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Spectroscopy and Photochemistry of Europium atoms in low temperature solids - an experimental and theoretical study

Byrne, Owen (2010) Spectroscopy and Photochemistry of Europium atoms in low temperature solids - an experimental and theoretical study. PhD thesis, National University of Ireland Maynooth.

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Abstract

This thesis presents an experimental and theoretical study of the luminescence, reactivity and ionisation of atomic europium isolated in cryogenic thin films of rare gases argon, krypton and xenon. Many studies are available concerning the spectroscopy of matrix-isolated main group metal atoms, however, the lanthanide series remains relatively undocumented. A thorough investigation of matrix-isolated europium is performed in this thesis in an effort to develop the spectroscopy of the fblock metal atom elements. Absorption spectroscopy identifies the visible s - p type y 8P - a 8S and the UV f - d type 8P - a 8S electronic transitions of atomic Eu. The absorption spectra are complicated showing numerous features due to multiple guest site occupancy in the host lattices upon deposition. Annealing simplifies spectra, removing one thermally unstable site of isolation in each RG. Site-specific excitation spectra allow further deconvolution of complicated absorption spectra and the identification of two lattice vacancy types occupied by the metal guest. The spherical nature of the [Xe]4f76s2 ground state of europium coupled with predicted Eu-RG diatomic bond lengths, allows attribution of these occupied sites to hexa-vacancies and tetra-vacancies. The complex visible y 8P state luminescence was probed using steady-state and time-resolved methods. Multiple emission features are identified with y 8P state excitation. Specific excitation in each particular site simplified the emission profiles yielding four emission bands. Based on their spectral location and temporal characteristics the observed emission features are assigned as y 8P resonance fluorescence, emission from the z 6P and a 8D excited states to the ground state and emission of the metastable a 10D electronic state. The solid state lifetimes of the a 8D and a 10D states are the first ever reported for these states. Excitation spectra in the region of the z 6P state identify the z 6P5/2, and z 6P7/2 levels. The z 6P3/2 level is not identified in accordance with the J = 0, ±1 electric-dipole selection rule. The effect of each hosts increased capability for distortion is obvious in the excitation spectra recorded in this region. The z 6P5/2 excitation band is observed to progress from a structureless feature in solid Ar to a resolved triplet in the Xe lattice. The greater capability for distortion of the Xe host causing Jahn-Teller threefold splitting of this state. The effects of y 8P state laser irradiation on the Eu/RG samples are then investigated. The atomic absorption features are completely removed by this process and replaced with multiple new higher energy absorptions. Based on their spectral location, intensity distribution and lifetime characteristics, these new features are attributed to singly ionised europium formed in the lattice during irradiation. The ionisation shows a strong power dependence suggesting the ion is formed via a multiphoton process. Annealing reforms the neutral species implying the free ionised electron is stabilised in the host lattice. The visible luminescence of the states of 4f7(8S°7/2)6p1/2 and 4f7(8S°7/2)6p3/2 configuration of Eu+/RG are investigated using time-resolved and steady-state techniques. Observed emission features are attributed to relaxation from particular electronic states of the ion. Absorption spectroscopy identifies two lattice trapping sites occupied in Ar and Kr and only one site in solid Xe. The site occupied in all three hosts is attributed to the interstitial octahedral site based on comparison to Eu+-RG predicted diatomic bond lengths. The additional site present in Ar and Kr results from occupancy of a single vacancy. Atomic Eu isolated in argon matrices doped with dinitrogen or carbon monoxide undergoes photochemistry during lamp irradiation. UV/Vis absorption spectra identify atomic isolation upon deposition. Following y 8P state irradiation all atomic features are completely removed. Infrared absorption spectra in the region of the N-N and C-O stretches show ligand complexation with the metal centre occurs. A vibrational frequency analysis of the Eu:N2 and Eu:CO complexes is performed using the hybrid B3LYP, the DFT BP86 and the post-Hartree-Fock MP2 methods. The appropriateness of each functional at describing lanthanide metal chemistry is compared. BP86 is found to be the most accurate. A vibrational frequency analysis at the BP86 level allows identification of the binary europium dinitrogen complexes in both the side-on and end-on orientation in low ligand concentration samples in solid argon. The side-on species is calculated to be in the region of 900 cm-1 lower in energy. The mono-ligated carbon bound Eu(CO) species is observed in samples of 0.1 % CO in Ar. Analysis of the bonding molecular orbitals shows europium in a high-spin [Xe]4f75d16s1 state is involved. Metal-ligand interactions occur via a classic synergistic donation/ back-bonding scheme. The high-spin state of europium is occupied as it reduces repulsion by removal of an s electron and increases the metal’s ability for back-bonding by promotion of an electron to a d-orbital allowing a greater degree of ligand co-ordination.

Item Type: Thesis (PhD)
Keywords: Spectroscopy; Photochemistry; Europium atoms; low temperature solids;
Subjects: Science & Engineering > Chemistry
Item ID: 2439
Depositing User: IR eTheses
Date Deposited: 14 Feb 2011 12:24
URI:

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