Inelastic electron scattering to negative parity states ofSi28

Negative-parity states of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ are studied by high-resolution inelastic electron scattering. The form factors of the $1^{-}$ (8.904), $5^{-}$ (9.702), $1^{-}$, $2^{-}$ (9.929), $3_2^{-}$ (10.180), and $4^{-}$ $T=1\mathrm{}$ (12.664) states are determined for the first time, for momentum transfers between 0.9 and 2.4 ${\mathrm{fm}}^{-1\mathrm{}}$. The $3_1^{-}$ (6.879) state is studied by subtracting off the theoretical contribution of the nearby $4^{+}$ (6.889) state. An upper limit for the $5^{-}$ $T=1\mathrm{}$ (13.248) is established. The $3^{-}$ $T=\frac{0}{6^{-}}$ $T=0\mathrm{}$ complex (11.58) remains unresolved. We present evidence for oblate-prolate deformation changes in the $3^{-}$ $T=0\mathrm{}$ states. The experimental data are compared with predictions of the open-shell random phase approximation of Rowe and Wong. In ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, the open-shell random phase approximation is demonstrated to be extremely sensitive to the ground state wave function used, and other possible limitations of the open-shell random phase approximation are discussed.