arXiv subjects

Gianfranco Vidali

Publications and source records attributed to Gianfranco Vidali.

26 records · Page 2Linked to original sources

arXiv2005

Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling

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.

astro-ph↗

arXiv2005

Molecular Hydrogen Formation on Ice Under Interstellar Conditions

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.

astro-ph↗

arXiv2000

Master Equation for Hydrogen Recombination on Grain Surfaces

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 that takes into account both the discrete nature of the H atoms and the fluctuations in the number of atoms on a grain. The hydrogen recombination rate on microscopic grains, as a function of grain size and temperature, is then calculated using the master equation. The results are compared to those obtained from the rate equations and the conditions under which the master equation is required are identified.

astro-ph↗

arXiv1996

Laboratory Synthesis of Molecular Hydrogen on Surfaces of Astrophysical Interest

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 $γ$) 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 $γ$) from astronomical observations, if the actual surface of an average grain is rougher, and its area bigger, than the one considered in models.

astro-ph↗

arXiv1993

Scaling of Island Growth in Pb Overlayers on Cu(001)

The growth and ordering of a Pb layer deposited on Cu(001) at 150 K has been studied using atom beam scattering. At low coverage, ordered Pb islands with a large square unit cell and nearly hexagonal internal structure are formed. This is a high order commensurate phase with 30 atoms in the unit cell. From the measurement of the island diffraction peak profiles we find a power law for the mean island - size versus coverage with an exponent $n=0.54 \pm 0.03$. A scaling behavior of growth is confirmed and a simple model describing island growth is presented. Due to the high degeneracy of the monolayer phase, different islands do not diffract coherently. Therefore, when islands merge they still diffract as separate islands and coalescence effects are thus negligible. From the result for $n$ we conclude that the island density is approximately a constant in the coverage range $0.1 < Θ< 0.5$ where the ordered islands are observed. We thus conclude that most islands nucleate at $Θ< 0.1$ and then grow in an approximately self similar fashion as $Θ$ increases.

cond-mat↗

arXiv1992

Growth of Metastable High-Order Commensurate Overlayers of Pb on Cu(001)

We studied the growth and ordering of a Pb layer deposited on Cu(001) at 150 K. Contrary to the case of adsorption of Pb at room temperature, islands readily form. These islands order in a high-order commensurate structure of symmetry ( sqrt(61) x sqrt(61) ) R arctan(5/6). We followed the growth of these islands using atom beam scattering. From the measurement of the island diffraction peak profiles we found a power growth law for the mean island- size versus coverage. Upon increasing the coverage another high-order commensurate structure is formed, (5 x 5) R arctan(3/4). All the ordered phases obtained by adsorption at low temperature are metastable.

cond-mat↗

arXiv1992

Metal Adatom Induced Corrugation of Cu(001)

We report the discovery of a metal adatom induced corrugation of Cu(001) as probed by atom beam scattering (ABS). At Pb or Bi coverages of 0.05 (fraction of Cu layer), and while a (1$\times$1) LEED pattern is still observed, the ABS diffraction pattern from \underbar{uncovered} Cu(001) areas has changed considerably from that of clean Cu(001). We show that these uncovered areas have become up to ten times more corrugated than the clean Cu(001). We performed several carefully designed experiments to test whether our data could be interpreted as a result of scattering from adsorbate islands or from a lattice of randomly occupied sites. Instead, we ascribe the nature of this metal adatom induced corrugation to the redistribution of the Cu(001) surface charge density due to the adsorption of Pb or Bi atoms. No effect is seen following adsorption of weakly chemisorbed adatoms, such as Hg. A model consistent with all our observations is presented.

cond-mat↗

arXiv1992

Models for Monolayers Adsorbed on a Square Substrate

Motivated by recent experimental studies of Hg and Pb monolayers on Cu(001) we introduce a zero temperature model of a monolayer adsorbed on a square substrate. Lennard-Jones potentials are used to describe the interaction between pairs of adlayer-adlayer and adlayer-substrate atoms. We study a special case in which the monolayer atoms form a perfect square structure and the lattice constant, position and orientation with respect to the substrate can vary to minimize the energy. We introduce a rule based on the Farey tree construction to generate systematically the most energetically favored phases and use it to calculate the phase diagram in this model.

cond-mat↗