Metal Transition in Sodium-Ammonia Nanodroplets.

The famous nonmetal-to-metal transition in Na-ammonia solutions is investigated in nanoscale solution droplets by photoelectron spectroscopy. In agreement with the bulk solutions, a strong indication for a transition to the metallic state is found at an average metal concentration of 8.8±2.2 mole%. The smallest entity for the phase transition to be observed consists of approximately 100-200 solvent molecules. The quantification of this critical entity size is a stepping stone toward a deeper understanding of these quantum-classical solutions through direct modeling at the molecular level.

Solvated Electrons in Clusters: Magic Numbers for the Photoelectron Anisotropy.

This paper reports on a curiosity concerning magic numbers in neutral molecular clusters, namely on magic numbers related to the photoelectron anisotropy in angle-resolved photoelectron spectra. With a combination of density functional calculations and experiment, we search for magic numbers in Na(H2O)n, Na(NH3)n, Na(CH3OH)n, and Na(CH3OCH3)n clusters. In clusters of high symmetry, the highest occupied molecular orbital can be delocalized over an extended region, forming a symmetric charge distribution of high s character, which results in a pronounced anisotropy in the photoelectron angular distribution. We find magic numbers at n = 6 and 4 for sodium-doped dimethyl ether and ammonia clusters, respectively, but not for sodium-doped water and methanol clusters, which is likely a consequence of the degree of hydrogen bonding and the number of structural isomers.

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 to be 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.

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. First, aerosol photoemission studies can be performed for many different materials, including liquids. Second, 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.

Barrierless proton transfer across weak CH⋯O hydrogen bonds in dimethyl ether dimer.

We present a combined computational and threshold photoelectron photoion coincidence study of two isotopologues of dimethyl ether, (DME - h6)n and (DME - d6)n n = 1 and 2, in the 9-14 eV photon energy range. Multiple isomers of neutral dimethyl ether dimer were considered, all of which may be present, and exhibited varying C-H⋯O interactions. Results from electronic structure calculations predict that all of them undergo barrierless proton transfer upon photoionization to the ground electronic state of the cation. In fact, all neutral isomers were found to relax to the same radical cation structure. The lowest energy dissociative photoionization channel of the dimer leads to CH3OHCH3 (+) by the loss of CH2OCH3 with a 0 K appearance energy of 9.71 ± 0.03 eV and 9.73 ± 0.03 eV for (DME - h6)2 and deuterated (DME - d6)2, respectively. The ground state threshold photoelectron spectrum band of the dimethyl ether dimer is broad and exhibits no vibrational structure. Dimerization results in a 350 meV decrease of the valence band appearance energy, a 140 meV decrease of the band maximum, thus an almost twofold increase in the ground state band width, compared with DME - d6 monomer.

Size-dependent velocity map photoelectron imaging of nanosized ammonia aerosol particles.

The size dependence of the band position, the bandwidth, and the asymmetry parameter in the valence photoelectron spectrum of ammonia clusters and ultrafine ammonia aerosol particles is investigated for aggregates with diameters up to ~7 nm. The neutral aggregates are ionized by single photons at 13.318 eV from a tabletop vacuum ultraviolet laser. The photoelectrons are detected in a velocity map imaging photoelectron spectrometer. The size distributions of the neutral aggregates are determined independently using the Na-doping method. Up to clusters with a few tens of molecules, the first ionization energy (value at band maximum) decreases, the width of the lowest band in the photoelectron spectrum broadens, and the corresponding asymmetry parameter decreases with increasing cluster size. Constant ("plateau") values of these parameters are found for larger aggregates, which might be explained by a predominantly noncrystalline structure of these aggregates with an average coordination similar to that of the liquid. This explanation is in agreement with previous infrared studies and theoretical predictions.

A velocity map imaging photoelectron spectrometer for the study of ultrafine aerosols with a table-top VUV laser and Na-doping for particle sizing applied to dimethyl ether condensation.

We present a new experimental configuration for the study of size-dependent, angle-resolved photoelectron and photoion spectra of weakly bound ultrafine aerosol particles targeted at particle sizes below ~20 nm. It combines single photon ionization by a tunable, table-top vacuum ultraviolet laser at energies up to 18 eV with velocity map imaging detection and independent size determination of the aerosol particles using the Na-doping method. As an example, the size-dependence of the valence photoelectron spectrum of dimethyl ether clusters and ultrafine aerosols is investigated. Up to a mean particle diameter of ~3-4 nm, the first ionization energy (value at band maximum) decreases systematically (up to ~1 eV) and the corresponding band broadens systematically (up to a factor of ~3) with increasing aggregate size. Plateau values for band positions and bandwidths are reached beyond a diameter of ~3-4 nm. Experimental evidence for the dominance of the fast intermolecular proton transfer over monomer fragmentation reactions upon ionization is presented via photoion imaging.