Fig. 2: Resonance profiles of the transition (n, ℓ) = (37, 35) → (38, 34) synthesized in various targets.
From: High-resolution laser resonances of antiprotonic helium in superfluid 4He
Spectral lines measured in gaseous and supercritical helium (green data points) at pressures p = 101 kPa, 231 kPa, 343 kPa and 424 kPa, and temperature T = 6.0–6.4 K show progressively larger collisional shifts and broadenings relative to the data9 measured in gas of p = 0.1 kPa and T ≈ 1.6 K (grey) with a linewidth ΓL = 0.19(3) GHz. The best fits of four overlapping Lorentzian functions are shown superimposed on each spectrum. Spectral lines observed in He I (blue) of T = 4.01 K, 3.51 K, 3.01 K and 2.20 K became narrower as the liquid temperature was reduced. A rapid reduction of the linewidth below the He II transition temperature revealed the hyperfine structure, with the sharpest spectral lines observed between T = 1.78(2) K and 1.88(2) K (red). Error bars, 1 s.d.
