Can an Isovector Dipole Mode of Nuclear Collective Motion Give a Signature Duplicating That of an Isoscalar Monopole Mode?

It is shown that inelastic $\alpha$-particle scattering to the well-known isovector giant dipole excitation, when computed with a neutron radius greater than the proton radius and including isovector Coulomb excitation, is predicted to give an angular dependence similar to that of an isoscalar monopole excitation claimed to exist at the same nuclear excitation energy. A much smaller fraction of the monopole sum-rule transition strength is then required by these data, perhaps altering the conclusions for nuclear compressibility.