\begin{table}%T2
\caption{\label{table2}{Sources for double-ionisation cross sections, either from theoretical estimates 
or from laboratory measurements}.}
	\small%\centering
\par
\begin{tabular}{lc}
\hline\hline
{Ionisation}	& {Photoionisation}  \\
\hline\noalign{\smallskip}
${\rm N_2}\rightarrow {\rm N_2}^{++} + 2 {\rm e}^-$         &{\citet{Simon2005} theoretical estimates} 	 \\
			  &{This work, based on \citet{Samson1990}}	\\
\hline\noalign{\smallskip}
${\rm O} \rightarrow {\rm O}^{++} + 2 {\rm e}^-$ & {\citet{Fennelly1992} }\\
			       & {\citet{Angel1988} } \\
\hline\noalign{\smallskip}
${\rm CO_2} \rightarrow {\rm CO_2}^{++} + 2 {\rm e}^-$ & {\citet{Masuoka1994} }\\
\hline\noalign{\smallskip}
			& {\citet{Baluja1988}}   	\\
			${\rm O}^+ \rightarrow {\rm O}^{++}+ {\rm e}^-$ & {\citet{Kjeldsen2002} }\\
			& {\cite{Aguilar2003} } \\
\hline\noalign{\smallskip}
{Ionisation}	 & {Electron impact}  \\
\hline        &{\citet{Krishnakumar1990} } \\
			 & {\citet{Mark1975}}\\
			 ${\rm N_2}\rightarrow {\rm N_2}^{++} + 2 {\rm e}^-$ & {\citet{Halas1972} }\\
			 & { \citet{Straub1996} }\\
\hline
			 & {\citet{Itikawa1990} }\\
			 ${\rm O} \rightarrow {\rm O}^{++} + 2 {\rm e}^-$ & {\citet{Zipf1985} }\\
			  	& {\citet{Ziegler1982}  }\\
\hline\noalign{\smallskip}
${\rm CO_2} \rightarrow {\rm CO_2}^{++} + 2{\rm e}^-$ & {\citet{Tian1998} }\\
\hline\noalign{\smallskip}
${\rm O}^+ \rightarrow {\rm O}^{++}+ {\rm e}^-$	   &	{\citet{Aitken1971} }	\\
	 & {\citet{Yamada1988} }\\
\hline
\end{tabular}
\par
\end{table}