\begin{table}%t2
      \caption{Collisional line width $\gamma$ $\rm [cm^{-1}~atm^{-1}]$ (at 300~K) and temperature dependence factor~$n$ for NH$_3$, PH$_3$ and CO with H$_2$ and He and for \object{Saturn} and \object{Uranus} (a blank space means that no data is available).}  	       \label{spec_param}       
\small%\centerline
 {
    \begin{tabular}{c c c c c}
      \hline \hline
      $\gamma$     & H$_2$  & He     & \object{Saturn} & \object{Uranus} \\	      
      \hline
      NH$_3$       & 0.069  &	     & 0.069  & 0.069  \\
      PH$_3$ (1--0) & 0.1078 & 0.0618 & 0.1011 & 0.1003 \\
      PH$_3$ (2--1) & 0.1064 & 0.0606 & 0.0997 & 0.0990 \\
      CO (1--0)     & 0.0749 & 0.0479 & 0.0709 & 0.0696 \\
      CO (2--1)     & 0.0715 & 0.0468 & 0.0679 & 0.0666 \\
       \hline
      $n$               & H$_2$ & He   & \object{Saturn} & \object{Uranus} \\  	   
      \hline 
      NH$_3$            & 0.67  &      & 0.67   & 0.67 \\
      PH$_3$            & 0.73  & 0.30 & 0.67   & 0.66 \\
      CO (1--0) \& (2--1) &       &      & 0.60   & 0.60 \\
      \hline			
    \end{tabular}}
    \medskip
 References: \citet{berge_1976} and \citet{brown_1996} for NH$_3$, \citet{levy_1993} and \citet{levy_1994} for PH$_3$ and \citet{draegert_1968}, \citet{1988JQSRT..39...13V}, \citet{beaky_1996} and \citet{1996Icar..123...23W} for CO.
\end{table}