\begin{table}%t4
\caption{\label{comparison_f}Comparison between theoretical $f$-values obtained in this work for Nb~II and those calculated previously.}
\par
%\centering
\par
\begin{tabular}{rrrrrrr}
\hline \hline \noalign{\smallskip}
\multicolumn{2}{c}{Lower level\tablefootmark{a}}       &   \multicolumn{2}{c}{Upper level\tablefootmark{a}}                    &  $f$-value     &   \multicolumn{2}{c}{$f$-values}       \\
Desig.            & $E$(cm$^{-1}$) &   Desig.             & $E$(cm$^{-1}$) &   this work   &   \multicolumn{2}{c}{RCI\tablefootmark{b} }       \\
                       &                    &                           &                    &               &   velocity    &   length  \\
\hline\noalign{\smallskip}
4d$^3$($^4$F)5s  $^5$F$_2$ &   2629.13          &   4d$^3$($^4$F)5p   $^5$G$_3^\circ$     &   33 919.24         &   0.277       &   0.239       &   0.315   \\
4d$^3$($^4$F)5s  $^5$F$_3$ &   3029.63          &   4d$^3$($^4$F)5p   $^5$G$_3^\circ$ &   33 919.24         &   0.072       &   0.065       &   0.074   \\
4d$^4$ $^5$D$_2$         &   438.36           &   4d$^3$($^4$F)5p   $^3$D$^{\circ}_3$ &   36 553.24         &   0.064       &   0.054       &   0.068   \\
4d$^3$($^4$F)5s  $^5$F$_2$ &   2629.13          &   4d$^3$($^4$F)5p   $^3$D$^{\circ}_3$ &   36 553.24         &   0.022       &   0.020       &   0.027   \\
4d$^3$($^4$F)5s  $^5$F$_3$ &   3029.63          &   4d$^3$($^4$F)5p   $^3$D$^{\circ}_3$ &   36 553.24         &   0.022       &   0.026       &   0.031   \\
4d$^3$($^4$F)5s  $^5$F$_4$ &   3542.56          &   4d$^3$($^4$F)5p   $^3$D$^{\circ}_3$ &   36 553.24         &   0.065       &   0.052       &   0.068   \\
4d$^4$ $^3$P$_2$         &   7261.32          &   4d$^3$($^4$F)5p   $^3$D$^{\circ}_3$ &   36 553.24         &   0.051       &   0.065       &   0.069   \\
4d$^4$ $^3$F$_4$         &   8320.37          &   4d$^3$($^4$F)5p   $^3$D$^{\circ}_3$ &   36 553.24         &   0.040       &   0.038       &   0.041   \\[2pt]
\hline
\end{tabular}
\tablefoot {\tablefoottext{a}{Ryabtsev et~al. (\cite{ryabtsev});}
\tablefoottext{b}{Beck \& Datta (\cite{beck}).}}  
\end{table}