The 4051 Å Comet Band of 13C3

The tricarbon C₃ molecule has been detected in a number of translucent interstellar clouds via its $A^1\Pi_u-X^1\Sigma_g⁺$ (000-000) electronic `comet' band around 4051 Å. So far, it is the largest molecule unambiguously identified in the diffuse interstellar medium. In this work, rotationally resolved laboratory spectra are presented for the corresponding transition of the ¹³C₃ isotopologue. The spectra are recorded in direct absorption using cavity ring-down spectroscopy in combination with a supersonic plasma jet. A rotational analysis yields accurate spectroscopic parameters. In contrast to ¹²C₃, no significant perturbations are found for (e- or f-parity) levels up to J' = 18 in the A ¹Π upper electronic state.