  Nuclear surface diffuseness reflects spectroscopic information near the Fermi
level. I propose a way to decompose the surface diffuseness into
single-particle (s.p.) contributions in a quantitative way. Systematic behavior
of the surface diffuseness of neutron-rich even-even O, Ca, Ni, Sn, and Pb
isotopes is analyzed with a phenomenological mean-field approach. The role of
the s.p. wave functions near the Fermi level is explored: The nodeless s.p.
orbits form a sharp nuclear surface, while the nodal s.p. orbits contribute to
diffusing the nuclear surface.
