\begin{table}%t2
\caption{\label{Photoionization}Photoionization and photodissociation reactions.}
\small%\centering
\par
\begin{tabular}[h]{cccc}
\hline\hline					Species 	& Process 			& Threshold (eV)	& References for cross sections 		\\
\hline
H		& ionization			&13.6 		& \citet{Samson66},	 \citet{Norwood90}	\\
H$_{2}$	& ionization			&15.4 		& \citet{Chung93}, \\
		&					&			&\citet{Yan98}, \citet{Yan98erratum} \\
		& dissociative ionization	& 18.1		&  \citet{Chung93} \\
		& dissociation			& $\sim$25 	& \citet{Glass-Maujean05} \\
He		& ionization			& 24.6 		& \citet{Yan98}		\\
		& double ionization		& 79.0 		&	\citet{Samson92} \\
CH$_{4}$& ionization			& 12.6		& \citet{Samson89}	\\
		& dissociative ionizations	& 14.3 and 15.7	&  \citet{Samson89}	\\
\hline
\end{tabular}
\end{table}