Low-lying dipole strength in 20O

Two 1− levels at 5.35(10) and 6.85(5) MeV were observed for the first time in 20O. The strong direct excitation and subsequent γ-ray decay of these states in virtual photon scattering at 100 MeV/nucleon, along with B(Eλ) predictions for 20O states in this energy region, established their dipole character. The extracted B(E1)↑ values of ≃0.062(16) e2 fm2 and ≃0.035(9) e2 fm2 for the 5.35 and 6.85 MeV states, respectively, are significantly larger than shell model calculations predict. Such large dipole strengths are not observed for low-lying 1− states in 18O, indicating a shift of dipole strength towards lower energies as one approaches the neutron drip-line.