Potential construction of the B(1)1 Î state in KCs based on Fourier-Transform spectroscopy data

•An empirical potential for first singlet Pi state of the KCs molecule was constructed.•Fourier-Transform spectroscopy yielded term values with 0.01 cm−1 accuracy.•Data set included vibrational levels up to v = 35 covering 85% of the potential well.•The potential reproduces the fitted data with a standard deviation 0.94 cm−1.•The empirical potential contains an inflection that reflects the avoided crossing.

The paper presents an empirical pointwise potential energy curve (PEC) of the extensively perturbed B(1)1Π state of the KCs molecule constructed by applying an Inverted Perturbation Approach routine. The experimental term values in the energy range E(v′,J′)∈[14071;15502]cm−1 involved in the fit were based on Fourier-Transform spectroscopy data obtained with 0.01 cm−1 accuracy from laser-induced B(1)1Π→X1Σ+ fluorescence spectra in the present work (654 term values) combined with 520 term values from Birzniece et al. (2012) . The data set included vibrational v′∈[0,35] and rotational J′∈[7,233] quantum numbers covering about 85% of the potential well. The present fit reproduces the data included in the fit with a standard deviation 0.94 cm−1. The empirical PEC contains an inflection at about R=4.1 Å that reflects the avoided crossing of two Ω=1 states corresponding to B(1)1Π and c(2)3Σ+ states as predicted by ab initio calculations.