4 research outputs found
Infrared Spectroscopy of Solvation in Small Zn<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> Complexes
Singly charged zinc-water cations
are produced in a pulsed supersonic
expansion source using laser vaporization. Zn<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> (<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<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>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<sup>+</sup>(H<sub>2</sub>O)<sub>2–4</sub>Ar complexes have multiple bands here, indicating
the presence of multiple low energy isomers differing in the attachment
position of argon. The Zn<sup>+</sup>(H<sub>2</sub>O)<sub>4</sub>Ar
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<sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> 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
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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 C<sub>2</sub>H<sub>2</sub><sup>+</sup> or a C<sub>2</sub>H<sub>4</sub><sup>+</sup> cation. Pure acetylene and ethylene expansions are separately carried
out to compare, contrast, and hence identify products from the mixed
expansion: these are C<sub>3</sub>H<sub>3</sub><sup>+</sup> (<i>m</i>/<i>z</i> = 39), C<sub>4</sub>H<sub>5</sub><sup>+</sup> (<i>m</i>/<i>z</i> = 53), and C<sub>5</sub>H<sub>5</sub><sup>+</sup> (<i>m</i>/<i>z</i> =
65). Intensity distributions of C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>H<sub>4</sub>, 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-TiO<sub>2</sub> Films
Mesoporous thin films of TiO<sub>2</sub> 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 Ag<sup>0</sup>, Ag<sup>+</sup>, 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