Fig. 4: Collisional shifts of the resonance frequency of the transition (n, ℓ) = (37, 35) → (38, 34) observed in gas, supercritical phase, 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 squares), He I (blue triangles) and He II (red circles) targets relative to the zero-density transition frequency of ν0 = 412,885.133(1) GHz (ref. 9) as a function of the atomic density. b, c, Magnified views of a, showing a deviation from a linear density dependence at target temperature 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 single error bar labelled as c.e. (see Methods). 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.
