\begin{table}%t2
\caption{References for the atomic data.}
\label{atomic_data}
%\centerline
{
\begin{tabular}{llll}
\hline
\hline
Ion          
                                  &  A              & C          &   E \\
\hline
{}[\ion{N}{ii}]                   &  [3]            & [9]	   & [1, 2] \\
{}[\ion{O}{ii}]                   &  [3]            & [6, 7]	   & [1, 4, 5] \\
{}[\ion{O}{iii}]                  &  [3]            & [9]	   & [1, 8] \\
{}[\ion{Ne}{iii}]                 &  [5, 10]         & [11]	   & [1]\\
{}[\ion{Ne}{iv}]                  &  [13, 14]        & [15]	   & [12]\\
{}[\ion{Ne}{v}]                   &  [5, 24]         & [9]          & [1, 10]\\
{}[\ion{S}{ii}]                   &  [16, 17]        & [18]	   & [1, 16]\\
{}[\ion{S}{iii}]                  &  [5, 19, 20, 21]  & [22]	   & [1] \\
{}[\ion{Cl}{iii}]                 &  [10, 5]         & [23]         & [1] \\
\hline
\end{tabular}} 
\medskip
References: 
[1]~\cite{bowen60}; [2]~\cite{williams95}; [3]~\cite{wiese96}; [4]~\cite{fawcett75}; [5]~\cite{kaufman86}; [6]~\cite{pradhan76}; [7]~\cite{mcLaughlin93}; [8]~\cite{moore85}; [9]~\cite{lennon94}; 
[10]~\cite{mendoza83}; [11]~\cite{butler94}; [12]~\cite{moore71}; [13]~\cite{becker89};
[14]~\cite{bhatia88}; [15]~\cite{giles81}; [16]~\cite{verner96}; [17]~\cite{keenan93};
[18]~\cite{ramsbottom96}; [19]~\cite{mendoza82}; [20]~\cite{heise95}; [21] \cite{lajohn93}; [22]~\cite{galavis95};
 [23]~\cite{butler89}; [24]~\cite{bhatia93}.
\end{table}