Strong population of excited0+states in even Zr isotopes observed with the (C14,O16) reaction

The two-proton pickup reaction (${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$) on the even Mo isotopes was investigated at $E_{\mathrm{lab}}=71\mathrm{}$ MeV. It was found that the ratio of the ground and first excite ${\mathrm{O}}^{+}$ state cross sections changes dramatically as neutrons are added to the $N=50\mathrm{}$ shell. While the reaction ${}_{}{}^{92}{}_{}{}^{}\mathrm{Mo}$(${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$)${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ populates the ground state three times more strongly than the first excited ${\mathrm{O}}^{+}$ state, for the heaviest isotope ${}_{}{}^{98}{}_{}{}^{}\mathrm{Zr}$, the excited ${\mathrm{O}}^{+}$ state has twice the population of the ground state. Attempts to explain the cross section ratios by means of distorted-wave Born approximation calculations, based on a complete Brody-Moshinsky decomposition of the two-proton cluster and spherical shell model wave functions, were not successful.