A. L. F. de Barros

Cosmic ray impact on astrophysical ices: laboratory studies on heavy ion irradiation of methane

Laboratory data of CH4 ice radiolysis promoted by fast heavy ions are obtained by infrared spectroscopy (FTIR). CH4 molecules are condensed on a CsI substrate at 15 K, and the ice layer is bombarded by 220 MeV 16 O 7+ ion beam. The ice thickness is thin enough to be traversed by projectiles at constant velocity close to the equilibrium charge state. The induced CH4 dissociation gives rise to the formation of molecular species CH3 ,C 2H2 ,C 2H4 ,C 2H6 ,a nd C 3H8. Their formation and dissociation cross sections are determined. C2H6 represent the most abundant daughter molecules. The carbon budget analysis of CH4 and its radiolysis products shows that the column density of carbon atoms contained in the methane destroyed during ion irradiation is 30−50% greater than the sum for the column densities of the newly formed species. As an astrophysical implication, the current results allow estimation of chemical reaction rates in ices covering interstellar grains.

Swift heavy ion irradiation of water ice from MeV to GeV energies

Context. Cosmic ray ion irradiation affects the chemical composition of and triggers physical changes in interstellar ice mantles in space. One of the primary structural changes induced is the loss of porosity, and the mantles evolve toward a more compact amorphous state. Previously, ice compaction was monitored at low to moderate ion energies. The existence of a compaction threshold in stopping power has been suggested.Aims. In this article we experimentally study the effect of heavy ion irradiation at energies closer to true cosmic rays. This minimises extrapolation and allows a regime where electronic interaction always dominates to be explored, providing the ice compaction cross section over a wide range of electronic stopping power.Methods. High-energy ion irradiations provided by the GANIL accelerator, from the MeV up to the GeV range, are combined with in-situ infrared spectroscopy monitoring of ice mantles. We follow the IR spectral evolution of the ice as a function of increasing fluence (induced compaction of the initial microporous amorphous ice into a more compact amorphous phase). We use the number of OH dangling bonds of the water molecule, i.e. pending OH bonds not engaged in a hydrogen bond in the initially porous ice structure as a probe of the phase transition. These high-energy experiments are combined with lower energy experiments using light ions (H, He) from other facilities in Catania, Italy, and Washington, USA.Results. We evaluated the cross section for the disappearance of OH dangling bonds as a function of electronic stopping power. A cross-section law in a large energy range that includes data from different ice deposition setups is established. The relevant phase structuring time scale for the ice network is compared to interstellar chemical time scales using an astrophysical model.Conclusions. The presence of a threshold in compaction at low stopping power suggested in some previous works seems not to be confirmed for the high-energy cosmic rays encountered in interstellar space. Ice mantle porosity or pending bonds monitored by the OH dangling bonds is removed efficiently by cosmic rays. As a consequence, this considerably reduces the specific surface area available for surface chemical reactions.

Cross-section measurements for the fragmentation of CHClF2 by electron impact

CFC compounds present in the upper atmosphere have a significant effect on the environment, strongly contributing to the increase of the hole in the ozone layer. Recent studies show that low-energy electron impact is an important process in the dissociation of these molecules, creating atomic chlorine, which breaks down ozone molecules. In this work, the CHClF2 fragmentation by electron impact in the 40–400 eV energy range is measured. Total and partial cross sections have been obtained, showing the predominance of the release of neutral chlorine, which amounts to around 60% of the total yield. There is a strong indication that this chlorine is being released as a result of the ionization of electrons from both chlorine and fluorine orbitals.

Radiolysis of astrophysical ices by heavy ion irradiation: Destruction cross section measurement

Many solar system objects, such as planets and their satellites, dust grains in rings, and comets, are known to either be made of ices or to have icy surfaces. These ices are exposed to ionizing radiation including keV, MeV and GeV ions from solar wind or cosmic rays. Moreover, icy dust grains are present in interstellar space and, in particular, in dense molecular clouds. Radiation effects include radiolysis (the destruction of molecules leading to formation of radicals), the formation of new molecules following radiolysis, the desorption or sputtering of atoms or molecules from the surface, compaction of porous ices, and phase changes. This review discusses the application of infrared spectroscopy FTIR to study the evolution of the chemical composition of ices containing the most abundant molecular species found in the solar system and interstellar medium, such as H2O, CO, CO2 and hydrocarbons. We focus on the evolution of chemical composition with ion fluence in order to deduce the corresponding destru...

The effect of photo-anode surface morphology and gel-polymer electrolyte on dye-sensitized solar cells with natural dyes

There is increasing demand for highly stable and solid-state dye-sensitized solar cells (DSSCs) for renewable energy resources. Following this concern, we have fabricated DSSCs using different thicknesses thin films of TiO2 photo-anode, graphite film as cathode and extracted natural dyes from fruits and vegetables as sensitizers. The effect of natural dye molecules on TiO2 particles and photon–electron conversion efficiencies were studied. Microstructures of photo-anodes, cathodes and the functional groups of the extracted dye molecules and their conjunction with TiO2 films were analyzed using scanning electron microscopy, atomic force microscopy and Fourier transform infrared spectroscopy, respectively. A mixture of polyvinylpyrrolidone and iodine has been used to prepare a polymer-gel electrolyte, and it confirms the promising behavior of highly stable and solid-state DSSCs. Amongst a variety of natural dyes extracted from fruits, jabuticaba skin dye shows better performance of the DSSCs in this work. The obtained results show that the thickness of the photo-anode thin film influences directly on the photovoltaic performance.

Negative atomic halogens incident on argon and molecular nitrogen: electron detachment studies

During the last years we have measured total detachment cross sections of atomic and cluster anions colliding with gases in the velocity range of 0.2 to 1.8 a.u. In particular, we measured negative atomic halogens incident on argon and molecular nitrogen. These last data are for the first time analyzed using the simple semi-classical model that we have developed. For that purpose, the values of elastic plus inelastic cross sections for impact of free electrons on Ar and N2, the latter showing a shape resonance, convoluted with the anions outermost electron momentum distribution yielded the overall shape of the anion cross sections. Inclusion of a velocity independent additive term, interpreted as an effective area of the collision region, led to accurate absolute cross section values. The high affinity of the halogens and the existence of a not well described resonance in the e-N2 collision, are characteristics that may be used to delimit the scope and validity of the model.

Biocompatible hydrophilic brushite coatings on AZX310 and AM50 alloys for orthopaedic implants

AbstractDicalcium phosphate dihydrate (DCPD) brushite coating with flake like crystal structure for the protection of AZX310 and AM50 magnesium (Mg) alloys was prepared through chemical deposition treatment. Chemical deposition treatment was employed using Ca(NO3)2·4H2O and KH2PO4 along with subsequent heat treatment. The morphological results revealed that the brushite coating with dense and uniform structures was successfully deposited on the surface of AZX310 and AM50 alloys. The X-ray diffraction (XRD) patterns and Attenuated total reflectance infrared (ATR-IR) spectrum also revealed the confirmation of DCPD layer formation. Hydrophilic nature of the DCPD coatings was confirmed by Contact angle (CA) measurements. Moreover, electrochemical immersion and in vitro studies were evaluated to measure the corrosion performance and biocompatibility performance. The deposition of DCPD coating for HTI AM50 enables a tenfold increase in the corrosion resistance compared with AZX310. Hence the ability to offer such significant improvement in corrosion resistance for HTI AM50 was coupled with more bioactive nature of the DCPD coating is a viable approach for the development of Mg-based degradable implant materials.

Effects on nitrogen-acetone ice induced by energetic heavy ion collisions

The radiolysis of a mixture of acetone and nitrogen condensed at 13 K by 40-MeV 58Ni11+ ions was studied. The bombardment with heavy ions is highly efficient for inducing chemical reactions in ices. The dissociation rate of acetone mixed with nitrogen as a function of the fluence is determined from a sequence of infrared spectra. The formation of new molecular species due to the irradiation is investigated and their cross-sections are calculated

Identification of the CHClF2 molecule fragmentation paths by electron impact

CFC compounds in the atmosphere play a crucial role to the environment, being the main responsible for the enlargement of the ozone hole. Recent studies show that electron impact can be a significant process for the fragmentation of these molecules; for this reason, the collisional processes of CHClF2 by electron impact were studied. Total and partial cross sections have been obtained, showing the predominance of the release of neutral chlorine, which is the main responsible for the breaking down of ozone molecules. There is a strong indication that this chlorine is being released as a result of the ionization of electrons from both chlorine and fluorine orbitals.

Energy distributions of H{sup +} fragments ejected by fast proton and electron projectiles in collision with H{sub 2}O molecules

Experimental measurements of the kinetic energy distribution spectra of H{sup +} fragment ions released during radiolysis of water molecules in collision with 20, 50, and 100 keV proton projectiles and 35, 200, 400, and 1000 eV electron projectiles are reported using a pulsed beam and drift tube time-of-flight based velocity measuring technique. The spectra show that H{sup +} fragments carrying a substantial amount of energy are released, some having energies well in excess of 20 eV. The majority of the ions lie within the 0-5 eV energy range with the proton spectra showing an almost constant profile between 1.5 and 5 eV and, below this, increasing gradually with decreasing ejection energy up to the near zero energy value while the electron spectra, in contrast, show a broad maximum between 1 and 3 eV and a pronounced dip around 0.25 eV. Beyond 5 eV, both projectile spectra show a decreasing profile with the electron spectra decreasing far more rapidly than the proton spectra. Our measured spectra thus indicate that major differences are present in the collision dynamics between the proton and the electron projectiles interacting with gas phase water molecules.