Formation of fullerenes in laser ablation of graphite in vacuum

The species yields and velocity distributions in the vacuum plume of laser ablated pirolitic graphite were studied in details by the time-of-flight mass spectrometry methods. Integral mass deposition on the opposite substrate was also investigated by use of quartz microbalance. At laser energy density near 2 J/cm(2) high yield of evaporation of clusters C-7 - C-27 is revealed. It is substantially higher in the case of preliminary ground graphite as compared with laser treated samples. First irradiation shots of a fresh graphite sample reveal the highest yield of C-7 - C-27 clusters and do not reveal detectable number of fullerenes in the plume. Fullerenes are more abundant in the case of target rotation or laser spot displacement. Fullerene evaporation yield increases during a certain time period of laser ablation of moving graphite target. Two-stage character of fullerenes appearance in the plume is proposed. Firstly high mass clusters are formed in the plume tail - the most cool part of the plume. As the pressure in the hot plume body is higher, most of fullerenes formed deposit back on the target surface. Next laser pulses evaporate these clusters, which are stable enough to stand in the hot plume body. The C-7 - C-27 cluster ions are proposed to be mainly produced as a result of ejection of the pieces of graphite slices.