\begin{table}%t2
\caption{Critical densities for $T_{{\rm kin}} = 500$~K.}
\label{tncrit}
%\centering
\par
\begin{tabular}{l c c c c}
\hline\hline
Species & $N=2\rightarrow 1$ & $N=3\rightarrow 2$ & $N=4\rightarrow 3$ & $N=7\rightarrow 6$ \\
        & $n_{{\rm crit}}$ [cm$^{-3}$] & $n_{{\rm crit}}$ [cm$^{-3}$] & $n_{{\rm crit}}$ [cm$^{-3}$] & $n_{{\rm crit}}$ [cm$^{-3}$] \\ 
\hline
H$_2$      & $4.2\times 10^5$ & $1.1\times 10^6$  & $2.1\times 10^6$  & $8.4\times 10^6$    \\
H       & $6.5\times 10^5$ & $1.6\times 10^6$  & $3.0\times 10^6$  & $1.4\times 10^7$    \\
e$^-$   & $3.9\times 10^2$ & $1.4\times 10^3$  & $3.4\times 10^3$  & $1.9\times 10^4$    \\
\hline
\end{tabular}
\end{table}