Precision determination of the fine-structure interval in the ground state of positronium. III

With an improved microwave magnetic resonance apparatus the Zeeman transition in orthopositronium has been remeasured and hence the ground-state fine-structure interval $\Delta \nu$ in positronium has been determined with higher precision: $\Delta \nu =203.384\mathrm{}\pm 0.004$ GHz (20 ppm). The measurements were made in ${\mathrm{N}}_2$ gas and gave the value $\left(\frac{1}{\Delta \nu }\right)\frac{\partial \left(\Delta \nu \right)}{\partial D}=(-5.7\mathrm{}\pm 1.4)\times {10}^{-5\mathrm{}}$ ${\mathrm{atm}}^{-1\mathrm{}}$ at 23°C for the coefficient of the fine-structure density shift. Relativistic binding corrections to the positronium Zeeman transition were taken into account.