\begin{table}%T2
\caption{\label{tbl:mol}Some low energy spectral lines observable by Odin.}
%\centerline
{
\begin{tabular}{lcccc}
\hline\hline\noalign{\smallskip}
Species  & Transition & Frequency & $E_{\rm u}\;^a$ & $n_{\rm crit}\;^b$ \\
         &            & (MHz)     & (K)  & (cm$^{-3}$) \\
\noalign{\smallskip}\hline\noalign{\smallskip}
H$_2$O        & $1_{10}-1_{01}$   & 556~936.00  &  27 & ~~~3$\times$10$^8\;\;^c$ \\
H$_2^{18}$O   & $1_{10}-1_{01}$   & 547~676.44 &  27 &  3$\times$10$^8$ \\
H$_2^{17}$O   & $1_{10}-1_{01}$   & 552~021.02 &  26 &  3$\times$10$^8$ \\
NH$_3$         & $1_0-0_0$         & 572~498.07 &  29 & ~~~ 5$\times$10$^7\;\;^c$ \\
$^{15}$NH$_3$  & $1_0-0_0$         & 572~112.79 &  29 &  5$\times$10$^7$ \\
O$_2$         & $3_3-1_2$         & 487~249.38 &  26 & 10$^3$ \\
O$_2$         & $1_1-1_0$         & 118~750.34 &   6 & 10$^3$ \\
CI            & $^3$P$_1-^3$P$_0$ & 492~160.70 &  24 & 10$^4$ \\
CO            & $5-4$           & 576~267.93 &  83 &   ~~~2$\times$10$^5\;\;^c$ \\
$^{13}$CO     & $5-4$           & 550~926.30 &  79  &  2$\times$10$^5$ \\
C$^{18}$O     & $5-4$           & 548~830.98 &  79 &  2$\times$10$^5$ \\
CS            & $10-9$          & 489~751.05 & 129 &  5$\times$10$^7$ \\
\hline
\end{tabular}}
\smallskip
$^a$ Upper state energy. \\
$^b$ Cloud density needed for excitation. \\
$^c$ Will be considerably reduced because of 
radiative trapping.
\end{table}