Review of some experiments in the 50 year saga of the E′ center and suggestions for future research

Fifty years after the first paper on the electron spin resonance (ESR) spectrum of the E′ center in silica and crystal was published, research on E′ centers is still an active topic of study. In the ensuing 50 years more than 3000 papers have been published in which the E′ center is the primary topic or an important part of the papers. More than 15 varieties of E′ centers have been reported. Early on there was a definitive correlation between the E′ center and the optical absorption at 5.83 eV leading to the established value of oscillator strength of f = 0.14. New results have called this assignment of oscillator strength into question and have raised the possibility of a diamagnetic band between 5.7 and 5.9 eV. Measurements of the temperature dependence of the spin-lattice relaxation time of the E 1 ′ and E 2 ′ centers in single crystals, and the E′ center in silica have been made from ∼1.24 K to 200 K. A function containing two Raman inelastic scattering terms due to two local vibration modes in each case and, in the case of the E 1 ′ and E 2 ′ in alpha quartz, a third term due to an optical mode fit the data to within ±10% from ∼5 K to 250 K. In the E 1 ′ and E γ ′ cases a fourth term due to elastic scattering, i.e. direct relaxation, at temperatures <5 K, is observed. The coefficients for the functions differ. The direct relaxation process is not observed in the E 2 ′ case because of cross relaxation effects. Some of the questions, concerning the E′ type centers, that remain are: what is the magnitude of the oscillator strength of the E′ band at 5.83 eV? what is the identity and the number of the other optical bands between 5.5 and 6.0 eV? and what are the causes of the temperature dependence of the spin relaxation time of various types of E′ centers?