The Experts below are selected from a list of 246 Experts worldwide ranked by ideXlab platform
Gianfranco Vidali - One of the best experts on this subject based on the ideXlab platform.
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FORMATION OF Molecular Hydrogen FROM METHANE ICE
The Astrophysical Journal, 2010Co-Authors: Kun Gao, Gianfranco Vidali, Chris J. Bennett, Ralf I. KaiserAbstract:To study the formation of Molecular Hydrogen in the wake of the processing of interstellar ices by energetic cosmic-ray particles, we investigated the interaction of energetic electrons, as formed in the track of galactic cosmic-ray particles, with deuterated methane ices (CD4) at 11 K. The energetic electrons mimic energy-transfer processes that occur in the track of the trajectories of energetic cosmic-ray particles; deuterated methane ice was utilized to discriminate the Molecular deuterium (m/z = 4) formed during the radiation exposure from the residual Molecular Hydrogen gas (m/z = 2) released inside the ultrahigh vacuum scattering chamber from outgassing of the stainless steel material. The ices were characterized online and in situ using Fourier transform infrared spectroscopy, while the evolution of the Molecular deuterium (D2) into the gas phase was monitored using a mass spectrometer. A mass spectrometric signal proportional to the number density of the deuterium molecules generated inside the ice and released during the irradiation was analyzed kinetically using a set of coupled rate equations. From the fit to the experimental data, we obtain activation energies for the diffusion of atomic deuterium (E 0 = 37 ± 1 meV), and for the desorption of atomic (E 1 = 32 ± 1 meV) and Molecular deuterium (E 2 = 32 ± 1 meV). These energies are placed in context and then transferred to atomic and Molecular Hydrogen to yield astrophysically relevant data. The experimental yield of Molecular deuterium is then used to calculate the formation rate of Molecular Hydrogen due to cosmic-ray interaction with ice-covered grains in dense clouds.
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Catalytic activity of interstellar grains: Formation of Molecular Hydrogen on amorphous silicates
Advances in Space Research, 2009Co-Authors: Gianfranco Vidali, J. E. Roser, Ryan P. BadmanAbstract:Abstract Silicates constitute an important class of interstellar grain material and are the site of catalytic activities, most notably the formation of Molecular Hydrogen. Here we report an analysis of experiments conducted in the laboratory to measure the efficiency of formation of Molecular Hydrogen on amorphous silicates, a realistic analogue of interstellar dust grains. From the measurements, we also obtain the energetics of key processes in the reaction and information on the mechanism of reaction. Comparison with earlier measurements of Molecular Hydrogen formation on a sample of polycrystalline olivine shows that amorphous materials are more efficient catalysts.
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Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions
The Astrophysical Journal, 2007Co-Authors: Hagai B. Perets, G. Manicò, E. Congiu, Joe Roser, Sol Swords, A. Lederhendler, Ofer Biham, Gianfranco Vidali, John Robert BrucatoAbstract:Experimental results on the formation of Molecular Hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of Molecular Hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H$_{2}$ formation within a temperature range which is relevant to diffuse interstellar clouds. The results also indicate that the Hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.
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Molecular Hydrogen Formation on Ice Under Interstellar Conditions
The Astrophysical Journal, 2005Co-Authors: Hagai B. Perets, G. Manicò, Joe Roser, Sol Swords, Ofer Biham, Valerio Pirronello, Gianfranco VidaliAbstract:The results of experiments on the formation of Molecular Hydrogen on low density and high density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low density ice gives rise to a broader spectrum of energy barriers compared to the high density ice. Inserting these parameters into the rate equation model under steady state conditions we evaluate the production rate of Molecular Hydrogen on ice-coated interstellar dust grains.
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Formation of Molecular Hydrogen on analogues of interstellar dust grains: experiments and modelling
Journal of Physics: Conference Series, 2005Co-Authors: Gianfranco Vidali, G. Manicò, Joe Roser, Hagai B. Perets, Valerio Pirronello, Ofer BihamAbstract:Molecular Hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of Molecular Hydrogen in simulated astrophysical environments. Since the formation of Molecular Hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.
Ofer Biham - One of the best experts on this subject based on the ideXlab platform.
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Formation of Molecular Hydrogen on amorphous silicate surfaces
arXiv: Astrophysics, 2007Co-Authors: G. Manicò, John Robert Brucato, E. Congiu, Joe Roser, Sol Swords, Hagai B. Perets, A. Lederhendler, Ofer Biham, Valerio PirronelloAbstract:Experimental results on the formation of Molecular Hydrogen on amorphous silicate surfaces are presented and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of Molecular Hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H_2 formation in diffuse interstellar clouds.
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Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions
The Astrophysical Journal, 2007Co-Authors: Hagai B. Perets, G. Manicò, E. Congiu, Joe Roser, Sol Swords, A. Lederhendler, Ofer Biham, Gianfranco Vidali, John Robert BrucatoAbstract:Experimental results on the formation of Molecular Hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of Molecular Hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H$_{2}$ formation within a temperature range which is relevant to diffuse interstellar clouds. The results also indicate that the Hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.
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Molecular Hydrogen Formation on Ice Under Interstellar Conditions
The Astrophysical Journal, 2005Co-Authors: Hagai B. Perets, G. Manicò, Joe Roser, Sol Swords, Ofer Biham, Valerio Pirronello, Gianfranco VidaliAbstract:The results of experiments on the formation of Molecular Hydrogen on low density and high density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low density ice gives rise to a broader spectrum of energy barriers compared to the high density ice. Inserting these parameters into the rate equation model under steady state conditions we evaluate the production rate of Molecular Hydrogen on ice-coated interstellar dust grains.
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Formation of Molecular Hydrogen on analogues of interstellar dust grains: experiments and modelling
Journal of Physics: Conference Series, 2005Co-Authors: Gianfranco Vidali, G. Manicò, Joe Roser, Hagai B. Perets, Valerio Pirronello, Ofer BihamAbstract:Molecular Hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of Molecular Hydrogen in simulated astrophysical environments. Since the formation of Molecular Hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.
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Molecular Hydrogen formation on astrophysically relevant surfaces
The Astrophysical Journal, 1999Co-Authors: Nadav Katz, Ofer Biham, Valerio Pirronello, Itay Furman, Gianfranco VidaliAbstract:Recent experimental results about the formation of Molecular Hydrogen on astrophysically relevant surfaces under conditions close to those encountered in the interstellar medium are analyzed using rate equations. The parameters of the rate equation model are fitted to temperature-programmed desorption curves obtained in the laboratory. These parameters are the activation energy barriers for atomic Hydrogen diffusion and desorption, the barrier for Molecular Hydrogen desorption, and the probability of spontaneous desorption of a Hydrogen molecule upon recombination. The model is a generalization of the Polanyi-Wigner equation and provides a description of both first- and second-order kinetic processes within a single model. Using the values of the parameters that best fit the experimental results, the efficiency of Hydrogen recombination on olivine and amorphous carbon surfaces is obtained for a range of Hydrogen flux and surface temperature pertinent to a wide range of interstellar conditions.
Valerio Pirronello - One of the best experts on this subject based on the ideXlab platform.
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Formation of Molecular Hydrogen on amorphous silicate surfaces
arXiv: Astrophysics, 2007Co-Authors: G. Manicò, John Robert Brucato, E. Congiu, Joe Roser, Sol Swords, Hagai B. Perets, A. Lederhendler, Ofer Biham, Valerio PirronelloAbstract:Experimental results on the formation of Molecular Hydrogen on amorphous silicate surfaces are presented and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of Molecular Hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H_2 formation in diffuse interstellar clouds.
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Molecular Hydrogen Formation on Ice Under Interstellar Conditions
The Astrophysical Journal, 2005Co-Authors: Hagai B. Perets, G. Manicò, Joe Roser, Sol Swords, Ofer Biham, Valerio Pirronello, Gianfranco VidaliAbstract:The results of experiments on the formation of Molecular Hydrogen on low density and high density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low density ice gives rise to a broader spectrum of energy barriers compared to the high density ice. Inserting these parameters into the rate equation model under steady state conditions we evaluate the production rate of Molecular Hydrogen on ice-coated interstellar dust grains.
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Formation of Molecular Hydrogen on analogues of interstellar dust grains: experiments and modelling
Journal of Physics: Conference Series, 2005Co-Authors: Gianfranco Vidali, G. Manicò, Joe Roser, Hagai B. Perets, Valerio Pirronello, Ofer BihamAbstract:Molecular Hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of Molecular Hydrogen in simulated astrophysical environments. Since the formation of Molecular Hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.
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Molecular Hydrogen formation on astrophysically relevant surfaces
The Astrophysical Journal, 1999Co-Authors: Nadav Katz, Ofer Biham, Valerio Pirronello, Itay Furman, Gianfranco VidaliAbstract:Recent experimental results about the formation of Molecular Hydrogen on astrophysically relevant surfaces under conditions close to those encountered in the interstellar medium are analyzed using rate equations. The parameters of the rate equation model are fitted to temperature-programmed desorption curves obtained in the laboratory. These parameters are the activation energy barriers for atomic Hydrogen diffusion and desorption, the barrier for Molecular Hydrogen desorption, and the probability of spontaneous desorption of a Hydrogen molecule upon recombination. The model is a generalization of the Polanyi-Wigner equation and provides a description of both first- and second-order kinetic processes within a single model. Using the values of the parameters that best fit the experimental results, the efficiency of Hydrogen recombination on olivine and amorphous carbon surfaces is obtained for a range of Hydrogen flux and surface temperature pertinent to a wide range of interstellar conditions.
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Formation of Molecular Hydrogen: The Mother of All Molecules
Exobiology: Matter Energy and Information in the Origin and Evolution of Life in the Universe, 1998Co-Authors: Valerio Pirronello, Ofer Biham, Chi Liu, Liyong Shen, Gianfranco VidaliAbstract:The problem of the formation of Molecular Hydrogen is certainly the most important one in astrophysical environments, and in interstellar clouds in particular, because H2 is the most abundant molecule in space and, as such, it is the most frequent collisional partner of all other particles in the gas phase. This relevant peculiarity makes of H2 what has been called (Duley and Williams, 1984) the “seminal molecule”, the mother of all other Molecular species; it, in fact once ionized by UV photons in diffuse clouds and by cosmic rays in dense, initiates ion-molecule reactions (the only ones that can efficiently take place in the cold interstellar medium because characterized by low energy activation barriers) chains that have been succesfully proposed to synthesize most of observed species in space. Furthermore Molecular Hydrogen, together with the other species, provides a very efficient mechanism for cooling clouds. Molecules, in fact, once are collisionally excited, deexcite in cascade through rotovibrational transitions emitting photons to which the cloud itself is transparent. This cooling mechanism helps the gravitational collapse of interstellar clouds, increases the formation rate of stars and has great impact in the structure and evolution of galaxies. In this note we report on measurements of Molecular Hydrogen synthesis for the first time performed in conditions and on materials close to interstellar reality (Pirronello et al., 1997a,b).
Hagai B. Perets - One of the best experts on this subject based on the ideXlab platform.
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Formation of Molecular Hydrogen on amorphous silicate surfaces
arXiv: Astrophysics, 2007Co-Authors: G. Manicò, John Robert Brucato, E. Congiu, Joe Roser, Sol Swords, Hagai B. Perets, A. Lederhendler, Ofer Biham, Valerio PirronelloAbstract:Experimental results on the formation of Molecular Hydrogen on amorphous silicate surfaces are presented and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of Molecular Hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H_2 formation in diffuse interstellar clouds.
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Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions
The Astrophysical Journal, 2007Co-Authors: Hagai B. Perets, G. Manicò, E. Congiu, Joe Roser, Sol Swords, A. Lederhendler, Ofer Biham, Gianfranco Vidali, John Robert BrucatoAbstract:Experimental results on the formation of Molecular Hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of Molecular Hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H$_{2}$ formation within a temperature range which is relevant to diffuse interstellar clouds. The results also indicate that the Hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.
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Molecular Hydrogen Formation on Ice Under Interstellar Conditions
The Astrophysical Journal, 2005Co-Authors: Hagai B. Perets, G. Manicò, Joe Roser, Sol Swords, Ofer Biham, Valerio Pirronello, Gianfranco VidaliAbstract:The results of experiments on the formation of Molecular Hydrogen on low density and high density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low density ice gives rise to a broader spectrum of energy barriers compared to the high density ice. Inserting these parameters into the rate equation model under steady state conditions we evaluate the production rate of Molecular Hydrogen on ice-coated interstellar dust grains.
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Formation of Molecular Hydrogen on analogues of interstellar dust grains: experiments and modelling
Journal of Physics: Conference Series, 2005Co-Authors: Gianfranco Vidali, G. Manicò, Joe Roser, Hagai B. Perets, Valerio Pirronello, Ofer BihamAbstract:Molecular Hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of Molecular Hydrogen in simulated astrophysical environments. Since the formation of Molecular Hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.
Patrick R. Briddon - One of the best experts on this subject based on the ideXlab platform.
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Molecular Hydrogen traps within silicon
Materials Science and Engineering B: Solid-State Materials for Advanced Technology, 1999Co-Authors: Ben Hourahine, Scott Oberg, R Jones, Patrick R. BriddonAbstract:We present the results of first principle calculations on the behaviour of Molecular Hydrogen within crystalline silicon, both as an isolated species, and within defects in the material. These results are compared with recent experimental infra-red and Raman data obtained for silicon treated by either Hydrogen plasma or soaked in Hydrogen gas. The effect of Fermi-level position on the diffusion barrier of Molecular Hydrogen within silicon is also discussed.
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Molecular Hydrogen traps within silicon
1998Co-Authors: Ben Hourahine, R Jones, Sven Öberg, Patrick R. BriddonAbstract:We present the results of rst principle calculations on the behaviour of Molecular Hydrogen within crystalline silicon, both as an isolated species, and within defects in the material. These result ...
