Time-of-Flight Mass Spectroscopy of Carbon Clusters and Hydrocarbons Produced by Laser Ablation of Graphite under H2 and He Buffer Gas-Formation and Stability of C10 and C2nH2 (n=2-5)-

Abstract

The products from laser ablated graphite under the flow of H2 and He bu#er gases were analyzed by time-of-flight (TOF) mass spectroscopy in combination with one-photon ionization of an energy of 10.5 eV/ photon. By changing the time interval between the ablation laser pulse and the photoionization laser pulse it was found that the neutral clusters ablated from the graphite have three distinctly di#erent velocities for one and the same value of m/z, which was detected as three separate bunches of TOF mass peaks. The e#ect of changing the time interval was examined extensively. As a result, it was found that the first component that appeared earlier in the ionization region consisted of various bared carbon clusters Cn with nῌ6 and hydrocarbons CnHm with n ῌ4 and mῌ1 through 4 while the second and third components that arrived later were C10 and C2nH2 (nῌ2 through 5) only. This persistence of C10 and C2nH2 at longer delay times was attributed to the robustness of these clusters against collisions occurring in ablated plumes. It was inferred that the robustness is due to the chemical inertness of a monocyclic form for C10 and of a linear polyynic form, H(῎C῎C)n῎H, for C2nH2. In the He bu#er gas heated to 600 K the yield of the third (slowest) component of C10 enhanced drastically, which indicates that thermal collisions in plumes favor the production of this monocycle, which further suggests that the monocycle may play a crucial role in the formation of C60, known as a favored product in hot plumes of ablated graphite. IntroductionCarbon clusters and their hydrogenated derivatives have long attracted attention in molecular spectroscopy, combustion chemistry, material sciences, and interstellar chemistry. 1), 2) Among various experimental techniques, laser ablation of graphite in bu#er gases has been employed to elucidate the structure and reactivity of carbon clusters and their derivatives.3)῎9) It is known that among the derivatives polyynes, H(῎C῎ C) n ῎H, are abundant products in the H 2 bu#er gas as are cyanopolyynes, H(῎C῎C) n ῎C῎N, in CH 3 CN and NH 3 gases. 5) These results in laboratories are useful for the understanding of the rich abundance of the molecules in the interstellar space. 10)῎14)The phenomenon of laser ablation itself is also an interesting subject in non-linear gas dynamics. Thus, ablated emissive products have been investigated by emission spectroscopy 15), 16) and by fast I(image intensifier)῎CCD photography.16) Ions directly produced by ablation also have been studied by mass spectrometry.17)῎20) All these techniques, however, are insensitive to neutral species in the ground electronic ῏