduced from the persistent low electron kinetic energy signal of an exponential, Boltzmann-type form, and the complete lack of angular structure in the low energy electrons for photon energies above ca. 20 eV. The coincidence technique permits to trace the appearance of fragmentation of the produced C₆₀⁺ ions and shows an energy dependence consistent with known appearance energies of the fullerene fragments.

The findings expand previous observations on multiphoton hot electron ionization of fullerenes [3], PAH molecules [4] and metal clusters [5]. The single photon nature of the process provides evidence that the creation of a highly and incoherently excited electronic subsystem does not depend on sequential absorption of a large number of subthreshold photons, but can be induced by absorption of a single high energy photon. This will have implications for the understanding of both short time electron dynamics and electron-photon interactions in molecular settings.

1. Klavs Hansen et al., Phys. Rev. Lett. 118 (2017) 103001
2. K. Hansen, Phys. Chem. Chem. Phys. 19 (2017), 19699
3. E.E.B. Campbell et al. Phys. Rev. Lett. 84 (2000) 2128-2131
4. M. Kjellberg, et al., J. Chem. Phys. 133 (2010) 074308
5. M. Maier et al., Phys. Rev. Lett. 96 (2006) 117405