A UV radiation source is tunable to optimize the process of densifying a carbon-doped silicon oxide film. The composition and relative concentration of stimulated gases stimulated within an airtight bulb is controlled to produce radiation optimized for absorption by undesirable chemical bonds of the carbon-doped silicon oxide film, leading to disruption of these bonds and their replacement by more desirable stable chemical bonds. The energy of radiation emitted by the source is determined by the identity of excited chemical species, and the intensity of the radiation emitted by the source is determined by the concentration of the excited chemical species. By exciting a specific mixture of gases, radiation is emitted at a combination of energies and intensities calculated to disrupt populations of unstable bonds in the carbon-doped silicon oxide film while leaving desirable bonds in the film unaffected.