\begin{table}%t4
%\centering
\par
\caption{\label{Table4}Peak position of IR bands of unirradiated CO:SO$_{2}$ ice as a mixture and layered sample.}
\small  \begin{tabular}{c c c c }
\hline\hline\noalign{\smallskip}
\multicolumn{2}{l}{Wavenumber (cm$^{-1}$)} & Molecule \\
mixture & layered & & \\
\hline
2139 & 2139 & CO \\
2092 & 2092 & $^{13}$CO \\
1341 & 1344 & SO$_2$ \\
1152 & 1152 & SO$_2$ \\
\hline    
\end{tabular}
\end{table}