Duo is a general, user-friendly program for computing rotational, rovibrational and rovibronic spectra of diatomic molecules. Duo solves the Schrödinger equation for the motion of the nuclei not only for the simple case of uncoupled, isolated electronic states (typical for the ground state of closed-shell diatomics) but also for the general case of an arbitrary number and type of couplings between electronic states (typical for open-shell diatomics and excited states). Possible couplings include spin-orbit, angular momenta, spin-rotational and spin-spin. Corrections due to non-adiabatic effects can be accounted for by introducing the relevant couplings using so-called Born-Oppenheimer breakdown curves.Duo requires user-specified potential energy curves and, if relevant, dipole moment, coupling and correction curves. From these it computes energy levels, line positions and line intensities. Several analytic forms plus interpolation and extrapolation options are available for representation of the curves. Duo can refine potential energy and coupling curves to best reproduce reference data such as experimental energy levels or line positions. Duo is provided as a Fortran 2003 program and has been tested under a variety of operating systems.

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The laser induced fluorescence (LIF) spectra A 1 Σ + ∼ b 3 Π(E J ) → X 1 Σ + of KCs dimer were recorded in near infrared region by Fourier Transform Spectrometer with a resolution of 0.03 cm −1 . Overall more than 200 collisionally enhanced LIF spectra were rotationally assigned to 39 K 133 Cs and 41 K 133 Cs isotopomers yielding with the uncertainty of 0.003-0.01 cm −1 more than 3400 rovibronic term values of the strongly mixed singlet A 1 Σ + and triplet b 3 Π states. Experimental data massive starts from the lowest vibrational level vA = 0 of the singlet and nonuniformly cover the energy range E J ∈ [10040, 13250] cm −1 with rotational quantum numbers J ∈ [7, 225]. Besides of the dominating regular A 1 Σ + ∼ b 3 ΠΩ=0 interactions the weak and local heterogenous A 1 Σ + ∼ b 3 ΠΩ=1 perturbations have been discovered and analyzed. Coupled-channel deperturbation analysis of the experimental 39 K 133 Cs e-parity termvalues of the A 1 Σ + ∼ b 3 ΠΩ=0,1,2 complex was accomplished in the framework of the phenomenological 4 × 4 Hamiltonian accounting implicitly for regular interactions with the remote 1 Π and 3 Σ + states manifold. The resulting diabatic potential energy curves of the interacting states and relevant spin-orbit coupling matrix elements defined analytically by Expanded Morse Oscillators model reproduce 95% of experimental data field of the 39 K 133 Cs isotopomer with a standard deviation of 0.004 cm −1 which is consistent with the uncertainty of the experiment. Reliability of the derived parameters was additionally confirmed by a good agreement between the predicted and experimental termvalues of 41 K 133 Cs isotopomer. Calculated relative intensity distributions in the A ∼ b → X LIF progressions are also consistent with their experimental counterparts. Finally, the deperturbation model was applied for a simulation of pump-dump optical cycle a 3 Σ + → A 1 Σ + ∼ b 3 Π → X 1 Σ + proposed for transformation of ultracold colliding K+Cs pairs to their ground molecular state vX = 0; JX = 0.

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