Fig. 1: Laser spectroscopy of \(\bar{p}{{}^{4}{\rm{H}}{\rm{e}}}^{+}\) synthesized in He I and He II targets.
From: High-resolution laser resonances of antiprotonic helium in superfluid 4He
a, Radial distribution functions of the antiproton (red curves) and electron (blue curves) for states (n, ℓ) = (37, 35) and (38, 34) of a single isolated \(\bar{p}{{}^{4}{\rm{H}}{\rm{e}}}^{+}\) atom. Here \({R}_{n\ell }(r)\) denotes the radial component of the antiproton or electron orbital at a distance r from the helium nucleus. Laser transitions between the two states involve an unusually small (Δre ≤ 2 pm) change in the root-mean-square radius of the 1s electronic orbital. b, Energy level diagram of \(\bar{p}{{}^{4}{\rm{H}}{\rm{e}}}^{+}\) indicating the positions of the transitions (37, 35) → (38, 34) and (39, 35) → (38, 34). c, Signal of the Cherenkov detector, which corresponds to the time distribution of \(\bar{p}{{\rm{H}}{\rm{e}}}^{+}\) annihilations. The laser resonance is induced at t ≈ 1.4 μs after \(\bar{p}{{}^{4}{\rm{H}}{\rm{e}}}^{+}\) formation. The large number of annihilations at t = 0 is not shown. d, Experimental layout. Antiprotons emerging from a radio frequency quadrupole (RFQ) decelerator came to rest in either a gaseous or supercritical phase helium, He I, or He II target. The produced \(\bar{p}{{\rm{H}}{\rm{e}}}^{+}\) atoms were irradiated with a resonant laser pulse (see text).
