Abstract
Accurate line lists for the carbon dimer, C2, are presented. These line lists cover rovibronic transitions between the eight lowest electronic states: |$X\, {}^{1}\Sigma _{g}^{+}$|, |$a\, {}^{3}\Pi _{u}$|, |$A\, {}^{1}\Pi _{u}$|, |$b\, ^{3}\Sigma _{g}^{-}$|, |$c\, ^{3}\Sigma _{u}^{+}$|, |$d\, {}^{3}\Pi _{g}$|, |$B\, {}^{1}\Delta _{g}$|, and |$B^\prime \, {}^{1}\Sigma _{g}^{+}$|. Potential energy curves (PECs) and transition dipole moment curves are computed on a large grid of geometries using the aug-cc-pwCVQZ-DK/MRCI level of theory including core and core–valence correlations and scalar relativistic energy corrections. The same level of theory is used to compute spin-orbit and electronic angular momentum couplings. The PECs and couplings are refined by fitting to the empirical (MARVEL) energies of 12C2 using the nuclear-motion program Duo. The transition dipole moment curves are represented as analytical functions to reduce the numerical noise when computing transition line strengths. Partition functions, full line lists, Landé-factors, and lifetimes for three main isotopologues of C2 (12C2, 13C2, and 12C13C) are made available in electronic form from the CDS (http://cdsarc.u-strasbg.fr) and ExoMol (www.exomol.com) data bases.
Highlights
The C2 molecule is a prominent species in a wide variety of astrophysical sources, including comets (Rousselot et al 2012), interstellar clouds (Hupe, Sheffer & Federman 2012), translucent clouds (Sonnentrucker et al 2007), protoplanetary nebulae (Wehres et al 2010), cool carbon stars (Goorvitch 1990), high-temperature stars (Vartya 1970), and the Sun (Lambert 1978; Brault et al 1982)
The spectroscopy of C2 is an important tool for stellar classifications (Keenan & Morgan 1941; Vartya 1970; Fujita 1980; Keenan 1993; De Mello et al 2009; Gonneau et al 2017) and determining the chemical composition of stars (Querci, Querci & Kunde 1971; Lambert et al 1984; Goorvitch 1990; Bakker et al 1996; Hall & Maxwell 2008; Zamora et al 2009; Ishigaki et al 2012; Green 2013; Schmidt et al 2013; Gonneau et al 2017) and of the Sun (Lambert 1968; Grevesse & Sauval 1973; Lambert 1978; Brault et al 1982)
The Swan bands of C2 have long been known in cometary spectra (Meunier 1911)
Summary
The C2 molecule is a prominent species in a wide variety of astrophysical sources, including comets (Rousselot et al 2012), interstellar clouds (Hupe, Sheffer & Federman 2012), translucent clouds (Sonnentrucker et al 2007), protoplanetary nebulae (Wehres et al 2010), cool carbon stars (Goorvitch 1990), high-temperature stars (Vartya 1970), and the Sun (Lambert 1978; Brault et al 1982). Brooke et al (2013) presented an empirical line list for the Swan system of C2 (d 3 g–a 3 u) which included vibrational bands with v = 0 − 10 and v = 0 − 9, and rotational states with J up to 96, based on an accurate ab initio (MRCI) transition dipole moment d–a curve. These empirical curves were obtained by refining ab initio curves using a recent set of experimentally derived (MARVEL) term values of C2 (Furtenbacher et al 2016) This methodology has been used for similar studies as part of the ExoMol project including the diatomic molecules AlO (Patrascu, Tennyson & Yurchenko 2015), ScH 2015), CaO (Yurchenko et al 2016b), PO and PS (Prajapat et al 2017), VO (McKemmish, Yurchenko & Tennyson 2016), NO (Wong et al 2017), NS and SH (Yurchenko et al 2018a), SiH (Yurchenko et al 2018c), and AlH (Yurchenko et al 2018b)
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