\begin{table}%t2
\caption{Ionization potentials of ions.\label{table:02}}
%\centerline
{\begin{tabular}{l c c c}
\hline\hline
 Ion &
 Ionization potential &
 Ionization potential &
 Classification \\
 &
 lower (eV) &
 upper (eV) &
 \\
\hline
O{\sc i}    &  0.0 & 13.6 & LIL \\
Al{\sc ii}  &  5.9 & 18.8 & LIL \\
Si{\sc ii}  &  8.1 & 16.3 & LIL \\
C{\sc ii}   & 11.3 & 24.4 & LIL \\
Si{\sc iii} & 16.3 & 33.5 & LIL \\
Al{\sc iii} & 18.8 & 28.4 & LIL \\
C{\sc iii}  & 24.4 & 47.9 & LIL \\
He{\sc ii}  & 24.6 & 54.4 & HIL$^{{\rm a}}$ \\
Si{\sc iv}  & 33.5 & 45.1 & LIL \\
O{\sc iii}  & 35.1 & 54.9 & LIL \\
N{\sc iv}   & 47.4 & 77.4 & HIL \\
C{\sc iv}   & 47.9 & 64.5 & HIL \\
O{\sc iv}   & 54.9 & 77.4 & HIL \\
N{\sc v}    & 77.4 & 97.9 & HIL \\
\hline
\end{tabular}}
\par
\smallskip
 $^{{\rm a}}$ 
         Classified as a HIL because 
         He{\sc ii}$\lambda$1640 is a recombination line
         and thus the $upper$ ionization potential is
         important rather than the $lower$ ionization
         potential.
\end{table}