  We show a marked reduction in the emission from nitrogen-vacancy (NV) color
centers in single crystal diamond due to exposure of the diamond to hydrogen
plasmas ranging from 700{\deg}C to 1000{\deg}C. Significant fluorescence
reduction was observed beneath the exposed surface to at least 80mm depth after
~10 minutes, and did not recover after post-annealing in vacuum for seven hours
at 1100{\deg}C. We attribute the fluorescence reduction to the formation of NVH
centers by the plasma induced diffusion of hydrogen. These results have
important implications for the formation of nitrogen-vacancy centers for
quantum applications, and inform our understanding of the conversion of
nitrogen-vacancy to NVH, whilst also providing the first experimental evidence
of long range hydrogen diffusion through intrinsic high-purity diamond
material.
