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Ultrashort laser pulses interacting with brittle dielectrics in the mid-infrared region of the spectrum produce a numberof novel effects which are potentially useful in materials processing and analysis. These include the texturing of the surface,the generation of hydrodynamic instabilities, and a surprisingly efficient and gentle ablation behavior. Nevertheless, themechanism of infrared laser ablation remains somewhat mysterious. Here we present evidence for a mechanism of explosivevaporization in fused silica, initiated by picosecond pulses from a tunable free-electron laser operating in the wavelength re-gion from 2-10 j.m. The unusual pulse structure of the free-electron laser — which produces I -Ps micropulses at intervals of350 ps in a macropulse lasting up to 4 ts — makes it possible to test separately the effects of intensity and fluence. We showin particular that thermal descriptions of the ablation process fail in the regime where there is high vibrational excitation den-sity in the solid due to resonant absorption ofmid-infrared laser light.Keywords: ablation, infrared, vibrational excitation, explosive vaporization, phase explosion