Comment on “New formulas for the $\left(-2\right)$ moment of the photoabsorption cross section, ${\sigma }_{-2}$”

Empirical formulas for the second inverse moment of the photoabsorption cross sections in nuclei are discussed by Orce [Phys. Rev. C 91, 064602 (2015)]. In this Comment I point out that the experimental values used are systematically too small in heavy nuclei by about 5–10% because of the neglection of the $E1$ strength below the neutron threshold. Furthermore, combining recently deduced values of the polarizability in heavy and total photoabsorption data in light nuclei, it is demonstrated that the mass number dependence of ${\sigma }_{-2}$ is sensitive to the volume and surface coefficients of the symmetry energy and parameters different to the ones chosen by Orce may be better suited.

Figure 1

Full squares: ${\sigma }_{-2}$ data deduced from relativistic Coulomb excitation experiments for ${}^{208}\mathrm{Pb}$ [18], ${}^{120}\mathrm{Sn}$ [17], and ${}^{68}\mathrm{Ni}$ [21, 22] and from total photoabsorption experiments for ${}^{40}\mathrm{Ca},{}^{27}\mathrm{Al},{}^{16}\mathrm{O}$, and ${}^{12}\mathrm{C}$ [19]. Error bars are partly smaller than the symbol size. Short-dashed (green) and long-dashed (blue) lines: Empirical formulas (1) and (2). Dotted (red) line: Equation (4) using symmetry energy parameters of Ref. [27]. Dashed-dotted and full black lines: Fit to Eq. (4) including and excluding the ${}^{12}\mathrm{C}$ data point.