Rydberg states of the CdAr van der Waals complex

Abstract

A theoretical study of the relatively unexplored area of low-lying Rydberg states of van der Waals molecules (vdW) in the particular case of the CdAr system is presented. The fully ab initio calculation of the potential energy curves (PECs) of the excited vdW complex CdAr above the $(5s6s)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}3}S_{1},^{\phantom{\rule{0.16em}{0ex}}1}S_{0}$ atomic asymptotes, reaching the $(5s6p)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}3}P_{0,1,2}, (5s5d)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}1}D_{2}{,}^{3}{D}_{1,2,3}, (5s6p)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}1}P_{1}$, and $(5s7s)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}3}S_{1},^{\phantom{\rule{0.16em}{0ex}}1}S_{0}$, is performed. All the calculated PECs of the Rydberg states of $\mathrm{\ensuremath{\Sigma}}$ symmetry exhibit undulations resulting in the double-well character that was already observed experimentally in the case of the lowest Rydberg states of various 12 group metal--rare gas vdW complexes. The outer wells obtained within the present work are shallow with depths not exceeding 20 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the atomic asymptote and reaching equilibrium distance of roughly 27 bohr in the case of states correlating with the $(5s7s)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}3}S_{1},^{\phantom{\rule{0.16em}{0ex}}1}S_{0}$ asymptotes. The largest calculated values of permanent electric dipoles at equilibrium distances exceed 10 D in the case of ${}^{1}\mathrm{\ensuremath{\Sigma}}$ Rydberg states correlating with $(5s6p)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}1}P_{1}$ and $(5s5d)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}1}D_{2}$ atomic asymptotes. The calculations were performed within the second-order multireference perturbation theory. The performance of this particular theoretical ab initio approach is discussed in the context of the considered type of systems and compared, wherever it was possible, with the benchmark results of the coupled-cluster method obtained in this work, with other ab initio calculations based on multireference perturbation theory and experimental data.