Ab initio configuration-interaction investigation of optical transitions in K+He and K+H2

The potassium resonance line (4s-->4p) centered around 770 nm is a major contributor to the optical extinction in the atmospheres of certain classes of brown dwarfs and extrasolar giant planets. The resonance line is significantly broadened by collisions with He and H2, and an accurate calculation of the line profile is needed for astrophysical models of these objects. As a first step, we report an accurate ab initio study of the K+He and K+H2 potential-energy curves correlating to the K 4s and 4p atomic energy levels, together with the dipole moments governing the transitions between these potential-energy curves. The molecular calculations have been carried out using a multireference configuration-interaction method, with the molecular orbitals expanded in a large Gaussian basis set. The transition dipole moments show significant variation with the molecular geometry. Calculations for the K+H2 system have been carried out for a range of H2 orientations and internuclear separations, so that the effect of H2 rotation and vibration may be explicitly included in future calculations of the pressure-broadened line profiles.