The high-energy particle bombardment of a molecular film adsorbed upon a metal substrate has been investigated via molecular dynamics computer simulations with an empirical many-body potential energy function constructed for studying reactive dynamics. The specific system modeled is the bombardment of an ethylidyne (C2H3) overlayer adsorbed on Pt(111) by a 500-eV Ar atom beam. Approximately 80% of the ejected hydrocarbon species originate from a single C2H3 adsorbate, while the others result from reactions between two C2H3 adsorbates. A study of the internal energies of all of the ejected hydrocarbon aggregates reveals that those originating from a single C2H3 adsorbate are generally stable to any further fragmentation or rearrangement. Examples of common ejection mechanisms for species which originate from a single adsorbate, such as CH3, C2H3, or HCCH, and those which originate from more than one adsorbate, such as CH4, are given.