Dry etching mechanisms of thermal and photochemical reactions with Cl-2 and HCl on GaAs(100) Ga-rich c(8x2) surfaces were investigated, using x-ray and ultraviolet photoelectron spectroscopy. At a substrate temperature of -100 degrees C, the etchant gases are chemisorbed on the GaAs surface. Cl-2 deposition on GaAs produces about five times more Cl-containing species than HCl deposition. The GaAs surface is disrupted with Cl-2 when the substrate temperature is increased to 250 degrees C after the saturated deposition of Cl-2 at -100 degrees C. A similar etching procedure with HCl farms an ordered surface of GaAs with As atoms on the top layer. After desorption of the etchant gases, the photoirradiation effect at room temperature was investigated by pulsed laser irradiation with excimer (193 and 248 nm) and YAG (266, 355, and 532 nm) lasers at an intensity of 2-10 mj/cm(2). Only the 193 nm irradiation removes Cl-containing species from the surface. With cw laser irradiation at 488 nm (60 mW/cm(2)), photochemical reactions take place on the GaAs substrate on which Cl-2 has been deposited.