Strong population of excited 0/sup +/ states in even Zr-isotopes observed with the (/sup 14/C,/sup 16/O) reaction

The two-proton pickup reaction ($^{14}\mathrm{C}$, $^{16}\mathrm{O}$) on the even Mo isotopes was investigated at ${E}_{\mathrm{lab}}=71$ 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 $N=50$ 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.