We have utilized an ArF excimer laser in a projection imaging geometry to etch GaAs layers grown in situ within a molecular beam epitaxy (MBE) system. Clean epitaxial surfaces were prepared using standard MBE growth procedures, then etched in a second chamber isolated by a gate valve. We report studies of the etching dependence on pulse rate (0.2 Hz to 150 Hz) and chlorine pressure (10−6, 10−4, and 5×10−3 Torr). We discuss the etching mechanism in terms of previous studies of the reaction between GaAs surfaces and molecular chlorine under high vacuum conditions. We show that the pulse rate and Cl2 pressure dependencies of the etching can be explained by the progress of the reaction at the instant of pulse exposure. Etch morphology is found to vary with the repetition rate, with the smoothest surfaces obtained at low repetition rates. We discuss the implications of the results in the context of in situ thin layer processing and epitaxial overgrowth.