Shedding light on a dark state: The energetically lowest quintet state of C$_2$2

Abstract

In this work we present a deperturbation study of the $d\,^3\Pi _g, v=6$d3Πg,v=6 state of C$_{2}$2 by double-resonant four-wave mixing spectroscopy. Accurate line positions of perturbed transitions are unambiguously assigned by intermediate level labeling. In addition, extra lines are accessible by taking advantage of the sensitivity and high dynamic range of the technique. These weak spectral features originate from nearby-lying dark states that gain transition strength through the perturbation process. The deperturbation analysis of the complex spectral region in the (6,5) and (6,4) bands of the Swan system ($d^3\Pi _g-a\,^3\Pi _u$d3Πg−a3Πu) unveils the presence of the energetically lowest high-spin state of C$_{2}$2 in the vicinity of the $d\,^3\Pi _g, v=6$d3Πg,v=6 state. The term energy curves of the three spin components of the d state cross the five terms of the $1\;^5\Pi _g$15Πg state at rotational quantum numbers N ⩽ 11. The spectral complexity for transitions to the v = 6 level of $d\,^3\Pi _g$d3Πg state is further enhanced by an additional perturbation at N = 19 and 21 owing to the $b\, ^3\Sigma _g^-, v=19$b3Σg−,v=19 state. The spectroscopic characterization of both dark states is accessible by the measurement of 122 “window” levels. A global fit of the positions to a conventional Hamiltonian for a linear diatomic molecule yields accurate molecular constants for the quintet and triplet perturber states for the first time. In addition, parameters for the spin-orbit and L-uncoupling interaction between the electronic levels are determined. The detailed deperturbation study unravels major issues of the so-called high-pressure bands of C$_{2}$2. The anomalous nonthermal emission initially observed by Fowler in 1910 [Mon. Not. R. Astron. Soc. 70, 484 (1910)] and later observed in numerous experimental environments are rationalized by taking into account “gateway” states, i.e., rotational levels of the $d\,^3\Pi _g, v=6$d3Πg,v=6 state that exhibit significant $^5\Pi _g$5Πg character through which all population flows from one electronic state to the other.

Spectroscopic observation of higher vibrational levels of C2 through visible band systems

“…8 The Dunham expansion coefficients listed in Table VI are used for evaluating the T v and B v values for the b 3 − g , B 1 g , and 1 5 g states. The coefficients for the b 3 − g state were re-determined including the recently reported term values and rotational constants of the v = 16 and 19 levels, which were derived from the perturbations observed in the Swan bands, 3,32 in order to accurately estimate the molecular constants for higher vibrational levels. Although the vibrational quantum number has not been identified for the reported vibrational level for the 1 5 g state at 29 258.6 cm −1 , 3 we assume that the reported value is of the v = 0 level, since the experimentally derived rotational constant is quite close to the rotational constant of v = 0 computed from a high-level ab initio potential of the 1 5 g state.…”

“…One is the c 3 + u state, 2 and another is the 1 5 g state. 3 Now, accurate electronic energies of C 2 have been obtained for all the electronic states located below 30 000 cm −1 . However, experimental information on C 2 has still been limited to relatively low energy regions of the potential curves for those electronic states, that is, relatively low-lying vibrational levels.…”

Active Thermochemical Tables: dissociation energies of several homonuclear first-row diatomics and related thermochemical values

“…In spite of the fact that the presence of C 2 is clearly visible in nearly every hydrocarbon flame (quoting Hoffmann [41]: 'the lovely blue color of hot hydrocarbon flames is due in large part to emission from excited C 2 molecules on their way to soot or CO 2 '), and in spite of numerous detailed spectroscopic studies of this molecule [42][43][44][45][46][47][48][49][50][51], there is very little in terms of accurate experimental determinations that could help define D 0 (C 2 ). Among the top fifty provenance contributors to the ATcT value, only four are experimental 6 Note that there is an inconsistency in the JANAF Tables [14,15]: the listed enthalpy of formation of C 2 was derived in the third edition [14] by combining the assumed bond dissociation energy of 589.7 ± 3.8 kJ/mol with an older value for the enthalpy of formation of C atom, which is lower by 1.7 kJ/mol than the listed value.…”

ExoMol: molecular line lists for exoplanet and other atmospheres

“…The next largest available diatomic line list is that for C 2 which is one of a number of diatomics species for which line lists have been provided by Kurucz. Recently, however, there have been several new experimental measurements on this system, including the characterization of entirely new, low‐lying electronic bands (Kokkin et al ; Joester et al ; Tanabashi et al ; Nakajima et al ; Bornhauser et al , ). This work has been accompanied by significantly improved ab initio electronic structure calculations (Kokkin, Bacskay & Schmidt ; Schmidt & Bacskay , ; Nakajima et al ).…”