First principles calculation of rotational and ro-vibrational line strengths

Abstract

Theory is developed for the calculation of dipole transition line strengths and frequencies for rotational and ro-vibrational transitions from wavefunctions expressed in the generalized body-fixed co-ordinates proposed by Sutcliffe and Tennyson (1986, Molec. Phys., 58, 1053). Computations using this theory produce calculated frequencies for the fundamental ro-vibrational transitions of H₂D⁺ and D₂H⁺ in very good agreement with experiment. These first principles calculations use the highly accurate ab initio electronic potential energy and dipole surface of Meyer, Botschwina and Burton, which has previously been shown by the authors to give ro-vibrational transition frequencies, rotational constants and vibrational fundamentals of spectroscopic accuracy. Three line reassignments are proposed on frequency considerations. Several transitions are proposed as candidates for observation on the grounds of computed relative intensities. Calculated pure rotational transitions in ground state for H₂D⁺ and D₂H⁺ are in excellent agreement with limited data available, and the full rotational spectra of these molecules are predicted.