  A systematic study of neutron-hole strength in the N = 81 nuclei 137Ba,
139Ce, 141Nd and 143Sm is reported. The single-neutron removal reactions (p,d)
and (3He,4He) were measured at energies of 23 and 34 MeV, respectively.
Spectroscopic factors were extracted from measured cross sections through a
distorted-wave Born approximation analysis and centroids of single-particle
strength have been established. The change in these centroid energies as a
function of proton number have been compared to calculations of the monopole
shift for the s1/2 and h11/2 orbitals, where the majority of the strength has
been observed. Significant fragmentation of strength was observed for the d and
g7/2 orbitals, particularly for the latter orbital which is deeply bound, with
summed strengths that indicate a significant amount lies outside of the
measured excitation energy range.
