Recently it has been observed that far-UV (193 nm) laser radiation can cleanly etch organic polymer films. In contrast to etching of the film by lower energy radiation, which leaves the walls of the etch region uneven due to melting, far-UV radiation ablates the material with minimum damage to the remaining sample. A model has been developed to explain the photochemical etching due to 193-nm laser light. The predictions from this model are (1) the reacted material ablates without melting the remainder of the sample; (2) the average perpendicular velocity of the ejected material is —1000-2000 m/s; (3) the angular spread is small and peaked in the direction normal to the surface; and (4) the material ablates layer by layer. Items 1, 2, and 3 are in excellent agreement with the experimental results. A model has also been developed to simulate the ablation of solids by the thermal (in contrast to the photochemical) process. The predicted pit shape and angular distribution are quite different from those obtained from the photochemical model. The effect of impurity or solvent molecules is also discussed. © 1985, American Vacuum Society. All rights reserved.