Highly charged carbon ions formed by femtosecond laser excitation of C60: A Step towards an x-ray laser

We study the interaction of C₆₀ molecules with intense (10–1000 TW/cm²) femtosecond pulses of 790-nm wavelength. High charge states of carbon up to C⁴⁺ are produced in this intensity range, as determined by time-of-flight spectroscopy. These high charge states of carbon are produced at intensities more than an order of magnitude lower than would be required based on optical field ionization of isolated carbon atoms. From a line-shape analysis of the time-of-flight data, we derive the kinetic-energy distribution of the C ions. We find average energies of up to a few hundred eV, indicating that these ions are released through Coulomb explosion of the C₆₀ molecules once the charge buildup due to ionization becomes sufficiently high. Ionic potentials of up to 200 V are derived from the kinetic-energy distributions. The ionic potential needed to retain electrons energetic enough to cause K-shell impact ionization is 392 V. This potential is not reached in case of ionization of C₆₀ with 790-nm, 45-fs pulses since the disintegration of the cluster occurs on a time scale of a few femtoseconds, much shorter than the optical pulse. In this short time span, the heating of the cluster is not efficient enough for higher charge states to be ionized.