Role of electronic structure in ionization and fragmentation of endohedral fullerenes Ho$_{\mathrm{3}}$N@C$_{\mathrm{80}}$ in an intense femtosecond laser field*

The ionization and fragmentation of gas phase endohedral fullerene Ho$_{\mathrm{3}}$N@C$_{\mathrm{80}}^{\mathrm{\thinspace }}$was investigated using ultrashort 800 nm laser pulses with an ion velocity map imaging (VMI) spectrometer. The power law's dependence I$^{\mathrm{n}}$ on laser intensity of the singly, doubly, and triply charged Ho$_{\mathrm{3}}$N@C$_{\mathrm{80}}$ molecule and Ho$^{\mathrm{+}}$ ion fragments have been experimentally determined. Theoretical calculation indicates that the superatom molecular orbitals (SAMOs) electronic states in Ho$_{\mathrm{3}}$N@C$_{\mathrm{80}}$ can be populated through direct multiphoton excitation. The ionization power law essentially reflects the photoexcitation step to the SAMOs. In addition to the molecular nuclear frame heating by electron-vibrational coupling, we observe a rapid heating process, which could be an `avalanche' process, produced via semi-free electrons impacting the molecular nuclear frame at high laser intensity.