MNRAS 497, 1081–1097 (2020) doi:10.1093/mnras/staa1954 Advance Access publication 2020 July 13 12 An update to the MARVEL data set and ExoMol line list for C2 Laura K. McKemmish ,1‹ Anna-Maree Syme ,1 Jasmin Borsovszky ,1 Sergei N. Yurchenko ,2 Jonathan Tennyson ,2 Tibor Furtenbacher 3 and Attila G. Csasz´ ar´ 3 1School of Chemistry, University of New South Wales, Sydney 2052, Australia 2 Department of Physics and Astronomy, University College London, London WC1E 6BT, UK Downloaded from https://academic.oup.com/mnras/article/497/1/1081/5870682 by University College London user on 27 August 2020 3Institute of Chemistry, ELTE Eotv¨ os¨ Lorand´ University and MTA-ELTE Complex Chemical Systems Research Group, H-1518 Budapest 112, PO Box 32, Hungary Accepted 2020 June 27. Received 2020 June 16; in original form 2020 May 14 ABSTRACT 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 12 of these bands. Recently, Furtenbacher et al. presented a set of 5699 empirical energy levels for C2, 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 3 electronic states, including the newly detected 3 g state. Older studies have been included following improvements in the 1 MARVEL procedure that allow their uncertainties to be estimated. These older works in particular determine levels in the C g 1 1 state, the upper state of the insufficiently characterized Deslandres–d’Azambuja (C g–A u) band. The new compilation considers a total of 31 323 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 per cent of the transitions with intensities over 10−18 cm molecule−1 at 1000 K have frequencies determined by empirical energy levels. Key words: molecular data – opacity – astronomical data bases: miscellaneous – comets: general – planets and satellites: atmo- spheres – stars: low-mass. Astronomically, C is unusual in that it has been studied via a large 1 INTRODUCTION 2 number of band systems including the Swan, Phillips, Deslandres– The spectroscopy of the dicarbon molecule, C2, has a long history. d’Azambuja, Ballik–Ramsay, Mulliken and Herzberg-F bands (see Interestingly, C2 was originally observed by Wollaston (1802), which Fig. 1 for the band designations). The Swan (Swings 1943;Gredel, represents the pre-history of spectroscopy. This observation was van Dishoeck & Black 1989; Lambert et al. 1990;Rousselotetal. followed by the identification (Swan 1857) of the well-known Swan 2000) and the Deslandres–d’Azambuja (Gredel et al. 1989)band 3 3 d g–a u band system. The last decade has seen the spectro- systems have been discovered in the spectra of comets when models scopic characterization of several new bands of C2, including the of cometary emission have been found to require no less than two 1 + 1 3 3 − 3 + first observation of multiplicity-changing ‘intercombination’ bands singlet, X g and A u, and four triplet, a u,b g ,c u ,and 3 linking both the singlet-triplet (Chen et al. 2015) and triplet-quintet d g, electronic states to explain the observations. Indeed, two of 3 → 1 + (Bornhauser et al. 2011) states. These observations have allowed the the intercombination bands mentioned above, a u X g and determination of reliable frequencies of singlet-triplet transitions, 3 +→ 1 + c u X g , are needed to explain the observed intensities in which are thought to be important in comets (Rousselot et al. the Swan band (Rousselot et al. 2000). 2000) and are candidates for observation in the interstellar medium C2 has a strong presence in the solar photosphere where it has been (Lebourlot & Roueff 1986). Detection of triplet-quintet transitions observed using the Swan (Asplund et al. 2005), the Phillips, and the has led to the spectroscopic characterization of a number of quintet Ballik–Ramsay (Brault et al. 1982) bands. The Phillips and Ballik– states for the first time (Schmidt & Bacskay 2011; Bornhauser Ramsay bands have also been observed in carbon stars (Goebel et al. et al. 2015, 2017). In addition, recent experiments detected and 1983; Goorvitch 1990), while Swan bands have been observed in characterized a number of new triplet bands (Krechkivska et al. peculiar white dwarfs (Hall & Maxwell 2008;Kowalski2010)and 2017; Welsh et al. 2017). the coronae borealis star V coronae australis (Rao & Lambert 2008). Interstellar C2 has been observed via the infrared Phillips band (Gredel, Black & Yan 2001; Iglesias-Groth 2011), while the Swan E-mail: [email protected] band emissions can be seen in the Red Rectangle (Wehres et al. C 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society 1082 L. K. McKemmish et al. the states involved (Yurchenko et al. 2016) and covers the band 1Π F u 3Δ systems linking the lowest eight electronic states, namely the Swan, g g 3 Phillips, Ballik–Ramsay, Duck, Bernath B and B’ bands, and the f Σ − g singlet–triplet intercombination lines. As a precursor to performing 70 Herzberg F these calculations, Furtenbacher et al. (2016) performed a MARVEL Herzberg g (Measured Active Rotational-Vibrational Energy Levels; Csasz´ ar´ Herzberg f et al. 2007; Furtenbacher, Csasz´ ar´ & Tennyson 2007; Furtenbacher & 12 Csasz´ ar´ 2012a; see Section 2 for a description) analysis for the C2 60 Goodwin− 1 Cool isotopologue. The empirical energies generated by MARVEL were 1 Δ Downloaded from https://academic.oup.com/mnras/article/497/1/1081/5870682 by University College London user on 27 August 2020 u 12 1 incorporated in the C2 8states line list, giving, for example, the Σ + E g 3 5 most accurate predictions available for the singlet-triplet intercom- 4 Π 1 Π g u bination lines. 50 12 Freymark 3 Krechkivska A number of advances has led us to review and update the C2 3 Π g −Schmidt MARVEL project. First, improvements in the MARVEL procedure, 1Σ + including significantly improved error handling (Tobi´ as´ et al. 2019), D u Radi− Mulliken 3Π Bornhauser was found to influence the results of the original study. Secondly, e g 40 1 C´ Π while the original MARVEL study considered 42 sources of spectro- g 12 −1 Kable− scopic C2 data, a number of largely older sources (Dieke & Lochte- 1 Schmidt 3 Π C g Holtgreven 1930; Fox & Herzberg 1937; Herzberg & Sutton 1940; Fox− 5 Phillips 1950; Herzberg, Lagerqvist & Malmberg 1969; Goodwin & 1 Π Herzberg g Cool 1988, 1989) were not considered in 2016, as they did not contain 30 Deslandres− any uncertainty estimates, a requirement for use in the MARVEL energy (x 10 cm ) d’Azambuja Messerle− procedure. New combination difference approaches implemented in Krauss MARVEL allow these uncertainties to be accurately estimated. These 3Π earlier works contain data on states that have not been observed in d g 20 Swan more recent studies; in particular, the studies of 40HeSu (Herzberg & 1Σ + Sutton 1940), 50Phillips (Phillips 1950), and 67Messerle (Messerle B´ g Duck 12 Bernath 1967) contain the only published high-resolution C2 spectra of 1 B Δ the Deslandres–d’Azambuja band. Finally, and most importantly, g 3Σ − 10 c u a series of new studies have provided additional data for known 1Π A u 3Σ + bands (Krechkivska et al. 2017; Welsh et al. 2017) and characterized b g Phillips Ballik− several new bands for the first time (Schmidt & Bacskay 2011; Ramsay 3Π Bornhauser et al. 2015, 2017). These sources are combined with a u 1Σ + 0 X g those considered previously to produce an updated set of empirical (MARVEL) rovibronic energy levels during this study. All (new and 12 old) data sources are referenced by band in Table 5 (vide infra). Figure 1. The band system of C2 with its well-established names. The dashed lines represent observed but unnamed intercombination bands; colours In this work, we also present an updated and improved version 12 indicate newly considered bands (red) and updates (blue). The Messerle– of the C2 8states ExoMol line list, which incorporates the new 1 Krauss band and the associated C g state are depicted in dots as doubts and extended MARVEL-derived set of empirical energy levels. have been raised about their correctness. 2 THE MARVEL PROCEDURE 2010). Absorption in the Phillips, Mulliken and Herzberg-F bands can be seen in translucent clouds (Sonnentrucker et al. 2007). Details about the MARVEL procedure (Furtenbacher et al. 2007; These astronomical observations require high-quality laboratory Furtenbacher & Csasz´ ar´ 2012a;Tobi´ as´ et al. 2019), built upon the data for their analysis and interpretation. Recent spectroscopic theory of spectroscopic networks (SN) (Csasz´ ar´ & Furtenbacher studies have probed new bands with well-known band systems 2011;Csasz´ ar,´ Furtenbacher & Arend´ as´ 2016), have been given in (Nakajima 2019; Krechkivska et al. 2018), providing new data on recent publications (Furtenbacher et al. 2014;Csasz´ ar´ et al. 2016; them. In addition, recent spectroscopic studies on C2 have used Tobi´ as´ et al. 2019, 2020). Therefore, only a brief discussion is given techniques yielding improved ionization (Krechkivska et al. 2016) here. and dissociation energies (Visser et al.