A three-dimensional Monte Carlo simulation of the laser desorbed flow of particles was performed. In addition to the gas-phase collisions, the chemical processes, i.e., recombination and dissociation are considered. The width of the angular distributions as a function of the radius of the evaporated area is analyzed. The comparison between the results obtained in the reactionless flow and in the flow with chemical reactions shows that in the presence of chemical processes the focusing effect of collisions is partly concealed and more wide angular distributions can be observed.