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, d3Πg → a3Πu), 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 e3Πg 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 e3Πg states, as predicted by Herzberg, before relaxing to the red manifold of d3Πg 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.)
