THE ORBITAL DESCRIPTION OF THE LOWER EXCITED STATES OF THE BH MOLECULE

Abstract

The electronic wave functions for the ground $(X^{1}\Sigma^{+})$ and the low-lying excited states $(A^{3}\Pi, A^{1}\Pi, ^{3}\Sigma^{+})$ 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 $A^{1}\Pi$ state in the zero rotational level is found to have a hump of 0.150 eV at R = 3.89 $a_{0}$ (experimentally a hump of unknown size is found at $3.9 \pm 0.4 a_{0}$); a smaller hump at larger R (0.026 eV at R = 4.92 $a_{0}$) is also found for the calculated $a^{3}\Pi$ 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 $R_{e}$ 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 $(X^{1}\Sigma^{+})$, 2.216 eV $(a^{3}\Pi)$, and 0.502 eV $(A^{1}\Pi)$. The $^{3}\Sigma^{+}$ state is unbound although it does exhibit a small unbound minimum.