First Observation of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>E</mml:mi><mml:mn>2</mml:mn><mml:mn/></mml:math>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 $E2$ nuclear-resonance effect was observed for the first time in $\overline{p}$ atoms, yielding information on the "hidden" $n=5$, $l=4$ level of the $\overline{p}$-molybdenum atom. Strong interaction widths and shifts affecting the 7 \ensuremath{\rightarrow} 6 transition were determined and the effective $\overline{p}$-nucleon scattering length was deduced. A unique strong-coupling effect was established in $\overline{p}\ensuremath{-}^{100}\mathrm{Mo}$.