\begin{table}%t6
%\centering
\par
\caption{\label{table6}Peak position of the IR bands of unirradiated CO:H$_2$S ice a 10:1 mixture (mixture~1) and a 1:10 mixture (mixture~2) and of a layered film.}
\small \begin{tabular}{c c c c }
\hline\hline\noalign{\smallskip}
\multicolumn{3}{c}{Wavenumber (cm$^{-1}$)} & Molecule \\
mixture 1 & mixture 2 & layered &  \\
\hline
2608 & 2640 & 2642 & H$_2$S \\
2571 & 2553 & 2546 & H$_2$S \\
2139 & 2138 & 2139 & CO \\
2092 & 2090 & 2092 & $^{13}$CO \\
\hline   
\end{tabular}
\end{table}