\begin{table}%t3
 \caption{\label{atomic_ref}References for default atomic parameters.}
 %\centerline
{
 \begin{tabular}{ccccc}
 \hline
 \hline
  Ion & \multicolumn{2}{c}{IP~(eV)} & \multicolumn{2}{c}{Reference}\\
\cline{2-3}
\cline{4-5}
X$^i$& X$^{i-1}$ & X$^i$  & Trans. prob. & Coll. str.  \\
 \hline
  O$^+$& 13.62 & 35.12 & [1] & [2] \\
  S$^+$& 10.36 & 23.34 & [3], [4] & [5] \\
 Cl$^{2+}$& 23.8 & 39.9  & [3]  & [6]  \\
 Ar$^{3+}$& 40.74 & 59.81 & [3]  & [7]  \\
 \hline
 \end{tabular}}
\smallskip
References: [1] Zeippen (\cite{zeippen1982}), [2]~Pradhan
(\cite{pradhan}), [3]~Mendoza \& Zeippen (\cite{mendoza1982}), [4]~Keenan et~al. (\cite{keenan1993}), [5]~Keenan et~al. (\cite{keenan1996}), [6]~Butler \&
Zeippen (\cite{butler1989}), [7]~Ramsbottom et~al. (\cite{Ramsbottom}).
\end{table}