Abstract: The specification discloses a process for modifying the environment of active atoms in a selected material by removing impurities from the environment. The material is heated at an elevated temperature in an atmosphere conducive to the formation of atomic oxygen so that the atomic oxygen can react with the atoms and molecules in the environment to remove the impurities in and thus alter selected properties of the material. In a preferred embodiment of the present invention, the lasing efficiency of a neodymium-doped yttrium aluminum garnet crystal is increased by heating the crystal at an elevated temperature in an atmosphere conducive to the formation of atomic oxygen so that the atomic oxygen can react with and remove impurities from the crystal to diminish the color center concentration therein and thus increase the fluorescence yield and lasing efficiency of the crystal.