  Recently, Nb superconducting radio frequency cavities vacuum heat treated
between 300-400 C for a few hours have exhibited very high quality factors
(~5x10^10 at 2.0 K). New secondary ion mass spectrometry measurements of O, N
and C show this enhancement in RF surface conductivity is primarily associated
with interstitial O alloying via dissolution and diffusion of the native oxide.
We use a theory of oxide decomposition and O diffusion to quantify previously
unknown parameters crucial in modeling this process. RF measurements of a
vacuum heat treated Nb superconducting radio frequency cavity confirm the
minimized surface resistance (higher Q0) previously expected only from 800 C
diffusive alloying with N.
