Focused ion beams are an established but inherently slow technique for many nanopatterning applications. One way to increase its processing speed is by gas-assisted ion-beam induced etching. However, to understand and improve this process, the ability to accurately simulate the precursor coverage is very important, because it strongly affects the efficiency of the process. In this paper, the authors present a recoils-based simulation model that considers precursor adsorption, decomposition, and diffusion. The authors provide a non-steady-state solution for translational symmetry, which they use to investigate the influence of the precursor diffusion coefficient on the etching process. They find that the diffusion coefficient influences the shape of the bottom of the irradiated structure. Furthermore, they compare the simulation results to experiments of SiO2 etched by XeF2 using a focused Ga ion beam, and extract model parameters such that the etching rate of numerous experiments with different current densities can be predicted accurately. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3481139]