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, the measurement yielding Δν = 203.384 9(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 equivalent to (1/Δν) par. deltaΔν/par. deltaD, have been measured for positronium in nitrogen and the noble gases: a(N2)=(-3.2+-0.5)x10-5 atm-1 (00C), a(He)=(+1.6+-0.3)x10-5 atm-1 (00C), a(Ne)=(-1.9+-0.4)x10-5 atm-1 (00C), a(Ar)=(-6.2+-0.3)x10-5 atm-1 (00C), a(Kr)=(-6.6+-0.5)x10-5 atm-1 (00C), and a(Xe)=(7.3+-1.97)x10-5 atm-1 (0degreeC). The positronium density shifts differ markedly from those of hydrogen