Sub-picosecond laser ionization of free C60 and C70 at 248 nm

Delayed ionization is found to be absent for sub-picosecond laser excitation of free C₆₀ and C₇₀ at 248 nm. The autocorrelation trace obtained for C ₆₀ ⁺ in a laser time-of-flight (TOF) mass spectrometer using two time-delayed and collinear 248 nm ultrashort laser pulses has a width of 1.1 ps (715 fs for sech² pulses), in agreement with the laser pulse duration measurement in NO gas. Both above observations can be explained by direct ionization of C₆₀ via coherent two-photon absorption by the high intensity sub-picosecond 248 nm laser excitation avoiding the channel leading to delayed ionization.