Exciplexes with Ionic Dopants: Stability, Structure, and Experimental Relevance of M⁺(²P)⁴He_<i>n</i> (M = Sr, Ba)

Abstract

M⁺(²P)⁴He_<i>n</i> species, possibly involved in the post ²P ← ²S excitation dynamics of Sr⁺ and Ba⁺ in cold ⁴He gas or droplets, are studied employing both high level <i>ab initio</i> calculations to determine the potential energy curves (PEC) and diffusion Monte Carlo (DMC) to obtain information on their ground state structure and energetics. PEC for the excited M⁺(²P)He dimers were obtained using MRCI calculations with extended basis sets. Potential energy surfaces (PES) for M⁺(²P)­He_<i>n</i> were built with the DIM model including spin–orbit coupling via a perturbative procedure. DMC simulations indicated several exciplexes (<i>n</i> > 2) to be stable against He dissociation whatever the ion state, a finding that is at variance with what was previously suggested for Ba⁺(²P_1/2) due to the repulsive nature of the interaction potential obtained in [Phys. Rev. A 2004, 69, 042505]. Our results, instead, support the suggestion made in [J. Chem. Phys. 2012, 137, 051102] for the existence of Ba⁺(²P_1/2)­He_<i>n</i> exciplexes emitted following the excitation of the barium cation solvated into He droplets. In the ²P_1/2 state, the Ba ion also shows a peculiar behavior as a function of <i>n</i> with respect to the location and binding strength of the attached He atoms compared to Sr⁺. Although the latter forms the usual equatorial He ring, Ba⁺ deviates from this geometry for 1 ≤ <i>n</i> ≤ 4, with the radial distribution functions strongly depending on the number of solvent atoms. Finally, a putative species is proposed to explain the emission band at 523 nm that follows D1 or D2 excitation of Ba⁺ in superfluid bulk helium