Infrared Spectroscopy of Solvation in Small Zn⁺(H₂O)_<i>n</i> Complexes

Singly charged zinc-water cations are produced in a pulsed supersonic expansion source using laser vaporization. Zn⁺(H₂O)_<i>n</i> (<i>n</i> = 1–4) complexes are mass selected and studied with infrared laser photodissociation spectroscopy, employing the method of argon tagging. Density functional theory (DFT) computations are used to obtain the structures and vibrational frequencies of these complexes and their isomers. Spectra in the O–H stretching region show sharp bands corresponding to the symmetric and asymmetric stretches, whose frequencies are lower than those in the isolated water molecule. Zn⁺(H₂O)_<i>n</i>Ar complexes with <i>n</i> = 1–3 have O–H stretches only in the higher frequency region, indicating direct coordination to the metal. The Zn⁺(H₂O)_2–4Ar complexes have multiple bands here, indicating the presence of multiple low energy isomers differing in the attachment position of argon. The Zn⁺(H₂O)₄Ar cluster uniquely exhibits a broad band in the hydrogen bonded stretch region, indicating the presence of a second sphere water molecule. The coordination of the Zn⁺(H₂O)_<i>n</i> complexes is therefore completed with three water molecules

Probing Methanol Cluster Growth by Vacuum Ultraviolet Ionization

The ability to probe the formation and growth of clusters is key to answering fundamental questions in solvation and nucleation phenomena. Here, we present a mass spectrometric study of methanol cluster dynamics to investigate these two major processes. The clusters are produced in a molecular beam and ionized by vacuum ultraviolet (VUV) radiation at intermediate distances between the nozzle and the skimmer sampling different regimes of the supersonic expansion. The resulting cluster distribution is studied by time-of-flight mass spectrometry. Experimental conditions are optimized to produce intermediate size protonated methanol and methanol–water clusters and mass spectra and photoionization onsets and obtained. These results demonstrate that intensity distributions vary significantly at various nozzle to ionization distances. Ion–molecule reactions closer to the nozzle tend to dominate leading to the formation of protonated species. The protonated trimer is found to be the most abundant ion at shorter distances because of a closed solvation shell, a larger photoionization cross section compared to the dimer, and an enhanced neutral tetramer precursor. On the other hand, the protonated dimer becomes the most abundant ion at farther distances because of low neutral density and an enhanced charged protonated monomer–neutral methanol interaction. Thomson’s liquid drop model is used to qualitatively explain the observed distributions

Probing Ionic Complexes of Ethylene and Acetylene with Vacuum-Ultraviolet Radiation

Mixed complexes of acetylene–ethylene are studied using vacuum-ultraviolet (VUV) photoionization mass spectrometry and theoretical calculations. These complexes are produced and ionized at different distances from the exit of a continuous nozzle followed by reflectron time-of-flight mass spectrometry detection. Acetylene, with a higher ionization energy (11.4 eV) than ethylene (10.6 eV), allows for tuning the VUV energy and initializing reactions either from a C₂H₂⁺ or a C₂H₄⁺ cation. Pure acetylene and ethylene expansions are separately carried out to compare, contrast, and hence identify products from the mixed expansion: these are C₃H₃⁺ (<i>m</i>/<i>z</i> = 39), C₄H₅⁺ (<i>m</i>/<i>z</i> = 53), and C₅H₅⁺ (<i>m</i>/<i>z</i> = 65). Intensity distributions of C₂H₂, C₂H₄, their dimers and reactions products are plotted as a function of ionization distance. These distributions suggest that association mechanisms play a crucial role in product formation closer to the nozzle. Photoionization efficiency (PIE) curves of the mixed complexes demonstrate rising edges closer to both ethylene and acetylene ionization energies. We use density functional theory (ωB97X-V/aug-cc-pVTZ) to study the structures of the neutral and ionized dimers, calculate their adiabatic and vertical ionization energies, as well as the energetics of different isomers on the potential energy surface (PES). Upon ionization, vibrationally excited clusters can use the extra energy to access different isomers on the PES. At farther ionization distances from the nozzle, where the number densities are lower, unimolecular decay is expected to be the dominant mechanism. We discuss the possible decay pathways from the different isomers on the PES and examine the ones that are energetically accessible

Understanding the Growth Mechanisms of Ag Nanoparticles Controlled by Plasmon-Induced Charge Transfers in Ag-TiO₂ Films

Mesoporous thin films of TiO₂ doped with silver can undergo spectacular microstructural modifications upon laser scanning at visible wavelengths through the excitation of a localized surface plasmon resonance in Ag nanoparticles (NPs). The latter can result in competitive physicochemical mechanisms, leading either to the shrinkage or to the growth of NPs depending on the exposure conditions. Contrary to intuition, we provide evidence that the speed of the laser scan controls the size of NPs as follows: low speeds lead to silver oxidation and a decrease in the NP size, whereas high speeds induce rapid temperature rises and a spectacular growth of NPs. Both regimes are separated by a speed threshold that depends on extrinsic and intrinsic parameters such as laser power, beam diameter, and initial size of Ag NPs. We propose here a comprehensive model based on a set of coupled differential equations describing the transformations of silver under laser excitation between the Ag⁰, Ag⁺, and metallic NP states, which provides a convincing physicochemical explanation of the experimental findings. This study constitutes a significant advance in the understanding of oxidation–reduction processes involved during laser exposure of metallic NPs and opens new directions to control their growth rate and their final size