Dry deposition and desorption of toxic gases to and from snow surfaces

A model describing toxic gas deposition to and desorption from a snow surface is presented. The model is based on the assumption that the deposition is caused by an adsorption of the toxic gas to small amounts of liquid water, which exist in the snow at temperatures equal to or below 0°C. It includes molecular diffusion in the snow, partition between gas and solution by use of Henry's law, drainage flow in melting snow and decomposition of agent. The interface to the atmosphere is defined by the flux to and from the surface with help of the aerodynamic resistance and the resistance in the viscous sub-layer. Deposition velocities to snow for some air pollutants are reviewed. The model is compared with sarin experiments in a test chamber, which verifies two main features of the model—primarily decreasing deposition with time and decreasing deposition with decreasing temperature. The model shows that the accumulation of sarin in the top layer of snow could be high enough to give lethal or severe injuries to people if the snow was used as drinking water. However, there is a tendency of the model to give too low deposition (too high surface resistance). Possible reasons for this observation are discussed.     

The desorption of condensed noble gases and gas mixtures from cryogenic surfaces

In accelerators, operating at liquid-helium temperature, cold surfaces are exposed to intense synchrotron radiation and bombardment by energetic electrons and ions. Molecular desorption yield and secondary electron yield can strongly influence the performance of the accelerator. In order to predict the gas density during the operation, the knowledge of electron-induced desorption yields of condensed gases and of its variation with the gas coverage is necessary. Desorption yields under electron impact of various noble gases and gas mixtures condensed on a copper surface cooled at 4.2 K have been measured.     

Surface tension of liquid inert gases

The surface tension of liquid inert gases, including ³He and ⁴He helium isotopes, has been calculated using a method based on determining the work that must be done to remove an atom from the liquid surface. This approach provides quite satisfactory agreement with the available experimental data. Calculations of the surface tension in liquid helium also take into account the quantum zero-point oscillations of atoms. The role of the dimensional effect in the surface tension for nanodroplets is discussed.     

Surface interaction and desorption of trimethyl phosphate from ozonized activated carbon fabric

Trimethyl phosphate (TMP) is a typical simulant of the CWA nerve agents and its adsorption on pristine and ozonized ACF has been studied by FT-IR and ESR spectroscopy. The FT-IR analysis is effective in putting in evidence the strong interaction of TMP with the ACF surfaces through the P = O stretching band shift toward lower frequencies after adsorption. Ozonized ACF ensures a stronger interaction with the adsorbate through the hydrogen bond formation between the adsorbed TMP and the oxygenated functional groups present on carbon surface. At room temperature the ozonized ACF is able to adsorb larger amounts of TMP than the pristine ACF and shows also a better retention of the adsorbate at room temperature. The TGA-FTIR analysis of TMP desorption from both ozonized and pristine ACF has shown that TMP undergoes a partial thermal decomposition into methanol and phosphoric acid above 175°C. The maximum methanol production rate was observed at 250°C in the TMP desorption process from ozonized ACF and at 280°C from TMP desorption from pristine ACF.     

Application of low temperature desorption in systems for adsorptive purification of cryogenic gases

The application of low temperature desorption and reactivation of the adsorbent in adsorptive systems for the purification of cryogenic gases from gas impurities is considered. The degree of purification obtained using low temperature desorption is evaluated. Experiments for the determination of the rate and the degree of desorption of nitrogen and hydrogen on charcoal at 77.4 K were carried out. Practical schemes for purification of gases using low temperature desorption of the main part of the adsorbed substances are proposed.     

Origin of the thermal desorption peaks of gases in NaI above 180°C

We analyze the origin of the water desorption peaks in NaI above the temperature stability range of the crystalline hydrate NaI · 2H₂O. The two water desorption peaks at t ≥ 180°C are shown to arise from the decomposition of aqua complexes based on hydroxyl groups. Such peaks in the pressure-temperature curve provide an effective and sensitive tool for detecting OH groups in raw materials. After prolonged room-temperature dehydration, no such peaks emerge. The dehydration process can be accelerated by microwave drying.     

High-numerical-aperture effects in photoresist

Two-beam and three-beam vector interference in thin photoresist films is used to illustrate the striking differences between s -polarized and p-polarized high-numerical-aperture illumination. Both simulations and experiments are performed for several cases, including undyed photoresist on silicon, dyed photoresist on silicon, and the addition of an antireflective layer between the photoresist and the silicon. A 0.85 numerical-aperture system is examined. The major differences between s- and p-polarized illumination include elliptical versus rectangular features and lower contrast for p-polarized images.     

Carbon based conductive photoresist

A conductive photoresist for photolithographic application was studied here. The negative near-UV sensitive epoxy-based photoresist was used as a polymer matrix and conductive carbon black was used as functional filler. DC electrical resistivity of composite as a function of filler concentration has a well-known S-shape. After UV-exposure the resistivity of the composite decreases for almost five orders of magnitude, mostly at percolation threshold (approx. 0.6 vol.%). This effect can be attributed to the fully cross-linked polymer structure formed during UV-exposure of the composite. The resistivity of prepared samples also depend on the state of dispersion of the functional filler obtained using different dispersing additives. Composites with better dispersed particles have lower resistivities. This effect remained below one order of magnitude and decreased after UV-exposure. The composites with carbon black concentration of up to 1.1 vol.% are suitable for spin-coating and photolithography.     

Thermal desorption of hydrogen from graphane

The process of hydrogen desorption from graphane (graphene sheet saturated by hydrogen adsorbed from both sides) has been studied using the method of molecular dynamics. The temperature dependences of the time of desorption onset for various hydrogen coverages on graphene are calculated and the corresponding activation energies in the Arrhenius equation are determined. It is established that graphane exhibits a rather high thermal stability that makes possible its use in two-dimensional electronics even at room temperature. For the same reason, graphane can hardly be considered as a promising hydrogen storage material for fuel cells.     

Dynamic holographic gratings with photoresist

The theoretical development of dynamic holographic gratings with piezoelectric crystals is presented. A thin layer of photoresist is applied over the surface of the piezoelectric crystal. A holographic grating was recorded over the photoresist-piezo device in such a way that when it is activated the changes in its dimensions produce a geometrical change in the grooves of the grating. Hence the initial diffraction pattern created may be dynamically modified.     

Surface scanning analysis of planar arrays of analytes with desorption electrospray ionization-mass spectrometry

The analysis of analytes deposited on, separated on, or otherwise distributed about a planar surface using desorption electrospray ionization mass spectrometry in a surface scanning sampling mode was investigated. The physical regions of the surface-impacting solvent/gas jet desorption/ionization plume were described. Under the conditions typical for desorption electrospray ionization used here, the impact plume formed an elliptical desorption/ionization region on the surface. Most effective desorption/ionization was obtained from a smaller elliptical area within the larger impact region that was centered on a point on-axis from the sprayer tip to the surface. Maximum signal from a given amount of material on a surface was observed with proper plume and sample alignment when the diameter of the sample spot was less than the diameter of the central high-efficiency desorption/ionization region of the impact plume. Solvent and gas flow out of this high-efficiency desorption/ionization region was found to limit analyte accessibility to this region via a "washing effect" when analytes were on smooth surfaces or on surfaces for which the analyte had little affinity. As such, the direction of surface scanning and scan speed during an analysis was found to be important for maximizing signal levels and signal reproducibility on particular surface types. Overall, the results presented illustrate means to improve analysis of sample spots on various types of surfaces using desorption electrospray ionization mass spectrometry in a surface scanning mode.     

Slow desorption of volatile organic compounds from soil: evidence of desorption step limitations.

Transient adsorption and desorption of 1,2 dichloroethane and toluene on dry Yolo silt loam soil were studied by continuously measuring the composition of the effluent from a soil-packed chromatography column with a mass spectrometer. After obtaining complete breakthrough at approximately 30% relative saturation of one chemical in nitrogen, pure nitrogen feed was initiated and maintained for several hours. Of the material adsorbed at breakthrough, 9.7% of the 1,2 dichloroethane and 14.2% of the toluene were highly resistant to desorption and remained sorbed on the soil even after 5 h of nitrogen flow. When a second chemical with a higher adsorption affinity was introduced into the soil column (water following toluene or toluene following 1,2 dichloroethane), the majority of the first chemical was quickly desorbed and began leaving the soil column before breakthrough of the second chemical. Conversely, when a second chemical with a smaller adsorption affinity was introduced into the soil column, only a small amount of the first chemical was displaced and began leaving the soil column after breakthrough of the second chemical. The results of this study indicate that the desorption step itself may be the rate-limiting step for sorbate which remains after prolonged exposure to sorbate-free gas.     

Developed profile of holographically exposed photoresist gratings

A simulation of the profile of holographically recorded structures in photoresists is performed. In addition to its simplicity this simulation can be used to take into account the effects that arise from exposure, photosensitization, development, and resolution of positive photoresists. We analyzed the effects of isotropy of wet development, nonlinearity of the photoresist response curve, background light, and standing waves produced by reflection at the film-substrate interface by using this simulation, and the results agree with the experimentally recorded profiles.