On the mechanism of C₆₀ thin film laser-induced desorption

The mechanism of thin film C₆₀ laser desorption has been investigated using nanosecond and picosecond UV laser pulses. The desorption experiments were performed under ultrahigh-vacuum conditions using reflectron time-of-flight mass spectroscopy from which the velocity distributions of the desorbed ions and the dependence of the ion yield on the laser fluence were obtained. A strong nonlinear dependence of the desorption yield on laser fluence in the threshold region, indicative of a thermal mechanism, was found for both ns and ps pulses. Typically, the C₆₀⁺ velocity distributions were bimodal and could be fitted by modified Maxwell-Boltzmann distributions. The fits to the slow contributions gave translational temperatures consistent with surface temperatures due to laser heating with ns pulses as estimated by solving the one-dimensional heat equation. In contrast, translational temperatures which are much too high to be consistent with purely thermal processes were obtained for the fast contributions. These fast contributions originate from different desorption mechanisms for the two different laser pulse lengths.