spin–orbit interaction

We have measured the hyperfine structure of mutually perturbing rovibrational levels of the 1sbd P0 and 2sAd S states of the NaK molecule, using the perturbation-facilitated optical-optical double resonance method with copropagating lasers. The unperturbed 1sbd P0 levels are split into four hyperfine components by the Fermi contact interaction bFI ·S. Mixing between the 1sbd P0 and 2sAd S levels imparts hyperfine structure to the nominally singlet component of the perturbed levels and reduces the hyperfine splitting of the nominally triplet component. Theoretical analysis relates these observations to the hyperfine splitting that each 1sbd P0 level would have if it were not perturbed by a 2sAd S level. Using this analysis, we demonstrate that significant hyperfine splitting arises because the 1sbd P0 state cannot be described as pure Hund’s case sad. We determine bF for the 1sbd P0 levels and also a more accurate value for the magnitude of the singlet-triplet spin–orbit coupling HSO= k1sbd P0svb ,JduHSOu2sAd SsvA ,Jdl. Using the known spectroscopic constants of the 1sbd P state, we obtain bF=0.009 89±0.000 27 cm−1. The values of uHSOu are found to be between 2 and 3 cm−1, depending on vb, vA, and J. Dividing uHSOu by calculated vibrational overlap integrals, and taking account of the 1sbd PV rotational mixing, we can determine the magnitude of the electronic part Hel of HSO. Our results yield uHelu = s16.33±0.15d cm−1, consistent with our previous determinations using different techniques. © 2005 American Institute of Physics. fDOI: 10.1063/1.1844293g