Energetics for carbon clusters produced directly by laser vaporization of graphite : dependence on laser power and wavelength

Carbon-cluster positive ions C+n have been produced by laser vaporization of a graphite target by using three wavelengths (1064, 532 and 266 nm). When the graphite target was placed between the acceleration electrodes of a time-of-flight mass spectrometer and irradiated, the mass distribution of C+n immediately after the laser vaporization was recorded as a mass spectrum. C+n with n= 1, 3, 5, 7 and 11 atoms were produced predominantly, and their distribution was found to be dependent on the laser power and its wavelength. When the laser power was not too high, the logarithm of the concentration of carbon-cluster ions is linearly dependent on the logarithm of the laser power density. This logarithmic relation is well explained by the mechanism that C+n with 1 ⩽n⩽ 20 is vaporized directly from the graphite through conversion of the multiphoton energy into thermal energy. Furthermore, contributions of photoelectronic excitation to the formation of carbon clusters become important for 266 nm laser irradiation.