Knowledge Bank

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 $B¯1\Pi$ and $C~1\Sigma +$ states. It turns out that there is very strong vibronic coupling through the bending vibration between the $C~1\Sigma +$ state and the $A\prime$ (lower) Born-Oppenheimer component of the $B~1\Pi$ state. The effect is that the bending frequency of the $C¯1\Sigma +$ state is increased by 50% relative to the ground state, while that of the $A\prime$ component of the $B~1\Pi$ state is reduced so far that the molecule becomes non-linear, with a potential barrier of about $120cm-1$ at the linear configuration; the $A\prime \prime$ (upper) component of the $B~1\Pi$ state is not affected. The principal evidence for the barrier is that the 010 $\Sigma +$ vibronic level lies $1.4cm-1$ below the 000 level (linear molecule notation) in the $B~1\Pi$ state of YOD. The density of the level structure in the $C~1\Sigma +$ state arises partly because higher levels of the $B~1\Pi$ state lying among the levels of the $C~1\Sigma +$ state gain intensity through the vibronic coupling.