Conspectus
Dicarbon, the molecule formed from two carbon
atoms, is among the
most abundant molecules in the universe. Said by some to exhibit a
quadruple bond, it is bound by more than 6 eV and supports
a large number of valence electronic states. It thus has a rich spectroscopy,
with 19 one-photon band systems, four of which were discovered by
the author and co-workers. Its spectrum was among the first to be
described: Wollaston reported the emission spectra from blue flames
in 1802.
C2 is observed in a variety of astronomical
objects,
including stars, circumstellar shells, nebulae, comets and the interstellar
medium. It is responsible for the green color of cometary comae but
is not observed in the comet tail. It can be observed in absorption
and emission by optical spectroscopy in the infrared, visible, and
ultraviolet regions of the spectrum, and because it has no electric-dipole-allowed
vibrational or rotational transitions, its spectral signature is a
sensitive probe of the local environment.
Before the work described
in this Account, models of C2 photophysics included the
thitherto-unobserved c
3Σ
u
+ state and parametrized the strength of spin-forbidden
intercombination transitions. Furthermore, they did not account for
photodissociation of C2, even though it was identified
in the 1930s as a key process. Inspired by the observation of C2 in the Red Rectangle nebula, the author was motivated to
instill rigor into C2 models and embarked on a spectroscopic
and computational journey that has lasted 15 years.
We were
the first to identify the c
3Σ
u
+ state through the d
3Π
g
–c
3Σ
u
+ transitions, which were to become known as
the “Duck” system. This minor partner to the well-known
Swan bands is a key part of astrophysical C2 models and
can now be included with rigor. We identified the e
3Π
g
–c
3Σ
u
+ system, and the c
3Σ
u
+ state is now well-studied. Meanwhile others
described the singlet–triplet and triplet–quintet interactions
in exquisite detail, allowing rigorous modeling of the a–X and c–X intercombination transitions.
The final piece of
the C2 puzzle would be understanding
how long it survives before being broken into carbon atom fragments.
Though predicted by Herzberg, predissociation in the e
3Π
g
state had never
been observed. To find it would require the complicated ultraviolet
spectroscopy of C2 to be disentangled. In so doing, we
identified the 43Π
g
and
33Π
g
states of C2, thus uncovering two new band systems. The 43Π
g
state allowed the first accurate determination
of the ionization energy of C2. With these new band systems
secure, we extracted new levels of the D
1Σ
u
+ state (Mulliken bands) and the e
3Π
g
state (Fox–Herzberg
bands) from our spectra. Upon climbing the energy ladder in the e
3Π
g
state
to v = 12, we finally identified the route to predissociation
of C2 via non-adiabatic coupling to the d
3Π
g
state. This observation
provided the first laboratory evidence for why C2 is observed
in the coma of a comet but not the tail.