ESR of Cl2− in various ion pairs at 4° K and the theory of V centers

ESR spectra have been measured of M+Cl2− ion pairs trapped in solid argon at 4° K, where M=Mg, Ca, Sr, Ba, Li, Na, and K. The derived g and hyperfine tensors of Cl2− are interpreted using the simple crystal field model of Schoemaker, and the spin densities and crystal field parameters E⊥ are compared with those for V centers in alkali halide crystals. From matrix data, it is proposed that the isotropic hyperfine coupling parameter Aσ for interaction with 35Cl in pseudo-’’free’’ Cl2− be taken as 37±1 G. A discussion is given pointing out an approximate linear correlation between the nearest intercation distance (d) and Δg⊥ (or Aσ) among all the VK centers. Constraints imposed by lattices of varying M+/Cl− radius ratios cause variations in d and in the s/pz character in the antibonding Cl2− electron, leading to corresponding changes in Aσ and inverse changes in Δg⊥. VF centers, with minimum constraints, have maximum values of Aσ.