The triatomic carbon radical, C3, is important in combustion processes, and features also in the spectra of various astronomical objects. It has a strong electronic transition in the 330-410 nm wavelength region. Working with jet-cooled laser-induced fluorescence spectra taken at Taipei, I was able to clarify the rotational assignments of the astrophysically-important 405 nm band, and to assign various perturbations at higher energy. At very low temperatures C3 forms van der Waals complexes with the noble gases. My analyses of high resolution spectra of C3Ar have shown that the complex is T-shaped, with the argon atom lying 3.8 Å from the centre of mass of the C3 radical. The excited electronic state of the C3 radical is orbitally degenerate, but the degeneracy is lifted in the lower symmetry of the C3Ar complex; however the orbital angular momentum does not go away, but re-appears as Coriolis (rotational) coupling between the two close-lying component electronic states. The emission spectra of C3Ar are very complicated because there is strong intramolecular vibrational relaxation in the upper states; the energy cascades to the lowest vibrational level of each of the vibrational potentials.
