Fourier transform spectroscopy and direct potential fit of a shelflike state: Application to E(4)¹Σ⁺ KCs

The paper presents high-resolution experimental study and a direct potential construction of a shelflike state E(4)¹Σ⁺ of the KCs molecule converging to K(4²S) + Cs(5²D) atomic limit; such data are of interest for selecting optical paths for producing and monitoring cold polar diatomics. The collisionally enhanced laser induced fluorescence (LIF) spectra corresponding to both spin-allowed E(4)¹Σ⁺ → X(1)¹Σ⁺ and spin-forbidden E(4)¹Σ⁺ → a(1)³Σ⁺ transitions of KCs were recorded in visible region by Fourier transform spectrometer with resolution of 0.03 cm⁻¹. Overall about 1650 rovibronic term values of the E(4)¹Σ⁺ state of ³⁹K¹³³Cs and ⁴¹K¹³³Cs isotopologues nonuniformly covering the energy range [16987, 18445] cm⁻¹ above the minimum of the ground X-state were determined with the uncertainty of 0.01 cm⁻¹. Experimental data field is limited by vibrational levels v′ ∈ [2, 74] with rotational quantum numbers J′ ∈ [1, 188]. The closed analytical form for potential energy curve (PEC) based on Chebyshev polynomial expansion (CPE) was implemented to a direct potential fit (DPF) of the experimental term values of the most abundant ³⁹K¹³³Cs isotopologue. Besides analyticity, regularity, correct long-range behavior, and nice convergence properties, the CPE form demonstrated optimal balance on flexibility and constraint for the DPF of a shelflike state aggravated by a limited data set. The mass-invariant properties of the CPE PEC were tested by the prediction of rovibronic term values of the ⁴¹K¹³³Cs isotopomer which coincided with their experimental counterparts with standard deviation of 0.0048 cm⁻¹. The CPE modeling is compared with the highly flexible pointwise inverted perturbation approach model, as well as with conventional Dunham analysis of restricted data set v′ ⩽ 50. Reliability of the empirical PEC is additionally confirmed by good agreement between the calculated and experimental relative intensity distributions in the long E(v′) → X(v″) LIF progressions.