Rotationally resolved spectra of the 4051 Å comet band of C 3 for all six 12 C and 13

Rotationally resolved absorption spectra of the e A 1 P u — e X 1 R þ g origin band of the tricarbon molecule (C 3) are reported for 12 C 3 and its 13 C-substitued isotopologues. The 4051 Å electronic transition, also identified in cometary tails and in translucent interstellar clouds, has been measured using cavity ring-down spec-troscopy and a supersonically expanding planar plasma. Previous spectroscopic studies of 12 C 3 are extended and the electronic origin band is re-investigated including a perturbation analysis of the upper state. The rotational analysis of spectra originating from 13 C mono-and di-substituted C 3 yields accurate spectroscopic parameters for all six isotopologues, including ground state molecular constants for 13 C 12 C 13 C and 12 C 13 C 13 C. The tricarbon molecule (C 3) is an important intermediate in many chemical processes involving hydrocarbons. As one of the simplest linear centro-symmetric poly-atomics with a low resistance to bending, C 3 serves as a benchmark molecule in quantum chemistry and molecular physics [1,2]. The 4051 Å band of C 3 was first detected in cometary emission in 1881 [3] and later in the laboratory in 1942 [4]. In 1951 C 3 was unambiguously identified as the emitter of this band following a 13 C-substitution experiment [5]. Later, Gausset et al. [6,7] assigned the spectrum as the e A 1 P u — e X 1 R þ g electronic transition based on a detailed rotational analysis of a number of vibronic bands. Due to the complexity of the spectrum, partly because of the existence of a low frequency bending mode, a large number of experimental and theoretical studies were needed to attain a comprehensive characterization of the e A 1 P u — e X 1 R þ g transition of C 3. C 3 has also attracted long-term interest from astronomers because of its important role in the chemistry of the interstellar medium (ISM). This small molecule is potentially involved in the formation of more complex molecules such as carbon chains and carbonaceous circumstellar grains, and as a photo-fragment of polycyclic aromatic hydrocarbons (PAHs) [8,9]. Different from most asymmetric carbon-chain species, C 3 cannot be detected in the interstellar medium by radio astronomy due to the lack of a permanent dipole moment. Detection of C 3 in circumstellar shells and in the dense ISM has been accomplished via ro-vibrational transitions …