\begin{table}%T4
\caption{\label{fig7_ref}References for O$^+$ atomic data used to
generate the loci plotted in Fig.~7. }
 %\centerline
{
 \begin{tabular}{ccc}
 \hline
 \hline
  Line &  \multicolumn{2}{c}{Reference}\\
\cline{2-3}
 &  Trans. prob. & Coll. str.  \\
  \hline
 Dotted-dotted-dashed & [2] & [4]\\
 Solid & [1] & [3]\\
 Short-dashed   & [2] & [3]\\
 Long-dashed  & [5] & [3]\\
 \hline
 \end{tabular}}
\smallskip
References: [1] Zeippen (\cite{zeippen1982}),  [2]~Zeippen
(\cite{zeippen1987a}),  [3]~Pradhan (\cite{pradhan}),  [4] McLaughlin \&
Bell (\cite{mclaughlin}),  [5] Wiese et~al. (\cite{wiese}).
\end{table}