The carbon dimer , the ^ { 12 } C _ { 2 } molecule , is ubiquitous in astronomical environments .
Experimental-quality rovibronic energy levels are reported for ^ { 12 } C _ { 2 } , based on rovibronic transitions measured for and among its singlet , triplet , and quintet electronic states , reported in 42 publications .
The determination utilizes the Measured Active Rotational-Vibrational Energy Levels ( MARVEL ) technique .
The 23,343 transitions measured experimentally and validated within this study determine 5,699 rovibronic energy levels , 1,325 , 4,309 , and 65 levels for the singlet , triplet , and quintet states investigated , respectively .
The MARVEL analysis provides rovibronic energies for six singlet , six triplet , and two quintet electronic states .
For example , the lowest measurable energy level of the a ^ { 3 } \Pi _ { u } state , corresponding to the J = 2 total angular momentum quantum number and the F _ { 1 } spin-multiplet component , is 603.817 ( 5 ) cm ^ { -1 } .
This well-determined energy difference should facilitate observations of singlet–triplet intercombination lines which are thought to occur in the interstellar medium and comets .
The large number of highly accurate and clearly labeled transitions that can be derived by combining MARVEL energy levels with computed temperature-dependent intensities should help a number of astrophysical observations as well as corresponding laboratory measurements .
The experimental rovibronic energy levels , augmented , where needed , with ab initio variational ones based on empirically adjusted and spin-orbit coupled potential energy curves obtained using the Duo code , are used to obtain a highly accurate partition function , and related thermodynamic data , for ^ { 12 } C _ { 2 } up to 4,000 K .