Martin J. Iedema

Direct observation of completely processed calcium carbonate dust particles

This study presents, for the first time, field evidence of complete, irreversible processing of solid calcium carbonate (calcite)-containing particles and quantitative formation of liquid calcium nitrate particles apparently as a result of heterogeneous reaction of calcium carbonate-containing mineral dust particles with gaseous nitric acid. Formation of nitrates from individual calcite and sea salt particles was followed as a function of time in aerosol samples collected at Shoresh, Israel. Morphology and compositional changes of individual particles were observed using conventional scanning electron microscopy with energy dispersive analysis of X-rays (SEM/EDX) and computer controlled SEM/EDX. Environmental scanning electron microscopy (ESEM) was utilized to determine and demonstrate the hygroscopic behavior of calcium nitrate particles found in some of the samples. Calcium nitrate particles are exceptionally hygroscopic and deliquesce even at very low relative humidity (RH) of 9-11% which is lower than typical atmospheric environments. Transformation of non-hygroscopic dry mineral dust particles into hygroscopic wet aerosol may have substantial impacts on light scattering properties, the ability to modify clouds and heterogeneous chemistry.

Immobility of protons in ice from 30 to 190 K

The anomalously fast motion of hydronium ions (H3O+) in water is often attributed to the Grotthuss mechanism, whereby protons tunnel from one water molecule to the next. This tunnelling is relevant to proton motion through water in restricted geometries, such as in ‘proton wires’ in proteins and in stratospheric ice particles. Transport of hydronium ions in ice is thought to be closely related to its transport in water,. But whereas claims have been made that such tunnelling can persist even at 0 K in ice, counter-claims suggest that the activation energy for hydronium motion in ice is non-zero. Here we use ‘soft-landing’ of hydronium ions on the surface of ice to show that the ions do not seem to move at all at temperatures below 190 K. This implies not only that hydronium motion is an activated process, but also that it does not occur at anything like the rate expected from the Grotthuss mechanism. We also observe the motion of an important kind of defect in ices hydrogen-bonded structure (the D defect). Extrapolation of our measurements to 0 K indicates that the defect is still mobile at this temperature, in an electric field of 1.6 × 108 V m−1.

Time-Resolved Aerosol Collector for CCSEM/EDX single-particle analysis

An automated Time-Resolved Aerosol Collector (TRAC) has been developed for sequential sampling of field-collected aerosols for laboratory-based Computer Controlled Scanning Electron Microscopy/Energy Dispersed X-ray (CCSEM/EDX) single-particle analysis. The collector is optimized for the use of grid-supported 50 nm carbon films as deposition substrates. The carbon films have low enough X-ray background to permit EDX analysis down to 0.1-0.2 w m particles, including detection of low-Z elements: C, N, and O. The TRAC provides unattended sampling onto a set of 151 individual grids, at sequential time intervals as short as 1 min. After collection, the samples are sealed and refrigerated pending analysis. The utility of the TRAC-CCSEM/EDX approach is exemplified using the aerosol samples collected during the Texas 2000 Air Quality Study (August 15-September 15, 2000). We are able to follow the time evolution in the relative contribution of nonvolatile particles such as ammonium sulfate, mineral dust, sea salt, and carbonaceous in the aerosol makeup. The results show, among other things, the diurnal cycles in appearance of fine carbonaceous and ammonium sulfate particles and substantial mixing/coating of mineral particles with ammonium sulfates.

Sticky Ice Grains Aid Planet Formation: Unusual Properties of Cryogenic Water Ice

There is limited time for the dust in the nebula around a newborn star to form planetesimals: in a few million years or less the stars stellar winds will disperse most of the unagglomerated dust. It has been difficult to explain the efficiency by which dust grains must have agglomerated to form planetesimals in circumstellar disks. A major obstacle is the fragility of aggregates, leading to collisional fragmentation, which makes it difficult for them to grow to, and beyond, meter-sized bodies. The distinct properties of cryogenic (5-100 K) amorphous water ice, which composes or coats the grains in the cooler parts of the nebulae (Jovian distances), may be able to account for the rapid agglomeration. Measurements are presented that show that this ice readily acquires persistent macroscopic electric dipoles, strongly enhancing grain-grain adhesion. In addition, measurements were made showing that vapor-deposited amorphous water ice is also highly mechanically inelastic (≈10% rebound). Together these may explain this efficient net sticking and net growth. Similar properties of higher temperature grains may aid agglomeration in the inner regions of the nebulae.

Soft-landed ions: a route to ionic solution studies

Abstract Solvated ions in condensed phase can be studied with new directness, using a very low energy (≤ 1 eV) mass-selected ion source, to ‘soft-land’ ions on or within surface films. The very low energy allows almost any ion to be studied without impact damage. Results for hydronium ions deposited on water ice are presented, where the lack of hydronium diffusion up to 190 K is evident, and intriguing information on dielectric behavior is measured. Cs + ions moving in n-hexane and 3-methyl pentane are also discussed.

Ion beam source for soft-landing deposition

“Soft-landing” deposition of molecular ions on various surfaces is important in making exotic radicals, modeling electrochemical double layers, and studying aqueous ion interactions. We have built a new mass-selected ion beam source for soft-landing deposition, designed to produce either positive or negative ions, including ions that depend on ion-neutral reactions (e.g., H3O+ and NH4+). The ionizer is a free jet crossed by an electron beam, producing a wide variety of positive and negative ions. The simple, short-length, planar ion deceleration minimizes defocusing and space charge effects. It currently delivers mass-selected ions with energies down to about 1 eV and currents of about 10 nA. The design allows easy maintenance. The performance of the ion beam compares favorably with previous low-energy positive ion systems.

Pyroelectricity of Water Ice

Water ice usually is thought to have zero pyroelectricity by symmetry. However, biasing it with ions breaks the symmetry because of the induced partial dipole alignment. This unmasks a large pyroelectricity. Ions were soft-landed upon 1 mum films of water ice at temperatures greater than 160 K. When cooled below 140-150 K, the dipole alignment locks in. Work function measurements of these films then show high and reversible pyroelectric activity from 30 to 150 K. For an initial approximately 10 V induced by the deposited ions at 160 K, the observed bias below 150 K varies approximately as 10 Vx(T/150 K)2. This implies that water has pyroelectric coefficients as large as that of many commercial pyroelectrics, such as lead zirconate titanate (PZT). The pyroelectricity of water ice, not previously reported, is in reasonable agreement with that predicted using harmonic analysis of a model system of SPC ice. The pyroelectricity is observed in crystalline and compact amorphous ice, deuterated or not. This implies that for water ice between 0 and 150 K (such as astrophysical ices), temperature changes can induce strong electric fields (approximately 10 MV/m) that can influence their chemistry, ion trajectories, or binding.

Soft-landed ion diffusion studies on vapor-deposited hydrocarbon films

Cesium and hydronium ions were deposited with a “soft-landing” ion beam (1 eV) on n-hexane and 3-methyl-pentane vapor-deposited thin films on a Pt (111) surface at 27 K. Dielectric properties and ion migration were studied during the ion deposition and during a temperature ramp up to the desorption temperature of the molecular films. The ions were found to migrate through amorphous versions of these films as expected by simple viscosity models near 90 K with ion mobilities of about 10−18 m2 V−1 s−1. No, or very limited, diffusion was observed through crystalline films. The n-hexane films crystallize during the ion motion. Analysis of this permits the estimation that average diffusional motion for a neutral hexane during crystallization is about 1 molecular diameter.

Probing aqueous-organic interfaces with soft-landed ions

Abstract A very low energy ion source (1 eV) is used to dose hydronium ions on or into epitaxially grown amorphous thin films of water and organic solvent, from tens to thousands of monolayers thick. This is done to probe the structure of the organic film, ion diffusion and solvation, and the kinetics of transfer of hydronium ions from an aqueous environment into a non-aqueous one. The organic solvent used is methyl cyclohexane. Water dosed from 0 to 0.5 monolayer on top of 60 monolayer films of methyl cyclohexane causes a solvation of ions co-adsorbed with the water, slowing the subsequent diffusion of ions through the solvent film. Water in excess of a monolayer causes extreme slowing of ion transfer into the organic phase. Structures in the film, due to intentional doping or crystallization, are also probed via the ion motion.

Low-energy deposition and collision-induced dissociation of water ions on Pt (111)

Abstract We studied collision-induced D2O+ dissociation on Pt(111) using a low-energy ion source (developed at PNNL and The University of Colorado) and temperature-programmed desorption (TPD). D2O+ ions were deposited on the surface at energies ranging from 3 to 27 eV. The molecular sticking probability decreases rapidly with kinetic energy, from 0.14 at 3 eV to 0.04 at 27 eV. The dissociative adsorption probability (in terms of surface D atoms produced) increases from 0 at 10 eV to 0.8 deuteriums per incident ion at 27 eV. The dissociation appears to involve oxygen ejection, since TPD after deposition shows D2 but no oxygen other than in water. This dissociation threshold is higher than for published results for CO+ and N+2 hitting metal surfaces. This is because metal electrons should neutralize D2O+ to the ground electronic state, limiting its internal energy available for dissociation.