15min:

A. G. ADAM, Department of Chemistry, University of New Brunswick, Bag Service 45222, Fredericton, N.B. E3B 6E2, Canada; K. ATHANASSENAS, C. T. KINGSTON, A. J. MERER, J. R. D. PEERS, S. J. RIXON, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C. V6T 1Z1, Canada; AND D. A. GILLETT, Lambda-Physik, Inc., 3201 West Commercial Blvd., Fort Lauderdale, FL 33309.

Extensive new spectra have been taken for the visible systems of YOH and YOD, in an attempt to establish the vibrational assignments in the very confused B¹ and C^{1 +} states. It turns out that there is very strong vibronic coupling through the bending vibration between the C^{1 +} state and the A' (lower) Born-Oppenheimer component of the B¹ state. The effect is that the bending frequency of the C^{1 +} state is increased by 50% relative to the ground state, while that of the A' component of the B¹ state is reduced so far that the molecule becomes non-linear, with a potential barrier of about 120 cm^-1 at the linear configuration; the A'' (upper) component of the B¹ state is not affected. The principal evidence for the barrier is that the 010 ⁺ vibronic level lies 1.4 cm^-1 below the 000 level (linear molecule notation) in the B¹ state of YOD. The density of the level structure in the C^{1 +} state arises partly because higher levels of the B¹ state lying among the levels of the C^{1 +} state gain intensity through the vibronic coupling.