\begin{table}%t2
\caption{\label{tab:adsorption_energy}Adsorption energies and photo-desorption yields.}
%\centerline
 {\begin{tabular}{lcc}
\hline\hline
Species & Adsorption energy    & Photo-desorption yield \\
        & $E^{\rm ads}_i$ [K]  & $Y_i$ [per UV photon] \\
\hline
&&\\[-2.2ex]
CO\#      &    960 $^a$   & 2.7 $\times$ 10$^{-3}$ $^c$\\
CO$_2$\#  &   2000 $^a$   & 1.0 $\times$ 10$^{-3}$ $^e$\\
H$_2$O\#  &   4800 $^b$   & 1.3 $\times$ 10$^{-3}$ $^d$\\
CH$_4$\#  &   1100 $^a$   & 1.0 $\times$ 10$^{-3}$ $^e$\\  
NH$_3$\#  &    880 $^a$   & 1.0 $\times$ 10$^{-3}$ $^e$\\
\hline
\end{tabular}}
\par
\smallskip
$^a$ \citet{Aikawa1996};
$^b$ \citet{Sandford1990};
$^c$ \citet{Oberg2007ApJ...662L..23O}; 
$^d$ \citet{Westley1995Natur.373..405W}; 
$^e$ assumed values. 
\end{table}