\begin{table}%t2
%\centering
\par
\caption{Unidentified strong lines in the spectrum of AFGL~2688.}
{\tiny\begin{tabular}{llll}
\hline\hline\noalign{\smallskip}
$\bar{\nu}_0$ (cm$^{-1}$) & $\lambda_0$ ($\mu$m) & $F$($\times$10$^{-18}$Wm$^{-2}$) & Possible carriers \\
\hline
15.657 & 638.69 &  201.3 $\pm$    9.0 & NH$_2$ \\
15.762 & 634.44 &  139.5 $\pm$    8.7 & H$^{13}$CN v2 ; $^{30}$SiS \\
15.931 & 627.71 &  116.1 $\pm$    8.4 & SiO 11--10 \\
16.547 & 604.34 &  193.8 $\pm$    6.3 & NH$_2$ \\
16.675 & 599.70 &  122.7 $\pm$    5.1 & H$^{13}$CN v1+v2 \\
16.856 & 593.26 &  304.8 $\pm$   12.0 & o-SH$_2$ \\
16.998 & 588.30 &  193.8 $\pm$   21.9 & CH$_2$ \\
17.927 & 557.82 &  194.1 $\pm$    8.4 & CH \\
18.766 & 532.88 &  122.1 $\pm$    3.9 & SiS ; SiO 13-12 \\
18.945 & 527.84 &  195.0 $\pm$    3.9 & p-SH$_2$ \\
19.440 & 514.40 &  159.9 $\pm$    3.6 &  \\
20.122 & 496.97 &  130.5 $\pm$    7.5 &  \\
21.208 & 471.52 &  201.6 $\pm$   12.0 &  \\
29.287 & 341.45 &  141.9 $\pm$    4.8 & HCN v2+2v3 ; C$_{18}$O \\
31.479 & 317.67 &  134.7 $\pm$   20.4 &  \\
31.778 & 314.68 &  114.9 $\pm$   20.4 & NH$_2$ \\
32.665 & 306.14 &  150.3 $\pm$    7.8 & HCN $\nu_2 = 0$ \\
36.432 & 274.48 &  109.5 $\pm$   15.6 &  \\
38.842 & 257.45 &  105.9 $\pm$    4.8 &  \\
39.685 & 251.98 &  100.5 $\pm$    6.0 &  \\
47.456 & 210.72 &  101.7 $\pm$   72.3 & HCN $\nu_2 = 1$ \\
47.522 & 210.43 &  128.1 $\pm$   85.5 &  \\
49.589 & 201.66 &  146.1 $\pm$   27.0 &  \\
50.814 & 196.80 &  114.9 $\pm$   22.2 & HCN 3v2 \\
\hline
\end{tabular}}
\end{table}