First Observation of the $E2\mathrm{}$ Nuclear-Resonance Effect in Antiprotonic Atoms

Antiprotonic x-ray spectra of ${}_{}{}^{92}{}_{}{}^{}\mathrm{Mo}$, ${}_{}{}^{94}{}_{}{}^{}\mathrm{Mo}$, ${}_{}{}^{95}{}_{}{}^{}\mathrm{Mo}$, ${}_{}{}^{98}{}_{}{}^{}\mathrm{Mo}$, and ${}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$ have been measured with Ge detectors. The $E2\mathrm{}$ nuclear-resonance effect was observed for the first time in $\overline{p}$ atoms, yielding information on the "hidden" $n=5\mathrm{}$, $l=4\mathrm{}$ level of the $\overline{p}$-molybdenum atom. Strong interaction widths and shifts affecting the 7 → 6 transition were determined and the effective $\overline{p}$-nucleon scattering length was deduced. A unique strong-coupling effect was established in $\overline{p}-{}_{}{}^{100}{}_{}{}^{}\mathrm{Mo}$.