Electron Spin Resonance of Radiation-Induced Defects in Potassium Azide

Abstract

Trapped nitrogen atoms and ${\mathrm{N}}_{2}^{\ensuremath{-}}$ molecule ions have been detected in potassium azide (K${\mathrm{N}}_{3}$) by electron spin resonance measurements at 77\ifmmode^\circ\else\textdegree\fi{}K following gamma-ray irradiation at this temperature. The nitrogen atom has effective spin $S=\frac{3}{2}$, and each fine structure component shows the hyperfine pattern of a single ${\mathrm{N}}^{14}$ nucleus. The atoms are trapped in two magnetic sites having rhombic symmetry which are equivalent under a 90\ifmmode^\circ\else\textdegree\fi{} rotation about [001]. Choosing $z\ensuremath{\parallel}[110]$, $x\ensuremath{\parallel}[1\overline{1}0]$, and $y\ensuremath{\parallel}[001]$, the results can be fitted to the spin-Hamiltonian, $\mathcal{H}=|\ensuremath{\beta}|\mathrm{H}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{g}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{S}+D{{S}_{z}}^{2}+E({{S}_{x}}^{2}\ensuremath{-}{{S}_{y}}^{2})+A\mathrm{I}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{S}$, where $g=2.001\ifmmode\pm\else\textpm\fi{}0.001$, $D=+0.0143\ifmmode\pm\else\textpm\fi{}0.0001$ ${\mathrm{cm}}^{\ensuremath{-}1}$, $E=\ensuremath{-}0.00199\ifmmode\pm\else\textpm\fi{}0.00002$ ${\mathrm{cm}}^{\ensuremath{-}1}$, and $A=0.00051\ifmmode\pm\else\textpm\fi{}0.00002$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The other defect, attributed to ${\mathrm{N}}_{2}^{\ensuremath{-}}$, 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\ifmmode^\circ\else\textdegree\fi{} rotation about [001]. For one site and the same choice of axes, ${g}_{x}=2.001\ifmmode\pm\else\textpm\fi{}0.002$, ${g}_{y}=2.001\ifmmode\pm\else\textpm\fi{}0.001$, and ${g}_{z}=1.984\ifmmode\pm\else\textpm\fi{}0.001$; ${A}_{x}=0.0006\ifmmode\pm\else\textpm\fi{}0.0002$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${A}_{y}={A}_{z}=0.00037\ifmmode\pm\else\textpm\fi{}0.00002$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The annealing behavior of these defects is related to the growth of the ${\mathrm{N}}_{4}^{\ensuremath{-}}$ molecule ion previously reported.