HIGHER VIBRATIONAL LEVELS OF THE $\tilde{A}{}^1\Pi_u$ STATE OF C${}_3$ OBSERVED BY LASER-INDUCED FLUORESCENCE

Abstract

The vibrational structure of the $\tilde{A}{}^1\Pi_u$ electronic state of C${}_3$ in the region 26000-31000 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 $\tilde{A}{}^1\Pi_u$ state, and by the effects of the double minimum potential in the $Q_3$ coordinate, recognized by Izuha and Yamanouchi. The present work shows that there is also strong anharmonic resonance between the overtones of the $\nu_1$ and $\nu_3$ vibrations. For instance, the $\Sigma^+_u$ vibronic 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${}^+$1 level. Similarly, the 202 level lies 60 cm${}^{-1}$ lower than expected because of interaction with the 400 and 004 levels. With these irregularities recognized, every observed vibrational level up to 29550 cm${}^{-1}$ (a vibrational energy of nearly 5000 cm${}^{-1}$) can now be assigned.