\begin{table}%t1
%\centering
\par
\caption{\label{det-parameter}Experimentally determined spectroscopic parameters$^a$ 
(cm$^{-1}$, MHz) of methylidynium, $^{12}$CH$^+$ in its 
$X\ ^1\Sigma ^+$ and $A\ ^1\Pi$ electronic states from a global fit 
involving four isotopologs.}
\begin{tabular}[t]{lc}
\hline \hline
Parameter & \multicolumn{1}{c}{Value} \\
\hline\noalign{\smallskip}
\multicolumn{2}{c}{$X\ ^1\Sigma ^+$} \\
\hline
$U_{10} \mu ^{-1/2}$                                              &     2~860.387~(22)      \\
$U_{10} \mu ^{-1/2}{\it \Delta} ^{\rm C} _{10} m_{\rm e}/M_{\rm C}$     &       $-$0.147~(18)      \\
$U_{10} \mu ^{-1/2}{\it \Delta} ^{\rm H} _{10} m_{\rm e}/M_{\rm H}$     &       $-$2.672~(13)      \\
$U_{20} \mu ^{-1}$                                                &      $-$59.369~7~(61)   \\
$U_{20} \mu ^{-1}{\it \Delta} ^{\rm H} _{20} m_{\rm e}/M_{\rm H}$       &          0.051~0~(62)   \\
$Y_{30}$                                                          &          0.225~46~(106) \\
$U_{01} \mu ^{-1}$                                                &   427~360.68~(121)      \\
$U_{01} \mu ^{-1}{\it \Delta} ^{\rm C} _{01} m_{\rm e}/M_{\rm C}$       &     $-$155.80~(21)       \\
$U_{01} \mu ^{-1}{\it \Delta} ^{\rm H} _{01} m_{\rm e}/M_{\rm H}$       &  $-$2~146.22~(177)      \\
$U_{11} \mu ^{-3/2}$                                              & $-$14~917.7~(57)        \\
$U_{11} \mu ^{-3/2}{\it \Delta} ^{\rm H} _{11} m_{\rm e}/M_{\rm H}$     &         58.6~(27)        \\
$Y_{21}$                                                          &         91.8~(41)        \\
$Y_{31}$                                                          &       $-$3.84~(66)       \\
$U_{02} \mu ^{-2}$                                                &      $-$42.468~(21)      \\
$U_{02} \mu ^{-2}{\it \Delta} ^{\rm H} _{02} m_{\rm e}/M_{\rm H}$       &          0.563~(20)      \\
$Y_{12}$                                                          &          0.960~(34)      \\
$Y_{22}$                                                          &       $-$0.045~(10)      \\
$Y_{03} \times 10^3$                                              &          4.264~(86)      \\
$Y_{13} \times 10^3$                                              &       $-$0.726~(53)      \\
$Y_{04} \times 10^6$\tablefootmark{c}                                         &       $-$0.4             \\
$C(^{13}$C)                                                       &          1.091~(28)      \\
\hline\noalign{\smallskip}
\multicolumn{2}{c}{$A\ ^1\Pi$} \\
\hline
$U_{00}$(H)\tablefootmark{b}                                                          &    23~639.326~(23)      \\
$U_{00}{\it \Delta} ^{\rm C} _{10} m_{\rm e}/M_{\rm C}$(H)\tablefootmark{b}         &      $-$19.620~(24)      \\
$U_{00}$(D)\tablefootmark{b}                                                   &    23~786.537~(24)      \\
$U_{00}{\it \Delta} ^{\rm C} _{10} m_{\rm e}/M_{\rm C}$(D)\tablefootmark{b}            &      $-$26.436~(26)      \\
$U_{10} \mu _c^{-1/2}$(H)                                          &     1~864.379~(14)      \\
$U_{10} \mu _c^{-1/2}{\it \Delta} ^{\rm C} _{10} m_{\rm e}/M_{\rm C}$(H) &          0.106~(12)      \\
$U_{10} \mu _c^{-1/2}$(D)                                          &     1~367.473~(17)      \\
$U_{10} \mu _c^{-1/2}{\it \Delta} ^{\rm C} _{10} m_{\rm e}/M_{\rm C}$(D) &       $-$0.180~(19)      \\
$Y_{20}$(H)                                                        &     $-$115.884~7~(48)   \\
$Y_{20}$(D)                                                        &      $-$60.815~4~(47)   \\
$Y_{30}$(H)                                                        &          2.639~48~(85)  \\
$Y_{30}$(D)                                                        &          0.776~92~(78)  \\
$U_{01} \mu _c^{-1}$                                               &   356~717.5~(63)        \\
$U_{01} \mu _c^{-1}{\it \Delta} ^{\rm C} _{01} m_{\rm e}/M_{\rm C}$      &       $-$9.2~(53)        \\
$U_{01} \mu _c^{-1}{\it \Delta} ^{\rm H} _{01} m_{\rm e}/M_{\rm H}$      &     $-$316.4~(24)        \\
$U_{11} \mu _c^{-3/2}$                                             & $-$27~562.7~(70)        \\
$U_{11} \mu _c^{-3/2}{\it \Delta} ^{\rm H} _{11} m_{\rm e}/M_{\rm H}$    &         24.06~(147)      \\
$Y_{21}$                                                           &     $-$634.8~(49)        \\
$Y_{31}$                                                           &        137.76~(95)       \\
$Y_{02}$                                                           &      $-$58.166~(26)      \\
$Y_{12}$                                                           &       $-$2.502~(61)      \\
$Y_{22}$                                                           &       $-$0.252~(44)      \\
$Y_{32}$                                                           &          0.149~3~(87)   \\
$Y_{03} \times 10^3$                                               &       $-$0.876~(84)      \\
$Y_{13} \times 10^3$                                               &       $-$0.911~(63)      \\
$q_{00}$                                                           &     1~234.25~(108)      \\
$q_{10}$                                                           &      $-$96.53~(82)       \\
$q_{01}$                                                           &       $-$0.643~6~(98)   \\
$q_{11}$                                                           &       $-$0.020~3~(79)   \\
\hline
\end{tabular}
\tablefoot{\tablefoottext{a}{The vibrational parameters $Y_{i0}$ as well as related parameters 
are given in cm$^{-1}$; all other $Y_{ij}$ as well as related parameters are given in MHz. Numbers in parentheses are one standard deviation in units of the least significant figures.}
\tablefoottext{b}{Determined with respect to the lowest allowed energy levels of each electronic state.}
\tablefoottext{c}{Estimated, see text.}}
\end{table}