Time-of-flight spectroscopy and fast imaging studies of carbon dimers in laser-produced plasmas

Abstract

Summary form only given. The interaction between laser ablation plumes and a background gas has recently received increased attention due to its importance in pulsed laser deposition and carbon-cluster production. Carbon clusters, like C60 and higher fullerenes are known to be produced by laser ablation of carbon in the presence of an ambient atmosphere. However, there is a great uncertainty as to when and where these clusters are formed within the laser-ablated plume. Moreover, understanding the particle generation in laser ablation plumes is very important for pulsed laser deposition, laser ablation sample introduction in ICP-MS, etc. In the context of carbon clusters, it is articulated that the molecular C2 plays a prominent role in their formation. Laser ablation has the unique advantage that most of these molecules are formed in their excited states, and hence, spectroscopic measurements offer an excellent means to investigate their evolution and dynamics. We investigated the effect of helium ambient gas on the dynamics of molecular C2 formation. For producing plasmas, graphite targets were irradiated with 1064 nm, 6 ns pulses from an Nd:YAG laser and the helium ambient gas pressure was varied from high vacuum to 5 Torr. The emission from C2 species were studied using spectrally-resolved fast imaging and space- and time-resolved emission spectroscopy. The ICCD imaging was also used to characterize the propagation and expansion dynamics of the laser generated graphite plasma. Space- and time-resolved spectroscopy provided the time-of-flight distribution of the C2 species in the plasma. The temporal evolution of carbon dimers are found to be significantly influenced by the pressure of the ambient gas. A multi-peak temporal intensity distribution is observed for carbon dimers at moderate pressure levels. The dynamics of C2 emission and the role of helium ambient pressure on its spatial and temporal evolution are analyzed and discussed.