Stress-Induced Polarization ofF-Center Luminescence

The degree of linear polarization of $F$-center luminescence induced by a uniaxial stress has been investigated in KCl, NaCl, RbCl, and NaF single crystals. The amount of polarization was found to be proportional to the magnitude of the applied stress and dependent upon the crystallographic direction along which the stress was applied. The effect was found to be independent of the crystal temperature, the wavelength of the luminescence, the $F$-center concentration, and the polarization of the exciting light. The results can be successfully analyzed in terms of emission from a $p$-like relaxed excited state which is very diffuse and largely unaffected by the stress, to an $s$-like ground state which is still mainly localized at the potential well and into which a small amount of $d$-like character is mixed by the stress.