Giant Dipole Resonance as a Fingerprint of $\alpha$ Clustering Configurations in ${}^{12}\mathrm{C}$ and ${}^{16}\mathrm{O}$

It is studied how the $\alpha$ cluster degrees of freedom, such as $\alpha$ clustering configurations close to the $\alpha$ decay threshold in ${}^{12}\mathrm{C}$ and ${}^{16}\mathrm{O}$, including the linear chain, triangle, square, kite, and tetrahedron, affect nuclear collective vibrations with a microscopic dynamical approach, which can describe properties of nuclear ground states well across the nuclide chart and reproduce the standard giant dipole resonance (GDR) of ${}^{16}\mathrm{O}$ quite nicely. It is found that the GDR spectrum is highly fragmented into several apparent peaks due to the $\alpha$ structure. The different $\alpha$ cluster configurations in ${}^{12}\mathrm{C}$ and ${}^{16}\mathrm{O}$ have corresponding characteristic spectra of GDR. The number and centroid energies of peaks in the GDR spectra can be reasonably explained by the geometrical and dynamical symmetries of $\alpha$ clustering configurations. Therefore, the GDR can be regarded as a very effective probe to diagnose the different $\alpha$ cluster configurations in light nuclei.

Figure 1

(a) Comparison of the GDR calculation for ${}^{16}\mathrm{O}$ (solid blue line, scaled by the left $Y$ axis) against experimental data (nuclear photoabsorption cross section on the oxygen target), Ref. [48] (empty triangles, scaled by the right $Y$ axis), and first principles calculation [49] (long dashed red line, scaled by the right $Y$ axis). (b) Time evolution of $D_G$ in the excited direction (solid blue line) and the nonexcited direction (short dashed black line).

Figure 2

${}^8\mathrm{Be}$, ${}^{12}\mathrm{C}$, and ${}^{16}\mathrm{O}$ GDR spectra with different cluster configurations. The corresponding $\alpha$ cluster configuration in the present eqmd model calculation is drawn in each panel, in which blue and red balls indicate protons and neutrons, respectively. The dynamical dipole evolution of ${}^8\mathrm{Be}$, ${}^{12}\mathrm{C}$, and ${}^{16}\mathrm{O}$ with linear-chain configurations are shown in [50].

Figure 3

Dependence of the GDR of ${}^{16}\mathrm{O}$ with a square configuration on a different binding energy. The left (right) peaks originate from the GDR parallel (perpendicular) to the configuration plane.