Anisotropic Coulomb explosion of $C60$ irradiated with a high-intensity femtosecond laser pulse

$C60$ vapor was irradiated with an intense femtosecond laser pulse $(1×1016$ W/cm² in 120 fs at a wavelength of 800 nm). Multiply charged carbon ions, up to $C4+,$ were detected in the time-of-flight spectra. We have measured both energy and angular distributions with respect to the laser polarization direction for ions with different charge number. The kinetic energy of ions are distributed above 1 keV. The angular distribution measurement shows that the highly charged ions of $C3+$ and $C4+$ are mostly distributed in the directions parallel to the laser polarization, and the $C+$ ions are slightly distributed in the direction perpendicular to it. These observations clearly indicate that an anisotropic explosion takes place. The average energy of ions with the different charge number is found to be proportional to the square of the charge number, while the maximum energy is proportional to the charge number. Classical molecular dynamics simulations have been successfully carried out reproducing not only the energy spectra but also the angular distributions of ions. The $C3+$ ions are shown to be produced by the cascade hopping of electrons induced by the intense laser light. The simulations suggest that the most crucial process for the anisotropic Coulomb explosion of $C60$ is not the electron impact ionization, but the cascade hopping of electrons.