Neutral carbon clusters produced from laser-ablated graphite in a supersonic pulsed-helium expansion source were studied by time-of-flight (TOF) mass analysis using single-photon ionization with 10.5 eV photons. Varying the delay time of an ionization laser pulse relative to a vaporization pulse, we found that a signal of C-10, along with a weaker signal of C-12, was intensified almost exclusively to the other C-n signals with relatively long delay times of 80-250 mus. We observed two distinctly different TOFs for one and the same size, a short TOF at shorter delay times and a long TOF at longer delay times. We attribute the difference in TOF to the difference in initial velocity of the neutral cluster. We also performed the experiment within a high vacuum to find a similar difference in TOF for clusters of the same mass. The bimodal arrival-time distribution from the source to the ionization region indicates that the bunch of laser-ablated clusters separates into two bunches with different group velocities. We attribute this separation to the formation of a relatively dense layer of clusters. During collisions behind this layer, the relatively stable neutral C-10, probably of a monocyclic structure, is formed preferentially. This must be the origin of the selective detection of C-10 at the longer delay times. Using He as a buffer gas, the signal of the C-10 was found to be of a magnitude two orders more pronounced than within the high vacuum. (C) 2003 American Institute of Physics.