Amorphous tantala (Ta2O5) thin films were deposited by reactive ion beam sputtering with simultaneous low energy assist Ar+ or Ar+/O2+ bombardment. Under the conditions of the experiment, the as-deposited thin films are amorphous and stoichiometric. The refractive index and optical band gap of thin films remain unchanged by ion bombardment. Around 20% improvement in room temperature mechanical loss and 60% decrease in absorption loss are found in samples bombarded with 100-eV Ar+. A detrimental influence from low energy O2+ bombardment on absorption loss and mechanical loss is observed. Low energy Ar+ bombardment removes excess oxygen point defects, while O2+ bombardment introduces defects into the tantala films.