\begin{table}%t3
\caption{Band strengths of molecules used for integration.
\label{strengths}}
%%\centerline
\par
{
\begin{tabular}{cccccc}   
\hline \hline
Species          & Vibration mode & Position         & Position  &           $A$          & Ref.\\
                 &                 & $\mu$m & cm$^{-1}$ &     cm group$^{-1}$    &     \\  \hline
silicates & SiO str. & 10 & $\sim$1000 & $2.0 \times 10^{-16}$ & $^a$ \\
aliphatics & CH str. & 3.4 &  2800$-$3000 & $1.2 \times 10^{-17}$ & $^b$ \\
CH$_2$ group & asym. CH$_2$ str. & 3.42 & 2922 & $8.4 \times 10^{-18}$ & $^c$ \\
CH$_3$ group & asym. CH$_3$ str. & 3.38 & 2958 & $1.25 \times 10^{-17}$ & $^c$\\
ketones & C=O str. & 5.83 & 1715 & $1.2 \times 10^{-17}$ &  $^d$\\
carboxylic acids & C=O str. &5.88 & 1700 & $3.2 \times 10^{-17}$ & $^d$ \\
carboxylic acids & OH str. &  2.77$-$4.35 & 3600$-$2300 & $2.8 \times 10^{-16}$ & $^e$ \\
carboxylic acid salts & COO$^-$ str. & 6.3 & 1586 &  $6.0 \times 10^{-17}$ & $^d$ \\
aromatics             & ring stretch & 6.2 & 1600 &   $  2 \times 10^{-19}$ & $^f$ \\
\hline
\end{tabular}}
\medskip
$^a$ \cite{jager1998}, in cm (SiO bond)$^{-1}$. \\
$^b$ \cite{dartois2004}, in~cm (C~atom)$^{-1}$, based on the value for hexane $7.2 \times 10^{-17}$~cm~molec$^{-1}$ dividing by 6 for the number of C~atoms (see text for details).\\
$^c$ \cite{dartois2004}, in cm (C~atom)$^{-1}$. \\
$^d$ \cite{wexler1967}. \\
$^e$ Using area(OH str.)/area(C=O str.) and for glycolic acid (carboxylic acid) we obtain
$A$(OH) $\approx$ $2.8 \times 10^{-16}$.\\
$^f$ \cite{schutte1998}, in cm (C~atom)$^{-1}$, corresponding to the average of neutral PAHs.\\
\end{table}