Download 2D-EE spectroscopy of heavier alkaline earth metals

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)