Luminescence Spectroscopy of Strontium and Barium atoms isolated in Low Temperature Solids – an experimental and theoretical study

This thesis presents an experimental and theoretical study of the alkaline earth atoms, strontium and barium, isolated in rare gas matrices (argon, krypton and xenon) at cryogenic temperatures. These solid-state M/RG (M = Sr and Ba; RG = Ar, Kr and Xe) samples are extensively characterised using time-integrated (steady-state) and time-resolved spectroscopic techniques. The primary goal of this work is to develop the spectroscopy of matrix-isolated Group II metals and augment the ongoing research of the Maynooth Group, which focuses on S metal atom – rare gas systems. Moreover, the literature on the spectroscopy of matrix-isolated Sr or Ba is quite sparse and the limited number of reports that do exist contain inaccuracies, which are in need of clarification and amendment. M/RG samples deposited at 10 K, under conditions of low metal loading, consist of well-isolated metals atoms. Conversely, matrices formed at elevated temperature contain a higher yield of metal clusters. Absorption spectra recorded of M/RG solids condensed onto a ‘warm’ sample window reveal the presence of additional non-atomic features – these are attributed to metal dimers based on the low metal fluxes employed during deposition. Further characterisation of the Group II diatomics was accomplished using steady-state and time-resolved luminescence spectroscopy. The luminescence produced with excitation of the metal dimer absorption bands reveals very different behaviour for Sr2/RG and Ba2/RG. Resonance fluorescence with a lifetime of 15 ns is observed for the lowest energy transition of Sr2. In contrast, the lowest energy transition of Ba2 is completely quenched – this behaviour is consistent with the absence of vibrational structure on the near-IR absorption band of Ba2, indicating that a lifetime broadening effect exists on this molecular transition. Moreover, the excitation/emission features identified in the visible spectral region (580 – 680 nm) for matrix-isolated Ba2, likely originate from molecular states of the dimer, which dissociate to atomic asymptotes existing above the (6s6p) 1P1 level of the atom. The (nsnp) 1P ← (ns2) 1S resonance transitions, which occur in the visible spectral region, are used to compare the isolation conditions of these two metal atom systems. Complex and structured absorption bands were recorded for both Sr and Ba in all three hosts, even after extensive sample annealing. Thus, these alkaline earth atoms occupy a myriad of thermally stable sites in solid Ar, Kr and Xe matrices. As a result, each M/RG solid exhibits numerous (nsnp) 1P emission bands and two dimensional – excitation and emission (2D-EE) spectra are required to fully unravel the luminescence. The excitation spectra extracted from the 2D-EE scans allow the complex absorption bands to be resolved into site-specific features. Both metal atom systems display a variety of excitation patterns, ranging from the classic Jahn-Teller threefold split, to asymmetric threefold (2+1) patterns and broad, unstructured bands. It is therefore concluded that the isolation of Sr and Ba atoms in rare gas solids occurs in cubo-octahedral, crystalline (fcc) sites on one hand and in non-crystalline sites on the other. To assist the interpretation of the site occupancy, novel ground state potential energy curves are computed for the 1:1 M·RG complexes (M=Ca, Sr, Ba and RG = Ar, Kr, Xe), using the empirical Tang-Toennies (TT) and ab initio CCSD(T) methods. The spectroscopic constants obtained from both approaches were in agreement. Long bond lengths (> 5 Å) and shallow bound regions (