4.2.1 Muonium in Solid Nitrogen

Muon spin relaxation experiments were carried out on samples of solid, pure nitrogen using conventional time-differential transverse field (TF) techniques. In the low temperature $\alpha$ phase a large muonium signal and a smaller diamagnetic muon signal together account for virtually all the muon polarization. In $\beta$-N₂ and $\ell$-N₂ some of the diamagnetic fraction appears as an additional fast-relaxing signal (with initial asymmetry $A_{\rm D}^f$ and relaxation rate $\lambda_{\rm D}^f$ ) and all the asymmetries are temperature dependent. The total asymmetry therefore has the form

The slowly-relaxing diamagnetic asymmetry $A_{\rm D}^s$and muonium asymmetry $A_{\rm Mu}$ obtained are shown in Fig. 4.11. The most striking feature in the temperature dependence of these is the obvious anticorrelation between them at temperatures near T$_{\alpha\beta}$,suggesting the presence of competing processes in which the stopping muon either captures an electron to form muonium, or eventually becomes incorporated into a molecular ion. Since only half of the muonium asymmetry is experimentally observable, (the other half oscillating too fast to be resolved - see Appendix A) loss of some of the diamagnetic species to muonium formation results in an increase in the muonium asymmetry half as large. The total $A_{\rm D}^s + A_{\rm D}^f + 2 A_{\rm Mu}$ is nearly temperature independent. Since the free electron mobility in $\beta$-N₂ increases with temperature below T$_{\alpha\beta}$ like the muonium fraction, this strongly suggests that transport of electrons through the lattice is involved in muonium formation in solid nitrogen. The electron mobility measured in solid nitrogen by Loveland et al. is shown in Fig. 4.12. From 63 K down to 53 K the mobility $\mu_{\rm e}(T)$ is constant at about 1.7$\times$10^-3 cm²s^-1V^-1; it then decreases gradually to half this value at T$_{\alpha\beta}$[19]. The available mobility data below T$_{\alpha\beta}$doesn't reveal the trend in $\alpha$-N₂ but it does indicate that the mobility is sharply increased to about $2.0 \times 10^{-3}{\rm cm}^2 {\rm s}^{-1} {\rm V}^{-1}$in the $\alpha$ phase. Over this range in T the muonium asymmetry changes by about the same fraction as the electron mobility, at least in the $\beta$ phase.

  

Figure 4.11: Muonium and slowly relaxing diamagnetic asymmetries measured in solid nitrogen.

  

Figure: Electron mobility in solid nitrogen measured by Loveland et al. [19] using a direct time-of-flight method. Various symbols indicate different samples.