Experimental studies of the structural, dynamical and electronic properties of clusters and nanoparticles

Abstract In this thesis, properties of freestanding atomic/molecular clusters are experimentally addressed using various synchrotron radiation and electron ionization based electron and ion spectroscopic methods. The work focuses on water clusters with and without dopant atoms/molecules, including ≲3nm sized [H₂O]N, [H₂O]N[KCl]M, [H₂O]NArM and ArN clusters. The surface properties of larger 100–200nm sized MgCl₂/CaCl₂ and MgBr₂/NaBr nanoparticles generated from precursor solution droplets are also investigated. The experiments provide fundamental level information on the particles’ chemical and structural composition in relation to their formation conditions, electronic properties, and dynamical response to ionizing radiation. Instrumentation was also developed for the generation and characterization of the clusters

Efficient neutralization of core ionized species in an aqueous environment

Abstract Core ionization dynamics of argon–water heteroclusters ArM[H₂O]N are investigated using a site and process selective experimental scheme combining 3 keV electron irradiation with Auger electron–ion–ion multi-coincidence detection. The formation of Ar 2p⁻¹ vacancies followed by non-radiative decay to intermediate one-site doubly ionized states Ar²⁺(3p⁻²)−ArM−1[H₂O]N and subsequent redistribution of charge to the cluster environment are monitored. At low argon concentrations the emission of an [H₂O]n′H⁺/[H₂O]n′′H⁺ ion pair is the dominant outcome, implying on high efficiency of charge transfer to the water network. Increasing the condensation fraction of argon in the mixed clusters and/or to pure argon clusters is reflected as a growing yield of Arm′+/Arm′′+ ion pairs, providing a fingerprint of the precursor heterocluster beam composition. The coincident Auger electron spectra, resolved with better than 1 eV resolution, show only subtle differences and thereby reflect the local nature of the initial Auger decay step. The results lead to better understanding of inner shell ionization processes in heterogeneous clusters and in aqueous environments in general

Solvent and cosolute dependence of Mg surface enrichment in submicron aerosol particles

Abstract The formation of multicomponent aerosol particles from precursor solution droplets often involves segregation and surface enrichment of the different solutes, resulting in non-homogeneous particle structures and diverse morphologies. In particular, these effects can have a significant influence on the chemical composition of the particle–vapor interface. In this work, we investigate the bulk/surface partitioning of inorganic ions, Na⁺, Mg² ⁺, Ca² ⁺, Cl⁻ and Br⁻, in atomiser-generated submicron aerosols using synchrotron radiation based X-ray photoelectron spectroscopy (XPS). Specifically, the chemical compositions of the outermost few nm thick surface layers of non-supported MgCl₂/CaCl₂ and NaBr/MgBr₂ particles are determined. It is found that in MgCl₂/CaCl₂ particles, the relative abundance of the two species in the particle surface correlates well with their mixing ratio in the parent aqueous solution. In stark contrast, extreme surface enrichment of Mg² ⁺ is observed in NaBr/MgBr₂ particles formed from both aqueous and organic solution droplets, indicative of core–shell structures. Structural properties and hydration state of the particles are discussed

Electron–ion coincidence spectroscopy of a large organic molecule:photofragmentation of avobenzone after valence and core ionisation

Abstract The Avobenzone (AVOB) molecule is very photoactive and undergoes irreversible degradation upon irradiation. We studied its valence and core-level (C1s and O1s) photoionisation and subsequent photofragmentation with photoelectron spectroscopy and photoelectron–photoion–photoion coincidence (PEPIPICO) spectroscopy. AVOB is one of the largest molecules studied with this technique. The results show that the AVOB molecule dissociates into an extensive range of fragments by different pathways with little element or site-selectivity. The coincident maps were used to determine selected fragment separation sequences by analysing the slopes of patterns from ion pairs after the core ionisation. Charge delocalisation over the benzene rings and their relative stability favor fragmentation by cleavage of the bridge between them

Core and valence level photoelectron spectroscopy of nanosolvated KCl

Abstract The solvation of alkali and halide ions in the aqueous environment has been a subject of intense experimental and theoretical research with multidisciplinary interests; yet, a comprehensive molecular-level understanding has still not been obtained. In recent years, electron spectroscopy has been increasingly applied to study the electronic and structural properties of aqueous ions with implications, especially in atmospheric chemistry. In this work, we report core and valence level (Cl 2p, Cl 3p, and K 3p) photoelectron spectra of the common alkali halide, KCl, doped in gas-phase water clusters in the size range of a few hundred water molecules. The results indicate that the electronic structure of these nanosolutions shows a distinct character from that observed at the liquid–vapor interface in liquid microjets and ambient pressure setups. Insights are provided into the unique solvation properties of ions in a nanoaqueous environment, emerging properties of bulk electrolyte solutions with growing cluster size, and sensitivity of the electronic structure to varying solvation configurations

Forming bonds while breaking old ones:isomer-dependent formation of H₃O⁺ from aminobenzoic acid during X-ray-induced fragmentation

Abstract Intramolecular hydrogen transfer, a reaction where donor and acceptor sites of a hydrogen atom are part of the same molecule, is a ubiquitous reaction in biochemistry and organic synthesis. In this work, we report hydronium ion (H₃O⁺) production from aminobenzoic acid (ABA) after core-level ionization with soft X-ray synchrotron radiation. The formation of H₃O⁺ during the fragmentation requires that at least two hydrogen atoms migrate to one of the oxygen atoms within the molecule. The comparison of two structural isomers, ortho- and meta-ABA, revealed that the production of H₃O⁺ depends strongly on the structure of the molecule, the ortho-isomer being much more prone to produce H₃O⁺. The isomer-dependency suggests that the amine group acts as a donor in the hydrogen transfer process. In the case of ortho-ABA, detailed H₃O⁺ production pathways were investigated using photoelectron-photoion-photoion coincidence (PEPIPICO) spectroscopy. It was found that H₃O⁺ can result from a direct two-body dissociation but also from sequential fragmentation processes

The effect of relative humidity on CaCl₂ nanoparticles studied by soft X-ray absorption spectroscopy

Abstract Ca- and Cl-containing nanoparticles are common in atmosphere, originating for example from desert dust and sea water. The properties and effects on atmospheric processes of these aerosol particles depend on the relative humidity (RH) as they are often both hygroscopic and deliquescent. We present here a study of surface structure of free-flying CaCl₂ nanoparticles (CaCl₂-NPs) in the 100 nm size regime prepared at different humidity levels (RH: 11–85%). We also created mixed nanoparticles by aerosolizing a solution of CaCl₂ and phenylalanine (Phe), which is a hydrophobic amino acid present in atmosphere. Information of hydration state of CaCl₂-NPs and production of mixed CaCl₂ + Phe nanoparticles was obtained using soft X-ray absorption spectroscopy (XAS) at Ca 2p, Cl 2p, C 1s, and O 1s edges. We also report Ca 2p and Cl 2p X-ray absorption spectra of an aqueous CaCl₂ solution. The O 1s X-ray absorption spectra measured from hydrated CaCl2-NPs resemble liquid-like water spectrum, which is heavily influenced by the presence of ions. Core level spectra of Ca²⁺ and Cl⁻ ions do not show a clear dependence of % RH, indicating that the first coordination shell remains similar in all measured hydrated CaCl₂-NPs, but they differ from aqueous solution and solid CaCl₂

Search for the interatomic Auger effect in nitrous oxide

Abstract The interatomic Auger effect following O 1s ionization in N₂O has been experimentally investigated using multi-electron coincidence spectroscopy. The expected transition energies have been established by comparison to the measured N 1s⁻¹v⁻¹ core-valence double ionization energies. We describe a procedure to eliminate the background of two competing processes contributing spectroscopic signatures to the same energy range, namely double Auger decay of the O 1s vacancy and direct single-photon double ionization into the N 1s⁻¹v⁻¹ states. While the interatomic Auger transitions could not be successfully isolated, we provide an upper boundary of the transition probability of 0.07% with respect to the dominant single Auger decay after O 1s ionization

Depth profiling of the chemical composition of free-standing carbon dots using X-ray photoelectron spectroscopy

Abstract The chemical and geometrical structure of free-standing carbon dots (Cdots) prepared from the pyrolysis of N-hydroxysuccinimide (NHS) have been characterized using X-ray photoelectron spectroscopy (XPS). An aerodynamic lens system was used to generate a sufficient particle density of monodispersed Cdots for XPS studies at the PLEIADES beamline at the SOLEIL synchrotron facility. Varying the X-ray excitation energy between 315 and 755 eV allows probing of the Cdots from the surface toward their core, owing to the kinetic energy dependence of the photoelectron inelastic mean free path. The C 1s, O 1s, and N 1s core-levels were recorded with high-spectral resolution to identify their main chemical components and branching ratios. While high-resolution transmission electron microscopy (HRTEM) reveals a defective graphitic core, the C 1s spectrum evidence two main peaks similar to those measured from the solid NHS. Their relative abundance as a function of the probing depth is strongly related to the chemical composition of the ligand shell that does not vary substantially over the first 3.4 nm. Combining the depth-resolved XPS and HRTEM studies, it was concluded that the Cdots possess a graphitic core surrounded by a relatively homogeneous shell of at least 3.4 nm thickness with a composition similar to that of the solid NHS