Electron Spin Resonance of Radiation-Induced Defects in Potassium Azide

Trapped nitrogen atoms and N₂^- molecule ions have been detected in potassium azide (KN₃) by electron spin resonance measurements at 77^∘{}K following gamma-ray irradiation at this temperature. The nitrogen atom has effective spin S=3/2, and each fine structure component shows the hyperfine pattern of a single N¹⁴ nucleus. The atoms are trapped in two magnetic sites having rhombic symmetry which are equivalent under a 90^∘{} rotation about [001]. Choosing z∥[110], x∥[11̄0], and y∥[001], the results can be fitted to the spin-Hamiltonian, $H=|{β}|H·{}g·{}S+D{{S}z}²+E({{S}ₓ}²{-}{{S}y}²)+AI·{}S$, where $g=2.001±{}0.001$, $D=+0.0143±{}0.0001$ ${cm}^{{-}1}$, $E={-}0.00199±{}0.00002$ ${cm}^{{-}1}$, and $A=0.00051±{}0.00002$ ${cm}^{{-}1}$. The other defect, attributed to ${N}₂^{{-}}$, has a spectrum characteristic of a single unpaired electron and a hyperfine pattern corresponding to two equivalent nitrogen nuclei, and is located in two equally populated magnetic sites related by a 90^∘{} rotation about [001]. For one site and the same choice of axes, gₓ=2.001±0.002, gy=2.001±0.001, and gz=1.984±0.001; Aₓ=0.0006±0.0002 cm^-1 and Ay=Az=0.00037±0.00002 cm^-1. The annealing behavior of these defects is related to the growth of the N₄^- molecule ion previously reported.