We have measured the hyperfine structure of mutually perturbing rovibrational levels of the 1(b) 3Pi0 and 2(A) 1Sigma+ states of the NaK molecule, using the perturbation-facilitated optical-optical double resonance method with copropagating lasers. The unperturbed 1(b) 3Pi0 levels are split into four hyperfine components by the Fermi contact interaction bFIS. Mixing between the 1(b) 3Pi0 and 2(A) 1Sigma+ 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 1(b) 3Pi0 level would have if it were not perturbed by a 2(A) 1Sigma+ level. Using this analysis, we demonstrate that significant hyperfine splitting arises because the 1(b) 3Pi0 state cannot be described as pure Hund's case (a). We determine bF for the 1(b) 3Pi0 levels and also a more accurate value for the magnitude of the singlet-triplet spin-orbit coupling HSO=[1(b) 3Pi0(vb,J)(H(SO))2(A) 1Sigma+(vA,J). Using the known spectroscopic constants of the 1(b) 3Pi state, we obtain bF=0.009 89+/-0.000 27 cm(-1). The values of (H(SO)) are found to be between 2 and 3 cm(-1), depending on vb, vA, and J. Dividing (H(SO)) by calculated vibrational overlap integrals, and taking account of the 1(b) 3Pi(Omega) rotational mixing, we can determine the magnitude of the electronic part H(el) of H(SO). Our results yield (H(el))=(16.33+/-0.15) cm(-1), consistent with our previous determinations using different techniques.

High-resolution spectra, including hyperfine structure, have been observed for numerous vibrational-rotational levels (v,N) of the 4 3 ⌺ ϩ Rydberg state of the NaK molecule. The data have been used to construct a Rydberg-Klein-Rees potential curve, and this molecular potential has been further refined using the inverse perturbation approximation method. Bound-free emission from the 4 3 ⌺ ϩ electronic state to the repulsive a(1) 3 ⌺ ϩ state has also been measured and used to determine both the absolute vibrational numbering and the transition dipole moment function M (R). The experimentally derived potential curve and M (R) are compared with recent theoretical calculations of Magnier et al.; the agreement is very good. Each of the levels (v,N) is typically split into three sets of sublevels by the Fermi contact interaction bI"S. Further splitting ͑of order 0.004 cm Ϫ1) has been attributed to the spin-rotation interaction ␥N"S. The patterns observed exhibit a clear transition from Hund's case b ␤S for small N toward Hund's case b ␤J for large N. The data can be fitted very well using a theoretical model based on setting up and diagonalizing a 12ϫ12 Hamiltonian matrix with two adjustable parameters (b and ␥͒. The values of b that fit the data best are ϳ(0.99 Ϯ0.04)ϫ10 Ϫ2 cm Ϫ1 , with a weak dependence on v. The best fit values of ␥ are in the range 1-6ϫ10 Ϫ4 cm Ϫ1 and depend strongly on v. The values of ␥ appear to exhibit anomalous structure for (v,N) levels perturbed by nearby levels of the 3 3 ⌸ state.

We have used the Doppler-free, perturbation-facilitated optical-optical double-resonance technique to investigate the vibrational, rotational, and hyperfine structure of the 3 (3)Pi double minimum state of NaK. Since this electronic state arises from an avoided crossing with the nearby 4 (3)Pi state, we observe striking patterns in the data that provide a sensitive probe of the electronic wave function in the various regions of the double well potential. A single-mode cw dye laser excites 2(A) (1)Sigma(+)(v(A),J) approximately 1(b) (3)Pi(Omega=0)(v(b),J) mixed singlet-triplet "window" levels from thermally populated rovibrational ground state levels, 1(X) (1)Sigma(+)(v(X),J+/-1). Further excitation by a single-mode cw Ti:sapphire laser selects various 3 (3)Pi(0)(v(Pi),J(Pi)) rovibrational levels, which are detected by observing direct 3 (3)Pi(0)-->1(a) (3)Sigma(+) fluorescence in the green spectral region. Using the inverse perturbation approximation method, we have determined a 3 (3)Pi(0) potential curve that reproduces the measured energies to approximately 0.24 cm(-1). In addition, the hyperfine and spin-orbit constants, b(F) and A(v), have been determined for each region of the potential curve.