Exciplexes with Ionic Dopants: Stability, Structure, and Experimental Relevance of M+(2P)4Hen (M = Sr, Ba)

M+(P-2)He-4(n) species, possibly involved in the post P-2 <- S-2 excitation dynamics of Sr+ and Ba+ in cold He-4 gas or droplets, are studied employing both high level ab initio 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+(P-2)He dimers were obtained using MRCI calculations with extended basis sets. Potential energy surfaces (PES) for M+(P-2)He-n were built with the DIM model including spin-orbit coupling via a perturbative procedure. DMC simulations indicated several exciplexes (n > 2) to be stable against He dissociation whatever the ion state, a finding that is at variance with what was previously suggested for Ba(P-2(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+(P-2(1/2))He-n exciplexes emitted following the excitation of the barium cation solvated into He droplets. In the P-2(1/2) state, the Ba ion also shows a peculiar behavior as a function of n 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 <= n <= 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.