The influence of molecular weight (M-w) in the KrF excimer laser (248 nm) ablation of doped poly(methyl methacrylate) (PMMA) is investigated by means of optical microscopy and micro-Raman spectroscopy. Upon laser irradiation of films of PMMA of three different average M-w, 2.5, 120 and 996 kDa, doped with iodonaphthalene and iodophenanthrene, gas filled bubbles were created in the films at irradiation fluences above 0.1 J/cm(2), well below the ablation threshold of the films. The size and density of bubbles depend on the fluence and on the polymer Mw, with larger bubbles being observed at higher fluences and lower M-w. The changes of intensity of the polymer Raman bands upon irradiation provide information on cleavage of the backbone and side chains of the polymer. The results provide direct support for the bulk photothermal model, according to which ejection requires that a critical number of bonds are broken. (c) 2005 Elsevier B.V. All rights reserved.