Observation of the 1154.9 nm transition of antiprotonic helium

The resonance transition $(n,l)=(40,36)\to(41,35)$ of the antiprotonic helium ($\overline{\mathrm{p}}^{4}$He$^{+}$) isotope at a wavelength of 1154.9 nm was detected by laser spectroscopy. The population of $\overline{\mathrm{p}}^{4}$He$^{+}$ occupying the resonance parent state $(40,36)$ was found to decay at a rate of $0.45\pm0.04$ $\mu$s$^{-1}$, which agreed with the theoretical radiative rate of this state. This implied that very few long-lived $\overline{\mathrm{p}}^{4}$He$^{+}$ are formed in the higher-lying states with principal quantum number $n\ge41$, in agreement with the results of previous experiments.