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Author
Date
2023Type
- Doctoral Thesis
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Abstract
The understanding of the solvation of electrons and their degree of delocalisation in that state have been of fundamental interest ever since first discovered in alkali metal ammonia solutions. Neutral, alkali metal-doped clusters serve as ideal model systems to study solvated electrons in their size-dependent properties. The presented work focuses on the characterisation of electronic properties via angle-resolved photoelectron spectroscopy and of their magnetic properties by Stern-Gerlach deflection. The combination of both experimantal techniques reveals information on the solvated electron binding energy, electron orbital and spin angular momenta as well as the cluster rotational angular momentum.
We report measurements of electron binding energies and photoelectron anisotropies in angle-dependent photoelectron spectra of lithium-doped dimethyl ether clusters as a function of cluster size distribution. Neutral singly doped lithium clusters are studied in the range from bare lithium atom to an average cluster size of 63 dimethyl ether molecules. We explain trends in electron binding energies and photoemission anisotropies on the bases of density functional calculations of LiCH3OCH3 clusters. The results of lithium-doped clusters are compared to a previous study performed in our group on NaCH3OCH3 clusters. The comparison reveals similar trends of electron binding energies and photoemission anisotropies with increasing cluster size. Yet, structural and electronic differences arise from the alkali metal substitution. In highly symmetric clusters, the highest occupied molecular orbital can delocalize over an extended region and form a symmetric charge distribution of mainly s-character, resulting in a pronounced photoemission anisotropy. The photoelectron angular distributions of Li(CH3OCH3)n reveal pronounced s-character at n=4, 5 and 6. These cluster sizes are refered to as magic clusters for the photoelectron anisotropy.
Photoelectron studies of cluster size distributions are limited in their details of cluster size-resolved photoelectron contributions. Size-resolved measurements of electron binding energies and photoemission anisotropies of neutral Na(NH3)n clusters are achieved with photoelectron-photoion-coincidence-spectroscopy, for photon energies in the range of 5.5 eV - 9.9 eV. Photoelectron kinetic energy spectra in combination with ab initio calculations show three distinct ionisation pathways of the single solvated electron, each of the pathways is dominant in a certain photon energy range. One of the ionisation pathways occurs via an autoionisation process after resonant excitation of the neutral state. The excitation process is proposed to be characterised by an electron transfer of an NH3 molecule lone pair to the singly occupied molecular orbital. This excited state is characterised by a doubly occupied orbital, the so called solvated dielectron. The excited, solvated dielectron subsequently decays by electron transfer processes with the emission of low kinetic energy electrons.
Besides studying the electronic states of the clusters, we exploit their magnetic character due to the unpaired electron in magnetic deflection experiments on molecular beams. The paramagnetic propeties of sodium-doped clusters with ammonia, water, methanol and dimethyl ether for cluster sizes n=1-4 are characterised by their deflection behaviour when traversing a magnetic field gradient. The spatial deflection is compared to molecular dynamics simulations based on the Zeeman interaction of a spin 1/2 system. The comparison of experiment and simulation reveals unperturbed magnetic properties of a spin 1/2 system for the smallest clusters NaNH3 and NaH2O. All larger clusters, in contrast show reduced deflection compared to simulations of a spin 1/2 system. Deviations from a spin 1/2 deflection behavior are attributed to a reduction of magnetic moment due to intracluster spin-relaxation processes. Determining effective magnetic moments for these clusters allows us to identify trends in their paramagnetic properties. The observed trends are discussed in terms of spin and rotational angular momentum interactions of the thermally accessible rovibrational density of states. We used density functional methods to determine the lowest energy cluster geometries and the rovibrational states with the harmonic oscillator and rigid rotor approximation. With this approach we find that the dominant mechanism of intracluster spin-relaxation processes in sodium-doped clusters is governed by the thermal population of spin-rotational coupled Zeeman states. Show more
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https://doi.org/10.3929/ethz-b-000612980Publication status
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Publisher
ETH ZurichSubject
Cluster chemistry; solvated electronsOrganisational unit
03961 - Signorell, Ruth / Signorell, Ruth
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Is cited by: https://doi.org/10.3929/ethz-b-000336606
References: https://doi.org/10.3929/ethz-b-000457854
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