\begin{table}%t2
\caption{\label{tab_fit}Hyperfine constants and central frequency of the $J_K = 1_0 \leftarrow 0_0$ transition of NH$_3$.}
%\centerline
$$
 {\small \begin{tabular}{lD{.}{.}{9}D{.}{.}{9}}
\hline\hline \\[-2ex]
\mcl{1}{l}{Hyperfine constant} & \mcl{1}{c}{Fit I$^{\rm a}$} & \mcl{1}{c}{Fit II$^{\rm b }$} \\
\hline  
\mcl{1}{l}{$(eQq)_{\rm N}$, N quadrupole coupling / MHz} & -4.08983(2) & -4.08984(70)\\
\mcl{1}{l}{$C_{\rm N}$, N spin-rotation / kHz} & 6.774(9)  & 6.91(17)\\
\mcl{1}{l}{$-\frac{1}{2} \left(M_{xx}^{\rm H} + M_{yy}^{\rm H}\right)$, H spin-rotation / kHz} & 17.859(7) & 17.20(23)\\
\mcl{1}{l}{$D_1$, N--H spin-spin / kHz} & -2.373(4) & -2.91(25)\\
\mcl{1}{l}{$D_3$, H--H spin-spin / kHz} & 27.55(6) & 31.6(15)\\
\mcl{1}{l}{Center frequency$^{\rm c}$ / MHz} & 572~498.15977(20) & 572~498.15994(19)\\ 
    \hline
  \end{tabular}}
  $$
\medskip
$^{\rm a}$ Hf-constants with their errors from \citet{Muenter-JMS81-NH3} were fixed in the fit; $^{\rm b }$ standard errors from the fit are reported in units of the last quoted digit; $^{\rm c}$ in parenthesis is reported the statistical error from the fit; in addition, a systematic error of $+$2~kHz due to pressure shift has to be considered, see text.
  \end{table}