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.

Abstract. Adiabatic and diabatic potential curves are obtained for linear HCNH. The electronic states of this molecule are characterized by strong mixing of valence and Rydberg configurations. Molecular orbitals for these two classes of configurations are determined separately and then combined. Large scale calculations at the level of single and double excitations from a multiconfiguration reference are performed. The calculations confirm the earlier results of Talbi & Ellinger (1998, Chem. Phys. Lett., 288, 155). Diabatic potential curves are determined using the block diagonalization method and are used to estimate the width Γ for electron capture by HCNH + . Rates for dissociative recombination through the CH bond and through the NH bond (which lead to HNC + H and HCN + H, respectively) are calculated using a simple quasi-diatomic model. None of the calculated results suggests a strong difference in the rates for production of HNC and HCN.

The excited 3 (3)Pi and 4 (3)Pi electronic states of the NaK molecule exhibit an avoided crossing, leading to the anomalous behavior of many features of the rovibrational energy levels belonging to each state. A joint experimental and theoretical investigation of these states has been carried out. Experimental measurements of the vibrational, rotational, and hyperfine structure of numerous levels of the 3 (3)Pi state were recently obtained using the Doppler-free, perturbation-facilitated optical-optical double resonance technique. Additional measurements for the 4 (3)Pi state as well as bound-free emission spectra from selected 3 (3)Pi, 4 (3)Pi, and mixed 3 (3)Pi to approximately 4 (3)Pi rovibrational levels are reported here. A model is also presented for calculating the mixed rovibrational level energies of the coupled 3 (3)Pi-4 (3)Pi system, starting from a 2x2 diabatic electronic Hamiltonian. The 3 (3)Pi and 4 (3)Pi potential curves and the coupling between them are simultaneously adjusted to fit the observed rovibrational levels of both states. The energy levels of the potential curves determined by the fit are in excellent agreement with experiment. The nonadiabatic coupling is sufficiently strong to cause an overall shift of 2-3 cm(-1) for many rovibrational levels as well as somewhat larger shifts for certain pairs of 3 (3)Pi to approximately 4 (3)Pi levels that would otherwise be very close together.