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Articles 1 - 11 of 11

Full-Text Articles in Physics

Interaction Of Atomic And Molecular Hydrogen With Tholin Surfaces At Low Temperatures, Gianfranco Vidali, Ling Li, Hui Zhao Oct 2010

Interaction Of Atomic And Molecular Hydrogen With Tholin Surfaces At Low Temperatures, Gianfranco Vidali, Ling Li, Hui Zhao

Physics

We study the interaction of atomic and molecular hydrogen with a surface of tholin, a man-made polymer considered to be an analogue of aerosol particles present in Titan's atmosphere, using thermal programmed desorption at low temperatures below 30 K. The results are fitted and analyzed using a fine-grained rate equation model that describes the diffusion, reaction and desorption processes. We obtain the energy barriers for diffusion and desorption of atomic and molecular hydrogen. These barriers are found to be in the range of 30 to 60 meV, indicating that atom/molecule-surface interactions in this temperature range are dominated by ...


Molecular Hydrogen Formation On Low Temperature Surfaces In Temperature Programmed Desorption Experiments, Gianfranco Vidali, Ling Li, J. Roser, E. Congiu Nov 2008

Molecular Hydrogen Formation On Low Temperature Surfaces In Temperature Programmed Desorption Experiments, Gianfranco Vidali, Ling Li, J. Roser, E. Congiu

Physics

The study of the formation of molecular hydrogen on low temperature surfaces is of interest both because it allows to explore elementary steps in the heterogeneous catalysis of a simple molecule and because of the applications in astrochemistry. Here we report results of experiments of molecular hydrogen formation on amorphous silicate surfaces using temperature-programmed desorption (TPD). In these experiments beams of H and D atoms are irradiated on the surface of an amorphous silicate sample. The desorption rate of HD molecules is monitored using a mass spectrometer during a subsequent TPD run. The results are analyzed using rate equations and ...


Formation Of Molecular Hydrogen On Amorphous Silicate Surfaces, Gianfranco Vidali, Ling Li, E. Congiu, S. Swords Sep 2007

Formation Of Molecular Hydrogen On Amorphous Silicate Surfaces, Gianfranco Vidali, Ling Li, E. Congiu, S. Swords

Physics

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.


Molecular Hydrogen Formation On Amorphous Silicates Under Interstellar Conditions, Gianfranco Vidali, Hagai B. Perets, Ling Li, S. Swords, E. Congiu Apr 2007

Molecular Hydrogen Formation On Amorphous Silicates Under Interstellar Conditions, Gianfranco Vidali, Hagai B. Perets, Ling Li, S. Swords, E. Congiu

Physics

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 ...


Formation Of Molecular Hydrogen On Analogues Of Interstellar Dust Grains: Experiments And Modelling, Gianfranco Vidali, Joe Roser Apr 2005

Formation Of Molecular Hydrogen On Analogues Of Interstellar Dust Grains: Experiments And Modelling, Gianfranco Vidali, Joe Roser

Physics

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 ...


Molecular Hydrogen Formation On Ice Under Interstellar Conditions, Hagai B. Perets, Gianfranco Vidali, Joe Roser, Sol Swords Mar 2005

Molecular Hydrogen Formation On Ice Under Interstellar Conditions, Hagai B. Perets, Gianfranco Vidali, Joe Roser, Sol Swords

Physics

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.


Master Equation For Hydrogen Recombination On Grain Surfaces, Gianfranco Vidali, Ofer Biham, Itay Furman, Valerio Pirronello Dec 2000

Master Equation For Hydrogen Recombination On Grain Surfaces, Gianfranco Vidali, Ofer Biham, Itay Furman, Valerio Pirronello

Physics

Recent experimental results on the formation of molecular hydrogen on astrophysically relevant surfaces under conditions similar to those encountered in the interstellar medium provided useful quantitative information about these processes. Rate equation analysis of experiments on olivine and amorphous carbon surfaces provided the activation energy barriers for the diffusion and desorption processes relevant to hydrogen recombination on these surfaces. However, the suitability of rate equations for the simulation of hydrogen recombination on interstellar grains, where there might be very few atoms on a grain at any given time, has been questioned. To resolve this problem, we introduce a master equation ...


H2 Formation On Interstellar Grains In Different Physical Regimes, Gianfranco Vidali, Ofer Biham, Itay Furman, N. Katz, Valerio Pirronello Jun 1998

H2 Formation On Interstellar Grains In Different Physical Regimes, Gianfranco Vidali, Ofer Biham, Itay Furman, N. Katz, Valerio Pirronello

Physics

An analysis of the kinetics of H2 formation on interstellar dust grains is presented using rate equations. It is shown that semi-empirical expressions that appeared in the literature represent two different physical regimes. In particular, it is shown that the expression given by Hollenbach, Werner and Salpeter [ApJ, 163, 165 (1971)] applies when high flux, or high mobility, of H atoms on the surface of a grain, makes it very unlikely that H atoms evaporate before they meet each other and recombine. The expression of Pirronello et al.\ [ApJ, 483, L131 (1997)] -- deduced on the basis of accurate measurements on ...


Efficiency Of Molecular Hydrogen Formation On Silicates, Gianfranco Vidali, Valerio Pirronello, Ofer Biham, Chi Liu, Lyiong Shen Apr 1997

Efficiency Of Molecular Hydrogen Formation On Silicates, Gianfranco Vidali, Valerio Pirronello, Ofer Biham, Chi Liu, Lyiong Shen

Physics

We report on laboratory measurements of molecular hydrogen formation and recombination on an olivine slab as a function of surface temperature under conditions relevant to those encountered in the interstellar medium. On the basis of our experimental evidence, we recognize that there are two main regimes of H coverage that are of astrophysical importance; for each of them we provide an expression giving the production rate of molecular hydrogen in interstellar clouds.


Laboratory Synthesis Of Molecular Hydrogen On Surfaces Of Astrophysical Interest, Gianfranco Vidali, V Pirronello, Chi Liu, Liyong Shen Nov 1996

Laboratory Synthesis Of Molecular Hydrogen On Surfaces Of Astrophysical Interest, Gianfranco Vidali, V Pirronello, Chi Liu, Liyong Shen

Physics

We report on the first results of experiments to measure the recombination rate of hydrogen on surfaces of astrophysical interest. Our measurements give lower values for the recombination efficiency (sticking probability S x probability of recombination upon H-H encounter \gamma) than model-based estimates. We propose that our results can be reconciled with average estimates of the recombination rate (1/2 n(H) n(g) v(H)A S \gamma) from astronomical observations, if the actual surface of an average grain is rougher, and its area bigger, than the one considered in models.


Molecular Hydrogen Formation On Astrophysically Relevant Surfaces, Gianfranco Vidali, N. Katz, Itay Furman, Ofer Biham, Valerio Pirronello Jun 1995

Molecular Hydrogen Formation On Astrophysically Relevant Surfaces, Gianfranco Vidali, N. Katz, Itay Furman, Ofer Biham, Valerio Pirronello

Physics

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 ...