Mass spectroscopic studies on a reactive laser ablation of graphite at 1064 nm in a pulsed oxygen jet have been performed. Significant changes in the relative enrichment of carbon ions including anomalous enrichment of C-2(+) ions as well as formation of CO+, C2O+, and C3O+ ions are observed. Collision-induced dissociation and reactive scattering of carbon ions prevail by injection of the oxygen jet to the carbon plume. Energetics of the ions is studied by using a time-of-flight quadrupole mass spectroscopy. Time-of-flight (TOF) spectra of each carbon ion consist of a fast and a slow component, which are considered to represent different formation mechanisms. By analyzing the changes of TOF spectra as a function of the delay time between the laser pulse and oxygen jet, we examine the effects of collisions on the formation of carbon ions.