Precision determination of the fine-structure interval in the ground state of positronium. IV. Measurement of positronium fine-structure density shifts in noble gases

The fine-structure interval Δν of ground state ($n=1$) positronium has been measured to high precision, yielding Δν=203.3849(12) GHz (6 ppm). Comparison with theory is at present limited by uncalculated terms of order α²Δν. Values for the linear coefficient of the fine-structure density shift, a≡(1Δν)∂Δν∂D, have been measured for positronium in nitrogen and the noble gases: a(N₂)=(-3.2±0.5)×10^-5 atm^-1 (0^∘{}C), a(He)=(+1.6±0.3)×10^-5 atm^-1 (0^∘{}C), a(Ne)=(-1.9±0.4)×10^-5 atm^-1 (0^∘{}C), a(Ar)=(-6.2±0.3)×10^-5 atm^-1 (0^∘{}C), |a(Kr)=(-6.6±0.5)×10^-5 atm^-1(0^∘{}C), and a(Xe)=(-7.3±1.7)×10^-5 atm^-1 (0^∘{}C). The positronium density shifts differ markedly from those of hydrogen.