Low-energy photoelectron transmission through aerosol overlayers

The transmission of low-energy (<1.8eV) photoelectrons through the shell of core-shell aerosol particles is studied for liquid squalane, squalene, and DEHS shells. The photoelectrons are exclusively formed in the core of the particles by two-photon ionization. The total photoelectron yield recorded as a function of shell thickness (1-80nm) shows a bi-exponential attenuation. For all substances, the damping parameter for shell thicknesses below 15nm lies between 8 and 9nm, and is tentatively assigned to the electron attenuation length at electron kinetic energies of ~0.5-1eV. The significantly larger damping parameters for thick shells (> 20nm) are presumably a consequence of distorted core-shell structures. A first comparison of aerosol and traditional thin film overlayer methods is provided

Electron mean free path from angle-dependent photoelectron spectroscopy of aerosol particles

We propose angle-resolved photoelectron spectroscopy of aerosol particles as an alternative way to determine the electron mean free path of low energy electrons in solid and liquid materials. The mean free path is obtained from fits of simulated photoemission images to experimental ones over a broad range of different aerosol particle sizes. The principal advantage of the aerosol approach is twofold. Firstly, aerosol photoemission studies can be performed for many different materials, including liquids. Secondly, the size-dependent anisotropy of the photoelectrons can be exploited in addition to size-dependent changes in their kinetic energy. These finite size effects depend in different ways on the mean free path and thus provide more information on the mean free path than corresponding liquid jet, thin film, or bulk data. The present contribution is a proof of principle employing a simple model for the photoemission of electrons and preliminary experimental data for potassium chloride aerosol particles

Angle-Resolved Photoemission of Solvated Electrons in Sodium-Doped Clusters

Angle-resolved photoelectron spectroscopy of the unpaired electron in sodium-doped water, methanol, ammonia, and dimethyl ether clusters is presented. The experimental observations and the complementary calculations are consistent with surface electrons for the cluster size range studied. Evidence against internally solvated electrons is provided by the photoelectron angular distribution. The trends in the ionization energies seem mainly determined by the degree of hydrogen bonding in the solvent and the solvation of the ion core. The onset ionization energies of water and methanol clusters do not level off at small cluster sizes, but decrease slightly with increasing cluster size

Magic Numbers for the Photoelectron Anisotropy in Li-Doped Dimethyl Ether Clusters

Photoelectron velocity map imaging of Li(CH$_3$OCH$_3$)$_n$ clusters (1 $\leq$ n $\leq$ 175) is used to search for magic numbers related to the photoelectron anisotropy. Comparison with density functional calculations reveals magic numbers at n=4, 5, and 6, resulting from the symmetric charge distribution with high s-character of the highest occupied molecular orbital. Since each of these three cluster sizes correspond to the completion of a first coordination shell, they can be considered as 'isomeric motifs of the first coordination shell'. Differences in the photoelectron anisotropy, the vertical ionization energies and the enthalpies of vaporization between Li(CH$_3$OCH$_3$)$_n$ and Na(CH$_3$OCH$_3$)$_n$ can be rationalized in terms of differences in their solvation shells, atomic ionization energies, polarizabilities, metal-oxygen bonds, ligand-ligand interactions, and by cooperative effects

Below band gap formation of solvated electrons in neutral water clusters?

Below band gap formation of solvated electrons in neutral water clusters using pump-probe photoelectron imaging is compared with recent data for liquid water and with above band gap excitation studies in the liquid and clusters. Similar relaxation times in the order of 200 fs and 1-2 ps are retrieved for below and above band gap excitation, in both clusters and liquid. The relaxation times independence from the generation process indicates that these times are dominated by the solvent response, which is significantly slower than the different solvated electron formation processes. The analysis of the temporal evolution of the vertical electron binding energy and the electron binding energy at half maximum suggests a dependence of the solvation time on the binding energy

Can we Observe Gas Phase Nucleation at the Molecular Level?

We propose and discuss an experiment for the study of neutral gas phase nucleation on a molecular level using propane as the condensable gas. The experiment combines a uniform Laval expansion with soft mass spectrometric detection. The uniform Laval expansion allows nucleation experiments under well-defined conditions while the mass spectrometric detection provides molecular-level information on the molecular aggregates formed. It is discussed how one could observe the onset of nucleation and retrieve the size of the critical nucleus from the mass spectra

New Particle Formation from the Vapor Phase : From Barrier-Controlled Nucleation to the Collisional Limit

Studies of vapor phase nucleation have largely been restricted to one of two limiting cases—nucleation controlled by a substantial free energy barrier or the collisional limit where the barrier is negligible. For weakly bound systems, exploring the transition between these regimes has been an experimental challenge, and how nucleation evolves in this transition remains an open question. We overcome these limitations by combining complementary Laval expansion experiments, providing new particle formation data for carbon dioxide over a uniquely broad range of conditions. Our experimental data together with a kinetic model using rate constants from high-level quantum chemical calculations provide a comprehensive picture of new particle formation as nucleation transitions from a barrier-dominated process to the collisional limit.Peer reviewe

EMSL Science Theme Advisory Panel Workshop - Atmospheric Aerosol Chemistry, Climate Change, and Air Quality

This report contains the workshop scope and recommendations from the workshop attendees in identifying scientific gaps in new particle formation, growth and properties of particles and reactions in and on particles as well as the laboratory-focused capabilities, field-deployable capabilities and modeling/theory tools along with linking of models to fundamental data