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 (1ν₀)(δνδρ)=A+B(T-T₀), the results for hydrogen, deuterium, and tritium are, respectively, AH=(-4.800±0.006)×10^-9 torr^-1 (0^∘{}C); BH=(+0.956±0.011)×10^-11 ^∘{}C^-1 torr^-1 (0^∘{}C); AD=(-4.802±0.022)×10^-9 torr^-1 (0^∘{}C); BD=(+1.091±0.047)×10^-11 torr^-1 (0^∘{}C); AT=(-4.795±0.012)×10^-9 torr^-1 (0^∘{}C); BT=(+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.