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2D-EE spectroscopy of heavier alkaline earth metals atoms Ba, Sr and Ca in rare gas matrices – CCSD(T) calculations and atomic site occupancies Barry M. Davis and John G. McCaffrey Department of Chemistry, Maynooth University, National University of Ireland – Maynooth, County Kildare, Ireland A two dimensional excitation-emission (2D-EE) spectroscopic investigation of Sr/RG and Ba/RG solids (RG = Ar, Kr and Xe) has revealed [1] the presence of these heavy alkaline earth atoms in unusual sites in the solid rare gases. Sr, identified as an impurity in the barium metal used, provided the initial impetus to examine the lighter alkaline earth atom in addition to barium and calcium. An overview of the UV/Vis absorption and luminescence, recorded for well-isolated barium and strontium atoms, is presented in this study. Both of these metal atoms exhibit complex absorption profiles even after extensive annealing, behaviour that arises from their occupancy in multiple sites of isolation. 2D-EE luminescence plots are used to identify the sample conditions required to extract all the site-specific emission features. Selected excitation scans reveal exotic band-profiles indicative of the occupancy of some atoms in cylindrical sites. Contrary to a recent report [2], no evidence is found for the existence of barium cations [3] in any of the RG solids. To advance the discussion of the possible sites occupied in the three M/RG matrix systems, the ground state potential energy curves were computed for the M∙RG diatomics at the coupled-cluster level of theory and are compared in the figure below. M-Ar M-Kr M-Xe A comparison of the three rare gas (RG2) dimer RG potentials (black curves) Ca Sr with those calculated Ba 200 with the coupled-cluster method for the nine M·RG diatomics (curves 100 in colour). The RG2 potentials were generated with Morse functions. 0 The large mismatch in the M·RG and RG2 potentials is immediately evident in -100 all three rare gases systems but especially so sv tv hv co in Ar. The grey vertical -200 lines show the sizes of the 4 6 8 4 6 8 4 6 8 cubic sites in the solid rare gases considered to R (Å) accommodate a spherical guest atom. The energies of the M·RG interactions at these sites have been extracted and will be discussed. It is only in the case of Ca-Xe that a possibility exists for single vacancy (SV) site occupancy, which has a very slight (64 cm-1) repulsive interaction. For all the other M/RG systems multi-vacancy (TV, HV and CO) sites would be expected to occur. 300 -1 Energy (cm ) 2 [1] Davis B.M. & McCaffrey J.G., J. Chem. Phys 144(5):044308 (2016). [2] Fairbank W et al. E. X. O. C. project, Phys. Rev. A 91, 022505 (2015). [3] Byrne O., Davis B.M. & McCaffrey J.G. J. Chem. Phys; 142(5):054307 (2015)