Rovibrationally-resolved photodissociation of NH and application to the solar UV opacity

Rovibrationally-resolved photodissociation cross sections of NH have been evaluated using a combination of ab initio and experimentally derived potential curves and dipole transition moments. Here we present results for the three electronic transitions: $2~^3\Sigma^- \leftarrow$ X$~^3\Sigma^-$, $2~^3\Pi \leftarrow$ X$~^3\Sigma^-$, A$~^3\Pi\leftarrow$ X$~^3\Sigma^-$. Partial cross sections for transitions from all 577 rovibrational levels obtained theoretically for the ground electronic state X$~^3\Sigma^-$, were computed for a wavelength range that extends from 500\AA\ to the dissociation threshold for each particular rovibrational level. Assuming a thermal Boltzmann distribution of the rovibrational levels in X$~^3\Sigma^-$, LTE cross sections are presented for gas temperatures between 500 and 10000~K. For applications to cold interstellar gas, cross sections for X$~^3\Sigma^-(v=0,J=0)$ to $2~^3\Sigma^-$ and $2~^3\Pi$ dominate, but for the high density and temperature conditions in stellar atmospheres, the LTE cross section to the A$~^3\Pi$ becomes competitive. Explicit application of the cross sections to the solar UV opacity will be presented. In particular, the NH photodissociation opacity is found to affect the non-LTE behavior of some species such as Cr I and V I.