Knowledge Bank

The electronic wave functions for the ground $(X1\Sigma +)$ and the low-lying excited states $(A3\Pi ,A1\Pi ,3\Sigma +)$ of the BH molecule have been calculated as a function of internuclear distance using the ab initio spin-coupling optimized GI (SOGI) method. The potential curve or the $A1\Pi$ state in the zero rotational level is found to have a hump of 0.150 eV at R = 3.89 $a0$ (experimentally a hump of unknown size is found at $3.9\pm 0.4a0$); a smaller hump at larger R (0.026 eV at R = 4.92 $a0$) is also found for the calculated $a3\Pi$ states. The presence of such humps is found to result from the recoupling of orbitals that must occur as R is decreased from $\infty$ to $Re$ and is comparable in origin to the activation barrier in a radical exchange reaction (e.g., $H_{2} + D \stackrel{\rightharpoonup}{\leftharpoondown} HD + $H). The calculated binding energies of the BH states are 3.272 eV $(X1\Sigma +)$, 2.216 eV $(a3\Pi )$, and 0.502 eV $(A1\Pi )$. The $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma +$ state is unbound although it does exhibit a small unbound minimum.