1. For a selection of important contributions see “Spin-Lattice Relaxation in Ionic Solids”. Eds.: A. A. and R. Orbach. New York: Harpers and Row 1969.
2. Van Vleck, J. H.: Phys. Rev.51, 426 (1940).
3. Orbach, R.: Proc. Roy. Soc. (London)A 264, 485 (1961).
4. The simplest and most important paramagnetic molecular ions observed have been the diatomic typeX 2 − whereX can be a halogen atom,O, N, orS atom. WhenX is the halogen host atom in alkali halide crystal the center is either an intrinsic VK-center or H-center (the H-center is an interstitial halogen atom, but withX 2 − occupying a substitutional halogen position and it usually possesses the same symmetry as the substitutionalO 2 −,N 2 −, orS 2 − impurity ions). Paramagnetic ions of types XY, (XY)−, (X2Y)− and X2 − have also been observed. For references the reader is referred to the review article by H. Seidel and H. C. Wolf in Physics of Color Centers. Ed.: W. Beall Fowler. New York: Academic Press 1968.
5. Känzig, W., Cohen, M. C.: Phys. Rev. Letters3, 509 (1959).