Bound states using the R-matrix method: Rydberg states of HeH

A method is presented for adapting scattering calculations performed with the molecular R-matrix method to find bound states based on the atomic method of Seaton. Quantum defect theory is used to determine initial energy grids and to determine whether all the bound states have been located. This method is particularly suited to the Rydberg states of electron plus molecular ion systems. The authors calculate and assign the lowest 33 electronic states of the HeH molecule. Previously on 14 of the lowest (n<5) bound states have been fully characterized, with several states omitted. They suggest that the omitted states give rise to some of the observed but previously unexplained weak transitions. Vibrational motion is included in their calculations within the adiabatic approximations. Effects arising from short-range correlations and nuclear motion are shown to be very significant for the lowest electronic states. Transition energies amongst the excited states agree with accurate spectroscopic determinations to better than 50 cm.