\begin{table}%t4
%%\centering

\par
\caption{\label{Gas/Surface model - fixed parameters}Gas/Surface model -- fixed parameters.}
\begin{tabular}{lllll} 
\hline\hline \noalign{\smallskip}
Parameter   & Value                  & Ref. \\
\hline\noalign{\smallskip}
$UV$ Radiation field  & $3.0 \times 10 ^{8}$~cm$^{-2}$~s$^{-1}$~Hz$^{-1}$ & 1 \\
Cosmic ray ionization rate & $1.3 \times 10^{-17}$~s$^{-1}$ & 2 \\
Grain albedo   & 0.5   & 3 \\ 
Sticking coefficient                               & 1.0                                            & 4 \\   
Sticking coefficient for hydrogen atoms            & 0.3                                            & 5 \\
Yield per UV photon on grain mantle impact         & 10$^{-6}$                                      & 6 \\
Surface density of sites                           & $1.5 \times 10^{15}$~cm$^{-2}$                & 4 \\
Number of sites per grain                          & 10$^{6}$                                       & 4 \\             
Fractional density of grains (w.r.t. Density of H) & $1.33 \times 10^{-12}$                         & 4 \\
Average grain radius                               & 10$^{-5}$~cm                                   & 4 \\
Barrier width between adjacent sites               & 10$^{-8}$~cm                                   & 4 \\
Grain velocity (non-thermal)                       & 10$^{4}$~cm~s$^{-1}$                            & 7 \\             
\hline
\end{tabular}
\tablebib{(1) Draine (\cite{Draine}); (2)~van Dishoeck \& Black  (\cite{Dishoeck}); (3) Whittet (\cite{Whittet92}); (4) Hasegawa et~al. (\cite{Hasegawa92});   (5) Brown (\cite{Brown}); (6) Willacy (\cite{Willacy93}); (7) V$\stackrel{..}{\rm o}$lk et~al. (\cite{Volk}).}  
\end{table}