\begin{table}%t5
%\centering
\par
\caption{\label{tab:halflife}Average dissociation rate~($R$) and the corresponding half-life ($t_{1/2}$) of water, ammonia, and CO~molecules under bombardment by 46~MeV~Ni~ions.} 
\par
\small \begin{tabular}{ l l l l }
\hline \hline
Species & Relative initial  & $R$ &  $t_{1/2}$ \\
 & abundance & ($10^{-15}$~s$^{-1}$)   &  ($10^6$~years)   \\
\hline
H$_2$O          & 50\%              & 8    & 2      \\
NH$_3$          & 30\%              & 7    & 3      \\
CO              & 20\%              & 9    & 2     \\
\hline  
\end{tabular}
\end{table}