Carbon clusters and hydrocarbon molecules and radicals produced by laser vaporization of graphite in a hydrogen (H-2 or D-2) gas expansion source were studied using 10.5 eV one-photon ionization followed by time-of-flight mass spectroscopy. By changing the delay time of the ionization pulse relative to the vaporization pulse, we found two distinctly different mass patterns: rather non-selective signals of C-n (n = 6-20) and CnHm (n = 3-20 and m = 1-4) at shorter delay times, while selective signals of C2nH2 (n = 2-5) and C-10 at longer delay times. The selective formation of the carbon cluster and hydrocarbon molecules at longer delay times is attributed to the relatively inert nature of polyynes H(-Cequivalent toC-)(n)H (n = 2-5) and monocyclic C-10. The formation mechanism of these hydrocarbons and carbon clusters by laser ablation under hydrogen gas is discussed. (C) 2003 Elsevier B.V. All rights reserved.