The optical emission from plasma produced by excimer laser (ArF, lambda = 193 nm) ablation of Si, SiO and Si3N4 targets was studied in the visible range. We present results of the time integrated optical spectroscopy measurements and time-of-flight analysis of the particle expansion dynamics. Both methods show the formation of diatomic molecules within the laser induced plasma plume. The velocities of the ejected species are strongly affected by a chemical reaction inside the plasma plume.