The eΠg3 state of C2: A pathway to dissociation

Abstract

The lowest 13 vibrational levels, v = 0-12, of the eΠg3 state of the C molecule have been measured by laser-induced fluorescence of new bands of the Fox-Herzberg system. The newly observed levels, v = 5-12, which span the eΠg3 electronic state up to and beyond the first dissociation threshold of C, were analyzed to afford highly accurate molecular constants, including band origins, and rotational and spin-orbit constants. The spin-orbit coupling constants of the previously published lowest five levels are revised in sign and magnitude, requiring an overhaul of previously published molecular constants. The analysis is supported by high level ab initio calculations. Lifetimes of all observed levels were recorded and found to be in excellent agreement with ab initio predicted values up to v = 11. v = 12 was found to exhibit a much reduced lifetime and fluorescence quantum yield, which is attributed to the onset of predissociation. This brackets the dissociation energy of ground state XΣg+1 C between 6.1803 and 6.2553 eV, in agreement with the Active Thermochemical Tables.

Photodissociation of dicarbon: How nature breaks an unusual multiple bond

“…A section of the spectrum of this band is shown in Fig. 2B; the assignments of the spectrum will be briefly discussed further below but essentially follow the assignments made previously (25). A larger portion of the (12 − 0) band is shown and briefly discussed in SI Appendix.…”

“…As opposed to our earlier work, where an electric discharge was used (25)(26)(27)(28)(29)(30)(31), C2 is here produced by photolysis of C2Cl4, roughly 1 mm in front of the orifice of the nozzle producing the molecular beam. This was done to minimize the production of carbon atoms at the molecular beam source (32).…”

“…In the course of revising the spectroscopy of the Fox-Herzberg system of C2 (e 3 Πg − a 3 Πu ), we determined that the v = 12 level was likely predissociated (25). Predissociation occurs when a bound level is above the lowest dissociation energy and accesses the continuum by nonadiabatic coupling, in this case to the isosymmetric d 3 Πg state.…”

An update to the MARVEL data set and ExoMol line list for 12C2

“…With these new included data, every observed band has MARVEL-compiled rotationally-resolved data with two exceptions: the Kable-Schmidt (Nakajima et al 2009) e 3 Π g − c 3 Σ + u band around 40 000 cm −1 and the Messerle-Krauss (Messerle & Krauss 1967) C 1 Π g − A 1 Π u band around 30 000 cm −1 . Initial errors in the e state constants prohibited a good fit to the Kable-Schmidt band in 2009; new constants (Welsh et al 2017) obtained from better data in the Fox-Herzberg band allowed a much better fit of the e 3 Π g − c 3 Σ + u (4−3) band in that paper's supplementary information, although a full experimental assigned line list was not produced. In the case of the Messerle-Krauss band, recent unpublished investigations (Nauta & Schmidt 2020) suggests that the observed lines of the Messerle-Krauss band are actually part of the Deslandres-d'Azambuja (C 1 Π g − A 1 Π u ) band and that the true C 1 Π g is much higher in energy, as consistently predicted by ab initio theory.…”

“…However, two of these three new spinforbidden bands were inconsistent with the rest of the MARVEL compilation without uncertainties of around 2.4 cm −1 and were thus excluded; in contrast, 67 spin-forbidden transitions between the same two electronic states in 11BoSyKnGe (Bornhauser et al 2011) all validated with smaller uncertainties. 17WeKrNaBa (Welsh et al 2017) Rotationally cool experimental conditions enabled detailed study of low-J ultraviolet rovibronic transitions in the Fox-Herzberg (e 3 Π g − a 3 Π u ) band. No explicit uncertainty was provided in this paper; anestimated source uncertainty of 0.035 cm −1 was used, though this seems to be a slight underestimation based on the uncertainties MARVEL found.…”