OH FORMATION THROUGH INVERSE PREDISSOCIATION

Author Institution: National Bureau of StandardsAlthough the species OH has long been known to be present in interstellar space, it is commonly believed that OH can not be formed by two body radiative recombination, since the usual Bates mechanism predicts a negligible rate constant for this process at thermal energies. We have calculated the rate constant for another mechanism of radiative recombination, namely, inverse predissociation. The continuum levels of the $^{2} H$ ground state of OH are mixed with the bound levels of the $^{2}\Sigma$ excited state by virtue of the non-diagonal electronic coupling matrix elements between them due to the breakdown of the Born-Oppenheimer approximation. We have calculated in the Hartree-Fock approximation the electronic matrix element $$, which is 0.94 at the $^{2}$ $^{2}\Sigma$ equilibrium position. The predissociation widths defined by configuration interaction theory were calculated using vibrational wavefunctions obtained numerically for the RKR potentials of the two states. Due to the small vibrational overlap, the width for the $^{2}\Sigma^{+}$ v = 1, K = 1 level is only $4 \times 10^{-7}$ $cm^{-1}$, which is about 10\% of the natural radiation width. Although the two body recombination rate constant is strongly temperature dependent, it is around $10^{-19}$ $cm^{3}$ $sec^{-1}$ between 20 and $200^{\circ} K$, sufficiently large to account for OH densities of $10^{-6}--10^{-8}$ $cm^{-3}$ under appropriate conditions. A non-thermal population of the ground state hyperfine levels is assured by this process