  The spectrum of dicarbon (C2) is important in astrophysics and for
spectroscopic studies of plasmas and flames. The C2 spectrum is characterized
by many band systems with new ones still being actively identified;
astronomical observations involve eight of these bands. Recently, Furtenbacher
et al. (2016, Astrophys. J. Suppl., 224, 44) presented a set of 5699 empirical
energy levels for 12C2, distributed among 11 electronic states and 98 vibronic
bands, derived from 42 experimental studies and obtained using the MARVEL
(Measured Active Rotational-Vibrational Energy Levels) procedure. Here, we add
data from 13 new sources and update data from 5 sources. Many of these data
sources characterize high-lying electronic states, including the newly detected
3Pig state. Older studies have been included following improvements in the
MARVEL procedure which allow their uncertainties to be estimated. These older
works in particular determine levels in the C1Pig state, the upper state of the
insufficiently characterized Deslandres-d'Azambuja (C1Pig-A1Piu) band. The new
compilation considers a total of 31323 transitions and derives 7047
empirical(MARVEL) energy levels spanning 20 electronic and 142 vibronic states.
These new empirical energy levels are used here to update the 8states C2 ExoMol
line list. This updated line list is highly suitable for high-resolution
cross-correlation studies in astronomical spectroscopy of, for example,
exoplanets, as 99.4% of the transitions with intensities over 10^(-18)
cm/molecule at 1000 K have frequencies determined by empirical energy levels.
