Formation of Positive and Negative Carbon Cluster Ions in the Initial Phase of Laser Ablation in Vacuum

In this paper, the formation of C n + and C n - cluster ions has been investigated. The carbon cluster ions were produced by the laser ablation of a rotating graphite target in vacuum. The study has been carried out using time-of-flight mass spectrometry. Mass spectra of positive and negative cluster ions were observed as a function of delay time between the irradiation of the laser pulse and the acceleration of ions. The mass distribution of clusters changed significantly in the initial phase (≤10 µs) after the irradiation of the laser pulse. The sizes of both positive and negative cluster ions grow with the increase in the delay time. These results indicate that clustering reactions progress even in vacuum without ambient gas. The mechanism of clustering is considered to be reactions among various carbon ions and neutral species in the laser ablation plume.