How dicarbon breaks apart

Like oxygen and nitrogen, dicarbon is a homonuclear diatomic molecule. But unlike its atmospheric counterparts, C₂ is so reactive that it exists only in rarefied or highly energetic environments. It can be found in flames, comets, stars, and the diffuse interstellar medium. In its lowest triplet state, C₂ announces its presence through the blue-green fluorescence of so-called Swan bands (green, d³Π_g → a³Π_u), shown here in the molecule’s energy-level diagram. In 1939, future Nobel laureate Gerhard Herzberg suggested that the excitation of C₂‘s electrons into the e³Π_g state by sunlight would result in the dissociation of the molecule. The suggestion would explain why a comet’s coma is often green, while its tail is not, as shown in the photograph of comet Lovejoy’s fluorescence. Sunlight first heats the comet’s ice and organic material to produce C₂ molecules, only to break them apart before they can ever reach the tail.

Eighty-two years later, researchers led by Timothy Schmidt , a University of New South Wales chemist, have finally observed the photodissociation of C₂ in the laboratory. They produced the C₂ from the photolysis of a beam of tetrachloroethylene (C₂Cl₄) molecules. A UV laser strips chlorine atoms from the parent molecules, leaving C₂ molecules behind to expand and cool in a vacuum chamber. That process also leaves them near the ground state in low rotational levels. The researchers then used a second and third UV laser beam to pump and probe the dicarbon molecules via two-photon ionization. From the speed of recoiling carbon atoms, the researchers deduced a bond-dissociation energy (602.804 kJ/mol) with a precision comparable to that for nitrogen and oxygen. Previous estimates of the bond energy for C₂ had uncertainties an order of magnitude higher than the other diatomic molecules.

The energy-level diagram outlines the mechanism: After excitation from the ground state (purple arrow), the molecule makes a brief stop on the black manifold of e³Π_g states, as predicted by Herzberg, before relaxing to the red manifold of d³Π_g states. It’s on those d³ states that the two carbon atoms dissociate (red dashed horizontal arrow). From that mechanism, the researchers then used quantum chemical calculations to predict the lifetime of cometary C₂ as 1.6 × 10⁵ seconds, a value reassuringly consistent with astronomical observations. Part of what makes the investigation so difficult is that dicarbon is unstable. To break a newly made C₂ bond, the molecule must absorb two photons. And during that absorption, C₂ undergoes two forbidden spectroscopic transitions: spin conservation and the Born–Oppenheimer approximation. (J. Borsovszky et al., Proc. Natl. Acad. Sci. USA 118, e2113315118, 2021 .)