The etch rate and surface SiCl and SiCl2 concentrations abruptly increase and the surface Si concentration abruptly decreases as the chlorine fraction is increased above a threshold value during Si etching by an inductively coupled Cl2–Ar mixture plasma. The surface species are detected by laser desorption of the chlorinated adlayer using laser-induced fluorescence and plasma-induced emission of the desorbed species. This threshold chlorine fraction is 75% for 80 eV average ion energy, and increases with the average energy of ions incident on the surface. This unexpected observation is attributed to a phase transition between two states of surface chlorination and morphology. © 2003 American Institute of Physics.