Spin-Alignment and Quasi-Molecular Resonances in Heavy-Ion Collision

Fragment-fragment-$\gamma$ triple coincidence measurements of the $^{28}$Si~$+$~$^{28}$Si reaction at E$_{\rm c.m.}$~=~55.8 MeV, carefully chosen to populate a well known quasi-molecular resonance in $^{56}$Ni, have been performed at the VIVITRON tandem facility by using the Eurogam Phase~II $\gamma$-ray spectrometer. In the $^{28}$Si~$+$~$^{28}$Si reaction, the resonant behavior of the $^{28}$Si $+$ $^{28}$Si exit-channel is clearly observed by the present fragment-fragment coincidence data. The more unexpected result is the spin disalignment of the $^{28}$Si $+$ $^{28}$Si resonance. This has been demonstrated first by the measured angular distributions of the elastic 0$^{+}$, inelastic 2$^{+}$ and mutual excitation channels 2$^{+}-2^{+}$, which are dominated by a unique and pure partial wave with L = 38 ${\rm \hbar}$, and has been confirmed by measuring their particle-$\gamma$ angular correlations with Eurogam Phase II. The spin disalignment supports new molecular model predictions, in which the observed resonance would correspond to the ''Butterfly mode''. A discussion concerning the {\it spin alignment and spin disalignment} for different systems : $^{12}$C~$+$~$^{12}$C, $^{24}$Mg~$+$~$^{24}$Mg and $^{28}$Si~$+$~$^{28}$Si will be given.}