Isoscalar giant dipole resonance in90Zr,116Sn,and208Pb

Strength functions for isoscalar dipole excitations in ${}^{90}\mathrm{Zr},$ ${}^{116}\mathrm{Sn},$ and ${}^{208}\mathrm{Pb}$ have been measured with inelastic scattering of 240 MeV α particles at small angles. The isoscalar $E1\mathrm{}$ strength distribution in each nucleus is found to consist of a broad component at $E_x\sim {114/A}^{1/3}\mathrm{MeV}$ containing approximately 100% of the $E1\mathrm{}$ EWSR and a narrower one at $E_x\sim {72/A}^{1/3}\mathrm{MeV}$ containing 15–28 % of the total isoscalar $E1\mathrm{}$ strength. The higher component is the compression mode $E1\mathrm{}$ strength previously reported only in ${}^{208}\mathrm{Pb},$ whereas the lower component may be a new mode not reported previously, but suggested by recent RPA-HF and relativistic mean field calculations.