Laser ablation mass spectrometry of pyrolyzed Koppers coal-tar pitch: A precursor for fullerenes and metallofullerenes

Pyrolyzed Koppers coal-tar pitch samples consisting mainly of polycyclic ar omatic hydrocarbons were studied by direct laser vaporization Fourier trans form ion cyclotron resonance and time-of-flight mass spectrometry. Fulleren e cations were readily formed from these carbonaceous precursors over a wid e range of low-laser ablation power densities. The laser-desorption ionizat ion of the pyrolyzed coal-tar pitches shows the presence of a range of poly aromatic hydrocarbons some containing as many as 70 carbon atoms, which sho uld require only small amounts of laser energy to convert them into fullere nes. Metallofullerene formation also occurs with low power laser ablation o f pyrolyzed mixtures of metal oxides, metal sulfides, or metal carbonates a nd the Koppers coal-tar pitch. Ablation with two different laser pulse widt hs of 230 mu s and 8 ns, at a laser wavelength of 1064 nm, shows that only the long-pulse low-power (200-2000 kW cm(-2)) laser radiation produces meta llofullerene ions. The short-pulse high-power (1-2300 MW cm(-2)) laser abla tion results only in the formation of small metal carbide cluster and fulle rene ions. Collision-induced dissociation experiments were used to study th e structures of the small metal carbide clusters and the endohedral metallo fullerene ion, La@ C-60(+).