Isotopic dependence of Born-Oppenheimer breakdown effects in diatomic hydrides: The X¹Σ⁺ states of HI DI and HBr DBr

Line positions available from literature data for HI DI and HBr DBr have been employed to determine corrections to the Born-Oppenheimer approximation. For the ground X¹Σ⁺ states of HI and DI, an analytical effective nonadiabatic Hamiltonian with 13 fitted parameters describes experimental observables within the precision of the measurements. The analytical operator for the X¹Σ⁺ states of HBr and DBr is defined by 12 fitted parameters and achieves similar success. Rotational and centrifugal distortion constants are determined a posteriori through perturbation theory; along with the rotationless eigenvalues, they represent the spectroscopic line positions to within the precision of the measurements. Quantum-mechanical molecular constants are also calculated for TI, T⁷⁹Br, and T⁸¹Br. A detailed study of the isotopic variation of R_e values in hydrogen halide ground states is made using the existing theory.