Onset of carbon cluster formation inferred from light emission in a laser-induced expansion

Space-time analysis of the light emitted from C, ${\mathrm{C}}_2$, and ${\mathrm{C}}_3$ has been performed in a laser-evaporated carbon expansion. The measured propagation velocities and concentration evolutions display the aggregation process as a half-collision governed by momentum conservation. ${\mathrm{C}}_2$ and ${\mathrm{C}}_3$ clusters are produced with strong electron excitation, the energy excess of which is removed by photon emission instead of collisional cooling.