Laser excitation spectrum of C₃ in the region 26 000-30 700 cm^-1

The vibrational structure of the Ã1Πu electronic state of C ₃ in the region 26 000-30 775 cm^-1 has been re-examined, using laser excitation spectra of jet-cooled molecules. Rotational constants and vibrational energies have been determined for over 60 previously-unreported vibronic levels; a number of other levels have been re-assigned. The vibrational structure is complicated by interactions between levels of the upper and lower Born-Oppenheimer components of the Ã1Πu state, and by the effects of the double minimum potential in the Q3 coordinate, recognized by Izuha and Yamanouchi [16]. The present work shows that there is also strong anharmonic resonance between the overtones of the ν1 and ν3 vibrations. For instance, the levels 2 1⁺ 1 and 0 1 ⁺ 3 are nearly degenerate in zero order, but as a result of the resonance they give rise to two levels 139 cm^-1 apart, centered about the expected position of the 2 1 ⁺ 1 level. With these irregularities recognized, every observed vibrational level up to 30 000 cm^-1 (a vibrational energy of over 5000 cm^-1) can now be assigned. A Σu+ vibronic level at 30181.4 cm^-1, which has a much lower B′ rotational constant than nearby levels of the Ã1Πu state, possibly represents the onset of vibronic perturbations by the B̃′1Δu electronic state; this state is so far unknown, but is predicted by the ab initio calculations of Ahmed et al. [36].