An effective Hamiltonian for diatomic molecules . Ab initio calculations of parameters of HCl+</SBT>
The form of the effective Hamiltonian for a diatomic molecule in an electronic state intermediate between Hund's cases (a) and (b) is reexamined. The derivation is performed in three stages: (i) an electronic contact transformation removes the effects of matrix elements which couple different electronic states; (ii) a vibrational contact transformation removes matrix elements coupling different vibrational states; and (iii) a spin-rotational contact transformation removes indeterminacies in the final Hamiltonian. Expressions for the effective parameters resulting from these transformations are presented. The Hamiltonian is in a convenient form for fitting to experimental data, and the formulas for the parameters assist in the comparison with ab initio calculations. The relations between the present parameters and those employed in the computer program of Zare, Schmeltekopf, Harrop, and Albritton [ J. Mol. Spectrosc.46, 37-66 (1973)] are given, and make it possible to correct a previous misunderstanding regarding the value of the spin-rotation parameter γe in the X2Π state of HCl +. The parameters obtained for this state are compared with ab initio calculations which take account of first-order perturbations within the state and second-order perturbations from the A2Σ + state and from the four states of the configuration … σ 2π 2σ ∗. The A2Σ + state is the dominant perturber, but the contributions from the other states are not negligible. For example, their contribution to γe is larger than the first-order contribution.