Renner-teller, spin-orbit and fermi-resonance interactions in x²Π ncs investigated by lif spectroscopy

The electronic spectrum of the NCS radical is obtained following UV excimer-laser photolysis of C₂H₅NCS in a large excess of inert gas in a supersonic free jet expansion. The spectrum is complicated by the presence of Renner-Teller and spin-orbit coupling and a Fermi resonance between the doubly excited bending vibration and v₃, the CS stretching vibration. Bending vibronic level structure in the X²π ground state of the radical is determined from a variety of laser-induced-fluorescence techniques, including stimulated-emission pumping. These energy levels are used to determine vibronic-coupling parameters for the radical. Coupling between close-lying bending levels with different v₂ and K (= ∆ + I) is found to be important because some of these levels are close together in NCS owing to the very large spin-orbit coupling. A parameter representing this effect is related to the dipolar contribution ε1 to the Renner-Teller coupling parameter ε by applying perturbation theory to a simple model involving the X²π and B²Σ⁺ electronic states of NCS. This procedure results in an estimate for ε₁ that agrees approximately with that determined from the vibronic-coupling parameter g_K, assuming the same model for the interacting electronic states.