Electroexcitation of rotational bands in O18

The fourth 2+ state in O18 at an excitation energy of 8.21 MeV has been studied by high-resolution electron scattering. The Coulomb form factor for this state was measured in the momentum-transfer range 0.9q2.1 fm-1 and the extracted B(C2) value was determined to be 7.3±4.2 e2fm4. The level at 8.21 MeV is identified as a four-particle, two-hole (4p2h) state of the type p-2(sd)4. Form-factor measurements in the range 1.0q1.9 fm-1 are also presented for a strongly excited normal-parity state at 9.36 MeV. Preliminary measurements of the differential cross section for exciting this state by 135-MeV protons suggest a 2+ assignment; the measured Coulomb form factor of this state allows it to be identified as the best experimental candidate for a predicted K=2 4p2h state with the four sd-shell nucleons coupled to T=0. The excitation of these states and other members of rotational bands in O18 are discussed within the framework of structure models. Shell-model calculations for the C4 form factors of the first three 4+ states in O18 are compared to existing data.