The perturbations of line frequencies in high-J manifolds of the ν3 band of SF6

In the diode laser spectra of several P- and R-branches of high-J manifolds in the ν₃ band of SF₆ it has been observed that the lines are shifted away from the positions predicted by the diagonal F_A1pp^(4,R,R) approximation of Moret-Bailly. The shifts form a smooth function of the line positions across each manifold; this function can be represented empirically by a parabola, to which the measured wavenumbers fit within experimental accuracy. This phenomenon is explained here by model calculations for P-, Q-, and R-branch excited state levels that perturb each other through higher-order vibration-rotation interactions. In the analysis, eigenvalues calculated by second-order perturbation theory are compared with those obtained by diagonalization of the Hamiltonian.