  Using Woods-Saxon potentials and the eigenphase formalism for one-particle
resonances, one-particle bound and resonant levels for neutrons as a function
of quadrupole deformation are presented, which are supposed to be useful for
the interpretation of spectroscopic properties of some light neutron-rich
nuclei with weakly-bound neutrons. Compared with Nilsson diagrams in text books
which are constructed using modified oscillator potentials, we point out a
systematic change of the shell structure in connection with both weakly-bound
and resonant one-particle levels related to small orbital angular momenta
$\ell$. Then, it is seen that weakly-bound neutrons in nuclei such as
$^{15-19}$C and $^{33-37}$Mg may prefer to being deformed as a result of
Jahn-Teller effect, due to the near degeneracy of the 1d$_{5/2}$-2s$_{1/2}$
levels and the 1f$_{7/2}$-2p$_{3/2}$ levels in the spherical potential,
respectively. Furthermore, the absence of some one-particle resonant levels
compared with the Nilsson diagrams in text books is illustrated.
