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Ultraviolet laser-induced etching is a method of machining and nanostructuring diamond surfaces in which carbon is removed from the surface via a photochemical process involving oxygen. We show here that using a dry source of oxygen at pressures in the range of 0.01-1 Torr leads to a 10-fold increase in the etch rate compared to etching in atmospheric air. The enhanced etch rate is also found to be accompanied by a marked change in the nanopatterned surface morphology. We developed a rate equation model for the etch rate that provides good agreement with measurements for pressures up to approximately 0.1 Torr. For higher pressures, the reduced etch rate and departure from the model are attributed to the contamination of the diamond surface by trace amounts of water vapor, introduced as an impurity from the gas sources. The results provide a method for markedly increasing the etch rate, as well as a better understanding of the role of gas impurities on the etch mechanism and emergent nanopattern formation.