Molecular dynamics simulations are performed in order to understand the fragmentation and emission mechanisms of kDa chain segments when a polymer (polystyrene) is bombarded by a keV particle (Ar). The calculated energy spectra of small fingerprint fragments and kDa chain segments are compared to experimental distributions. The results show that the ejection of small fragments is primarily induced by interatomic collisions. In contrast, vibrational excitation of the polymer plays a major role in the emission of kDa chain segments. (C) 2002 Elsevier Science B.V.. All rights reserved.