\begin{table}%[b]%ta1
\caption{\label{tab:reach}List of reactions for the hydrogen chemistry.}
%%\centering

\par
\small
%
\par
\begin{tabular}{l l l l}
\hline
\hline
& Reaction & & Reaction\\
\hline
H1 & ${\rm H^+ + e^- \rightarrow H + \gamma}$ & H2 & ${\rm H + \gamma \rightarrow H^+ + e^-}$\\
H3 & ${\rm H + e^- \rightarrow H^- + \gamma}$ & H4 & ${\rm H^- + \gamma \rightarrow H + e^-}$\\
H5 & ${\rm H^- + H \rightarrow H_2 + e^-}$ & H6 & ${\rm H^- + H^+ \rightarrow H_2^+ + e^-}$\\
H7 & ${\rm H^- + H^+ \rightarrow H + H}$ & H8 & ${\rm H + H^+ \rightarrow H_2^+ + \gamma}$\\
H9 & ${\rm H_2^+ + \gamma \rightarrow H + H^+}$ & H10 & ${\rm H_2^+ + H \rightarrow H_2 + H^+}$\\
H11 & ${\rm H_2^+ + e^- \rightarrow H + H}$ & H12 & ${\rm H_2^+ + \gamma \rightarrow H^+ + H^+ + e^-}$\\
H13 & ${\rm H_2^+ + H_2 \rightarrow H_3^+ + H}$ & H15 & ${\rm H_2 + H^+ \rightarrow H_2^+ + H}$\\
H16 & ${\rm H_2 + e^- \rightarrow H + H^-}$ & H17 & ${\rm H_2 + e^- \rightarrow H + H + e^-}$\\
H18 & ${\rm H_2 + \gamma \rightarrow H_2^+ + e^-}$ & H19 & ${\rm H_3^+ + H \rightarrow H_2^+ + H_2}$\\
H20 & ${\rm H_3^+ + e^- \rightarrow H_2 + H}$ & H21 & ${\rm H_2 + H^+ \rightarrow H_3^+ + \gamma}$\\
\hline
\end{tabular}
\end{table}