Extended Data Fig. 1: Collisional shifts of the resonance frequency of the transition (n, ℓ) = (39, 35) → (38, 34) observed in gas and supercritical phase helium, He I and He II targets.
From: High-resolution laser resonances of antiprotonic helium in superfluid 4He
a, Shifts in gaseous and supercritical helium (indicated by green square markers), He I (blue triangles), and He II (red circles) targets relative to the zero-density transition frequency of ν0 = 501,948.753(2) GHz9 as a function of the atomic density. b,c Magnified views of a, showing a deviation from a linear density dependence at target termperature T < 2.5 K. The results of the best fits of linear functions on the gaseous and supercritical helium (green line) and He I (blue line) data at T ≥ 2.7 K are shown superimposed. The size of the correlated uncertainty arising from the conversion of the liquid helium pressure and temperature into atomic density based on the International Temperature Scale of 1990 is indicated separately by the error bar labelled as c.e. (see text). d, The temperature dependence shows an abrupt change in the gradient dν/dT at the He II transition temperature Tλ = 2.18 K. Error bars, 1 s.d.
