15min:

CHIAO-WEI CHEN, ANTHONY J. MERER, JUN-MEI CHAO AND YEN-CHU HSU, Institute of Atomic and Molecular Sciences, Academia Sinica, Box 23-166, Taipei, Taiwan 10617.

The vibrational structure of the A¹ _u electronic state of C₃ 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 A¹ _u state, and by the effects of the double minimum potential in the Q₃ coordinate, recognized by Izuha and Yamanouchi. The present work shows that there is also strong anharmonic resonance between the overtones of the ₁ and ₃ vibrations. For instance, the ⁺_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.