\begin{table}%t4 %\centering \par \caption{\label{tab_energy_ar}The level energies (Ryd) of Ar$^{8+}$ from different calculations along with the compilation of NIST v3 and CHIANTI~v6.} \begin{tabular}{cccccccc} \hline\hline\noalign{\smallskip} ID & Level~specification & NIST/CHIANTI\tablefootmark{a} & AS & FAC & CHIANTI\tablefootmark{b} & MCHF\tablefootmark{c} \\ \hline\noalign{\smallskip} 1 & ${\rm 2s^2 2p^6 ~ ^1S_0}$ & & 0.00000 & & & \\ 2 & ${\rm 2s^2 2p^5 3s ~ ^3P_2}$ & 18.4672 & 18.4701 & 18.4879 & 18.4267 & 18.4745 \\ 3 & ${\rm 2s^2 2p^5 3s ~ ^3P_1}$ & 18.5271 & 18.5316 & 18.5510 & 18.4823 & 18.5346 \\ 4 & ${\rm 2s^2 2p^5 3s ~ ^3P_0}$ & 18.6307 & 18.6318 & 18.6492 & 18.5898 & 18.6377 \\ 5 & ${\rm 2s^2 2p^5 3s ~ ^1P_1}$ & 18.6967 & 18.7031 & 18.7249 & 18.6463 & 18.7045 \\ 6 & ${\rm 2s^2 2p^5 3p ~ ^3S_1}$ & 19.5859 & 19.5907 & 19.6006 & 19.5898 & 19.6043 \\ 7 & ${\rm 2s^2 2p^5 3p ~ ^3D_3}$ & 19.7739 & 19.7862 & 19.8001 & 19.7404 & 19.7945 \\ 8 & ${\rm 2s^2 2p^5 3p ~ ^3D_2}$ & 19.7826 & 19.7955 & 19.8113 & 19.7475 & 19.8025 \\ 9 & ${\rm 2s^2 2p^5 3p ~ ^3D_1}$ & 19.8049 & 19.8493 & 19.8647 & 19.7960 & 19.8549 \\ 10 & ${\rm 2s^2 2p^5 3p ~ ^3P_2}$ & 19.8855 & 19.8968 & 19.9149 & 19.8358 & 19.9050 \\ 11 & ${\rm 2s^2 2p^5 3p ~ ^1P_1}$ & 19.9489 & 19.9644 & 19.9775 & 19.9104 & 19.9965 \\ 12 & ${\rm 2s^2 2p^5 3p ~ ^3P_0}$ & 19.9751 & 19.9910 & 20.0055 & 19.9271 & 20.0314 \\ 13 & ${\rm 2s^2 2p^5 3p ~ ^1D_2}$ & 20.0026 & 20.0131 & 20.0305 & 19.9587 & 19.9685 \\ 14 & ${\rm 2s^2 2p^5 3p ~ ^3P_1}$ & 20.0104 & 20.0245 & 20.0413 & 19.9645 & 20.0225 \\ 15 & ${\rm 2s^2 2p^5 3p ~ ^1S_0}$ & 20.7851 & 20.7670 & 20.8484 & 20.6289 & 20.6624 \\ 16 & ${\rm 2s^2 2p^5 3d ~ ^3P_0}$ & 21.4090 & 21.4282 & 21.4273 & 21.3865 & 21.4359 \\ 17 & ${\rm 2s^2 2p^5 3d ~ ^3P_1}$ & 21.4276 & 21.4478 & 21.4451 & 21.4038 & 21.4539 \\ 18 & ${\rm 2s^2 2p^5 3d ~ ^3P_2}$ & 21.4650 & 21.4876 & 21.4831 & 21.4403 & 21.4914 \\ 19 & ${\rm 2s^2 2p^5 3d ~ ^3F_4}$ & 21.4945 & 21.5229 & 21.5169 & 21.4640 & 21.5230 \\ 20 & ${\rm 2s^2 2p^5 3d ~ ^3F_3}$ & 21.5215 & 21.5488 & 21.5475 & 21.4885 & 21.5496 \\ 21 & ${\rm 2s^2 2p^5 3d ~ ^3F_2}$ & 21.5794 & 21.5962 & 21.5951 & 21.5314 & 21.5962 \\ 22 & ${\rm 2s^2 2p^5 3d ~ ^1F_3}$ & 21.6027 & 21.6352 & 21.6325 & 21.5614 & 21.6305 \\ 23 & ${\rm 2s^2 2p^5 3d ~ ^3D_1}$ & 21.6963 & 21.7272 & 21.7266 & 21.6480 & 21.7240 \\ 24 & ${\rm 2s^2 2p^5 3d ~ ^1D_2}$ & 21.7091 & 21.7380 & 21.7358 & 21.6690 & 21.7594 \\ 25 & ${\rm 2s^2 2p^5 3d ~ ^3D_3}$ & 21.7312 & 21.7616 & 21.7594 & 21.6918 & 21.7622 \\ 26 & ${\rm 2s^2 2p^5 3d ~ ^3D_2}$ & 21.7334 & 21.7652 & 21.7632 & 21.6909 & 21.7367 \\ 27 & ${\rm 2s^2 2p^5 3d ~ ^1P_1}$ & 21.9709 & 22.0317 & 22.0372 & 21.9433 & 22.0010 \\ 28 & ${\rm 2s 2p^6 3s ~ ^3S_1}$ & & 24.2031 & 24.2183 & 24.1764 & 24.6381 \\ 29 & ${\rm 2s 2p^6 3s ~ ^1S_0}$ & & 24.4742 & 24.4911 & 24.4184 & 24.7995 \\ 30 & ${\rm 2s^2 2p^5 4s ~ ^3P_2}$ & 24.6131 & 24.6443 & 24.6311 & 24.5969 & 24.8207 \\ 31 & ${\rm 2s^2 2p^5 4s ~ ^1P_1}$ & 24.6402 & 24.6718 & 24.6630 & 24.6187 & 24.6671 \\ 32 & ${\rm 2s^2 2p^5 4s ~ ^3P_0}$ & 24.7794 & 24.8031 & 24.7905 & 24.7609 & \\ 33 & ${\rm 2s^2 2p^5 4s ~ ^3P_1}$ & 24.7936 & 24.8230 & 24.8145 & 24.7746 & \\ 34 & ${\rm 2s^2 2p^5 4p ~ ^3S_1}$ & 25.1071 & 25.1267 & 25.1214 & 25.0596 & \\ 35 & ${\rm 2s^2 2p^5 4p ~ ^3D_3}$ & 25.1591 & 25.1444 & 25.1408 & 25.0981 & \\ 36 & ${\rm 2s^2 2p^5 4p ~ ^3D_2}$ & 25.1472 & 25.1500 & 25.1479 & 25.1014 & \\ 37 & ${\rm 2s^2 2p^5 4p ~ ^1P_1}$ & 25.1598 & 25.1727 & 25.1722 & 25.1203 & \\ 38 & ${\rm 2s^2 2p^5 4p ~ ^3P_2}$ & 25.1817 & 25.1843 & 25.1849 & 25.1307 & \\ 39 & ${\rm 2s^2 2p^5 4p ~ ^3P_0}$ & 25.2565 & 25.2714 & 25.2739 & 25.2097 & \\ 40 & ${\rm 2s^2 2p^5 4p ~ ^3D_1}$ & 25.3016 & 25.3050 & 25.3021 & 25.2590 & \\ 41 & ${\rm 2s^2 2p^5 4p ~ ^1D_2}$ & 25.3225 & 25.3263 & 25.3258 & 25.2787 & \\ 42 & ${\rm 2s^2 2p^5 4p ~ ^3P_1}$ & 25.3253 & 25.3285 & 25.3284 & 25.2750 & \\ 43 & ${\rm 2s 2p^6 3p ~ ^3P_0}$ & & 25.5061 & 25.5425 & 25.4653 & \\ 44 & ${\rm 2s 2p^6 3p ~ ^3P_1}$ & 25.3515 & 25.5140 & 25.5497 & 25.4727 & \\ 45 & ${\rm 2s 2p^6 3p ~ ^3P_2}$ & & 25.5355 & 25.5696 & 25.4927 & \\ 46 & ${\rm 2s^2 2p^5 4p ~ ^1S_0}$ & 25.5795 & 25.5741 & 25.6269 & 25.4414 & \\ 47 & ${\rm 2s 2p^6 3p ~ ^1P_1}$ & 25.4298 & 25.6165 & 25.6467 & 25.5656 & \\ 48 & ${\rm 2s^2 2p^5 4d ~ ^3P_0}$ & 25.7158 & 25.7514 & 25.7320 & 25.6922 & \\ 49 & ${\rm 2s^2 2p^5 4d ~ ^3P_1}$ & 25.7260 & 25.7622 & 25.7427 & 25.7023 & \\ 50 & ${\rm 2s^2 2p^5 4d ~ ^3F_4}$ & 25.7417 & 25.7728 & 25.7537 & 25.7187 & \\ 51 & ${\rm 2s^2 2p^5 4d ~ ^3P_2}$ & 25.7448 & 25.7801 & 25.7607 & 25.7205 & \\ 52 & ${\rm 2s^2 2p^5 4d ~ ^3F_3}$ & 25.7552 & 25.7857 & 25.7692 & 25.7305 & \\ 53 & ${\rm 2s^2 2p^5 4d ~ ^1D_2}$ & 25.7753 & 25.8057 & 25.7905 & 25.7478 & \\ 54 & ${\rm 2s^2 2p^5 4d ~ ^3D_3}$ & 25.7859 & 25.8163 & 25.8009 & 25.7560 & \\ 55 & ${\rm 2s^2 2p^5 4d ~ ^3D_1}$ & 25.8443 & 25.8824 & 25.8706 & 25.8135 & \\ 56 & ${\rm 2s^2 2p^5 4d ~ ^3F_2}$ & 25.9173 & 25.9460 & 25.9300 & 25.8927 & \\ 57 & ${\rm 2s^2 2p^5 4d ~ ^3D_2}$ & 25.9255 & 25.9546 & 25.9383 & 25.8973 & \\ 58 & ${\rm 2s^2 2p^5 4d ~ ^1F_3}$ & 25.9312 & 25.9595 & 25.9439 & 25.9037 & \\ 59 & ${\rm 2s^2 2p^5 4f ~ ^3D_1}$ & 25.9443 & 25.9702 & 25.9638 & 25.9250 & \\ 60 & ${\rm 2s^2 2p^5 4f ~ ^3D_2}$ & 25.9487 & 25.9754 & 25.9693 & 25.9276 & \\ \hline \end{tabular} \tablefoot{\tablefoottext{a}{Sources of NIST v3 are from the unpublished work of %Shirai et~al. ( \citet{SSM99}, and references therein, while that of CHIANTI~v6 is from the work of %Lepson et~al. ( \citet{LBB03}.} \tablefoottext{b}{Theoretical energies from %Zhang et~al. ( \citet{ZSC87}.} \tablefoottext{c}{MCHF data available from the website: \protect\url{http://atoms.vuse.vanderbilt.edu/}}} \end{table}