Precise hyperfine pressure-shift measurements for hydrogen isotopes in argon

The pressure shift of the hyperfine splitting ν in the ground state of the hydrogen isotopes for an argon buffer gas has been measured with about 0.1% accuracy by an optical-pumping method with improvements in sample design, temperature control, and magnetic field stabilization. If the shift is expressed as $\left(\frac{1}{{\nu }_0}\right)\left(\frac{\delta \nu }{\delta \rho }\right)=A+B(T-T_0)$, the results for hydrogen, deuterium, and tritium are, respectively, $A_H=(-4.800\pm 0.006)\times {10}^{-9}$ torr⁻¹ (0°C); $B_H=(+0.956\pm 0.011)\times {10}^{-11}$ °C⁻¹ torr⁻¹ (0°C); $A_D=(-4.802\pm 0.022)\times {10}^{-9}$ torr⁻¹ (0°C); $B_D=(+1.091\pm 0.047)\times {10}^{-11}$ torr⁻¹ (0°C); A_T=(-4.795±0.012)×10⁻⁹ torr⁻¹ (0°C); B_T=(+0.929±0.021)×10⁻¹¹ torr⁻¹ (0°C); where the arbitrary reference temperature T₀=26°C has been used in every case. The density ρ is quoted in units of torr (0°C), the equivalent of 3.536×10¹⁶ atoms/cm³. The absence of isotope dependence supports existing theories of the shift and encourages comparison with the measurements of muonium hyperfine splitting.