The early stages of the surface microstructural modification of silicon, induced by single pulses of CO2 laser irradiation (lambda = 10.6 mum), have been studied, in both vacuum and air, by contact mode AFM. The laser pulse was found to be absorbed at the front surface of the sample, facing the laser; this was shown to be due to the presence of native oxide, which absorbs at this wavelength. We found that this absorption of energy caused the stress-induced formation of vertically oriented, square-shaped fragments, 400-700 nm in length, often with short branches, that form a wall around the impact site; they oriented toward the plane of the sample with distance from the impact site, aligning more in the electric field direction of the pulse. In addition, electrically charged, branched fragments were redeposited at the outer extremities of the pulse site. (C) 2003 Elsevier B.V. All rights reserved.