Spectroscopy of $^{35}$P using the one-proton knockout reaction

The structure of $^{35}$P was studied with a one-proton knockout reaction at 88~MeV/u from a $^{36}$S projectile beam at NSCL. The $\gamma$ rays from the depopulation of excited states in $^{35}$P were detected with GRETINA, while the $^{35}$P nuclei were identified event-by-event in the focal plane of the S800 spectrograph. The level scheme of $^{35}$P was deduced up to 7.5 MeV using $\gamma-\gamma$ coincidences. The observed levels were attributed to proton removals from the $sd$-shell and also from the deeply-bound $p_{1/2}$ orbital. The orbital angular momentum of each state was derived from the comparison between experimental and calculated shapes of individual ($\gamma$-gated) parallel momentum distributions. Despite the use of different reactions and their associate models, spectroscopic factors, $C^2S$, derived from the $^{36}$S $(-1p)$ knockout reaction agree with those obtained earlier from $^{36}$S($d$,\nuc{3}{He}) transfer, if a reduction factor $R_s$, as deduced from inclusive one-nucleon removal cross sections, is applied to the knockout transitions. In addition to the expected proton-hole configurations, other states were observed with individual cross sections of the order of 0.5~mb. Based on their shifted parallel momentum distributions, their decay modes to negative parity states, their high excitation energy (around 4.7~MeV) and the fact that they were not observed in the ($d$,\nuc{3}{He}) reaction, we propose that they may result from a two-step mechanism or a nucleon-exchange reaction with subsequent neutron evaporation. Regardless of the mechanism, that could not yet be clarified, these states likely correspond to neutron core excitations in \nuc{35}{P}. This newly-identified pathway, although weak, offers the possibility to selectively populate certain intruder configurations that are otherwise hard to produce and identify.