Dunham Treatment Including Effects of the Born-Oppenheimer Breakdown for Open-Shell Diatomic Molecules in 2Sigma...

A Dunham treatment of an effective Hamiltonian for diatomic molecules in 2Sigma states that includes contributions of the Born-Oppenheimer breakdown has yielded an expression for vibrational-rotational energy by an expansion in powers of (v + (1/2))i[N(N + 1)]j-k(N'/2)k, where N' is N for the J = N + (1/2) spin-doublet state and -(N + 1) for the J = N - (1/2) state and k = 0, 1, ellipsis, j. Analytic expressions for the expansion coefficients Y*vJijk provide a potential model appropriate for the spectral fits for molecules in 2Sigma states. The expansion coefficients Y*vJij0 for the k = 0 terms are exactly the same as the Y*vNij coefficients given for 1Sigma states, the Dunham coefficients Yij modified by the contributions of the Born-Oppenheimer breakdown, provided that a spin contribution originating from off-diagonal electronic corrections of the electronic part of the spin-orbit interaction can be ignored. Spin-rotation energy is given by the terms with coefficients Y*vJijk (k >/= 1). There are no Y*vJijk (k >/= 2) terms in the previously known methods of analysis for 2Sigma molecules. The present potential model is applied to the reported vibrational-rotational spectra of CaH and all spectral lines of 40CaH and 40CaD isotopomers are fitted to a single set of molecular constants simultaneously with significantly smaller standard deviations. Copyright 1998 Academic Press.