Mass spectrometric study of carbon cluster formation in laser ablation of graphite at 355 nm

The ablation dynamics and cluster formation of C-n(+) ions ejected from 355 nm laser ablation of a graphite target in vacuum are investigated using a reflectron time-of-flight (RTOF) mass spectrometer. At low laser fluence. odd-numbered cluster ions with 3 less than or equal to n less than or equal to 15 are predominantly produced. Increasing the laser fluence shifts the maximum size distribution towards small cluster ions, implying the fragmentation of larger clusters within the hot plume. The temporal evolution of C-n(+) ions was measured by varying the delay time of the ion extraction pulse with respect to the laser irradiation, providing significant information on the characteristics of the ablated plume. Above a laser fluence of 0.2 J/cm(2), large cluster ions (n greater than or equal to 30) are produced at relatively long delay times, indicating that atoms or small carbon clusters aggregate during plume propagation. The dependence of the intensity of ablated C-n(+) ions on delay time after laser irradiation shows that the most probable velocity of each cluster ion decreases with cluster size.