Photometry of the cometary atmosphere: A review

Photometry and polarimetry of the cometary heads one of the most important sources of information about the physical processes in comets is reviewed. Methods of inspection discussed include: narrow band photometry, wide band photometry, color photography, tilting filter techniques, and photoelectric spectrum scanning. Results of photoelectric observations of comets are described including photoelectric and infrared measurements of comet Kohoutek. Photometric profiles of the coma in monochromatic light are used to determine the lifetime of the parent molecules for the observed radicals, CN and C2

The role of organic polymers in the structure of cometary dust

Several phenomena observed in P/Halley and other comets indicate additional fragmentation of dust particles or dust aggregates in cometary atmospheres. The disintegration of dust aggregates may be explained by sublimation of polymerized formaldehyde - POM - which play a role as binding material between submicron individual particles

Coma imaging of comet P/Brorsen-Metcalf at Calar Alto in late July to mid August 1989

Comet P/Brorsen-Metcalf was observed on 1989/07/28+30 and on 1989/08/04+12(+14) with the 3.5 m telescope and the 0.8 m Schmidt camera at Calar Alto/Spain. The images exhibit a narrow plasma tail pointing into anti-solar direction. On 1989/07/30 a triple tail was found which can be interpreted as tail ray event. The coma isophotes show prominent asymmetries with the nucleus located on the tailward side of the isophote foci and with a slightly higher brightness in the Northern Hemisphere of the coma. A strong curved jet feature was detected in the coma on 1989/07/30. The jet extended at least 30,000 km into the sunward coma hemisphere. The rotation period of about 1.3 days, estimated from the curvature of the coma jet, needs verification by other observations

Monovalent Ion Condensation at the Electrified Liquid/Liquid Interface

X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified interface between electrolyte solutions of water and 1,2-dichloroethane. Predictions of the ion distributions by standard Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at higher applied interfacial electric potentials. Calculations from a Poisson-Boltzmann equation that incorporates a non-monotonic ion-specific potential of mean force are in good agreement with the data.Comment: 4 pages, 4 figure

Reversibility of Ferri-/Ferrocyanide Redox during Operando Soft X-ray Spectroscopy

The ferri-/ferrocyanide redox couple is ubiquitous in many fields of physical chemistry. We studied its photochemical response to intense synchrotron radiation by in situ X-ray absorption spectroscopy (XAS). For photon flux densities equal to and above 2 × 1011 s–1 mm–2, precipitation of ferric (hydr)oxide from both ferricyanide and ferrocyanide solutions was clearly detectable, despite flowing fast enough to replace the solution in the flow cell every 0.4 s (flow rate 1.5 mL/min). During cyclic voltammetry, precipitation of ferric (hydr)oxide was promoted at reducing voltages and observed below 1011 s–1 mm–2. This was accompanied by inhibition of the ferri-/ferrocyanide redox, which we probed by time-resolved operando XAS. Our study highlights the importance of considering both electrochemical and spectroscopic conditions when designing in situ experiments

Holistic approach to dissolution kinetics : linking direction-specific microscopic fluxes, local mass transport effects and global macroscopic rates from gypsum etch pit analysis

Dissolution processes at single crystal surfaces often involve the initial formation and expansion of localized, characteristic (faceted) etch-pits at defects, in an otherwise comparatively unreactive surface. Using natural gypsum single crystal as an example, a simple but powerful morphological analysis of these characteristic etch pit features is proposed that allows important questions concerning dissolution kinetics to be addressed. Significantly, quantitative mass transport associated with reactive microscale interfaces in quiescent solution (well known in the field of electrochemistry at ultramicroelectrodes) allows the relative importance of diffusion compared to surface kinetics to be assessed. Furthermore, because such mass transport rates are high, much faster surface kinetics can be determined than with existing dissolution methods. For the case of gypsum, surface processes are found to dominate the kinetics at early stages of the dissolution process (small etch pits) on the cleaved (010) surface. However, the contribution from mass transport becomes more important with time due to the increased area of the reactive zones and associated decrease in mass transport rate. Significantly, spatial heterogeneities in both surface kinetics and mass transport effects are identified, and the morphology of the characteristic etch features reveal direction-dependent dissolution kinetics that can be quantified. Effective dissolution velocities normal to the main basal (010) face are determined, along with velocities for the movement of [001] and [100] oriented steps. Inert electrolyte enhances dissolution velocities in all directions (salting in), but a striking new observation is that the effect is direction-dependent. Studies of common ion effects reveal that Ca2+ has a much greater impact in reducing dissolution rates compared to SO42−. With this approach, the new microscopic observations can be further analysed to obtain macroscopic dissolution rates, which are found to be wholly consistent with previous bulk measurements. The studies are thus important in bridging the gap between microscopic phenomena and macroscopic measurements

Broadband infrared photometry of comet Hale-Bopp with ISOPHOT

Comet Hale-Bopp was observed five times with ISOPHOT, the photometer on board ESA's Infrared Space Observatory (ISO) between 4.6 and 2.8 AU. Each time, broadband photometry was performed using 4 different detectors, 5 apertures and 10 filters covering the range between 3.6 and 170 μm. Background observations were performed with identical instrument settings at the same positions on the sky several days after the comet observations. The observation strategy and the data reduction steps are described in some detail, including the techniques to correct for variable detector responsivity. The resulting inband power values of the Hale-Bopp observations and their uncertainties are given. The mean uncertainty is 25%. The final fluxes were computed, taking into account the zodiacal background, possible offset of the comet's position from the center of the aperture, the brightness distribution within the coma, and the spectral energy distribution of the comet's emission. Strong thermal emission from a broad size distribution of dust particles was detected in all of the data sets, even at r = 4.6-4.9 AU pre-perihelion and 3.9 AU post-perihelion; the total thermal energy varied as r-3. The 7.3-12.8 μm color temperature was ~1.5 times the blackbody temperature, higher than that observed in any other comet. Silicate features at 10 and 25 μm were prominent in all 5 data sets, the largest heliocentric distances that silicate emission has been detected in a comet. The presence of crystalline water ice grains is suggested from the 60 μm excess emission at 4.6-4.9 AU, consistent with the observed QOH if the icy grains were slightly warmer than an equilibrium blackbody. The average albedo of the dust is higher than that of comet P/Halley, but lower than other albedo measurements for Hale-Bopp nearer perihelion. There is no evidence for a component of cold, bright icy grains enhancing the scattered light at 4.6 AU. Simple models for a mixture of silicate and absorbing grains were fit to the ISO spectra and photometry at 2.8 AU. The observed flux at λ >100 μm requires a size distribution in which most of the mass is concentrated in large particles. Dust production rates of order 1.5 x 105 kg s-1 at 2.8 AU and 3 x 104 kg s-1 at 4.6 AU have been found. They correspond to dust to gas mass ratios of 6 to 10

Electrodeposition of Gold Nanostructures at the Interface of a Pickering Emulsion

The controlled electrodeposition of nanoparticles at the surface of an emulsion droplet offers enticing possibilities in regards to the formation of intricate structures or fine control over the locus or duration of nanoparticle growth. In this work we develop electrochemical control over the spontaneous reduction of aqueous phase Au(III) by heterogeneous electron trans-fer from decamethylferrocene present in an emulsion droplet –resulting in the growth of nanoparticles. As gold is a highly effective conduit for the passage of electrical current, even on the nanoscale, the deposition significantly enhances the current response for the single electron transfer of decamethylferrocene when acting as a redox indicator. The nanostructures formed at the surface of the emulsion droplets were imaged by cryo-TEM,providing an insight into the types of structures that may form when stabilised by the interface alone, and how the structures are able to conduct electrons