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Experimental Research in Professor Vidali's Laboratory

Currently, there are two main lines of research being pursued in our laboratory:

• In astrophysics: studies of physical and chemical processes in the interstellar medium
• In surface physics: static and dynamic characterization of surfaces and thin film growth. Currently this line of research is geared in support of our studies of processes in the interstellar medium

One of the most important questions in astrophysics is about the formation and destruction of molecular hydrogen in interstellar space. Atomic and molecular hydrogen enter virtually every reaction leading to the formation of molecules.

In this project, currently under way in our laboratory, we investigate how molecular hydrogen is formed in the interstellar medium. Atomic hydrogen is the most abundant element in the Universe, and molecular hydrogen (two atoms of hydrogen bound together) is the most abundant molecule. Theoretical arguments suggest that the largest percentage of molecular hydrogen is formed in interstellar space when hydrogen atoms recombine on surfaces of interstellar material. The importance of knowing how hydrogen recombination occurs stems from the fact that hydrogen assumes very important roles in the interstellar medium (ISM), from shielding the core of an interstellar cloud from UV radiation and hastening its collapse, to presiding to the chemistry of virtually every molecule formed in space.

How does one study such processes?

In our laboratory we set-up an apparatus to study hydrogen recombination reactions on surfaces of materials under conditions that are made as close as technically possible to the conditions prevalent in the interstellar medium.

The interstellar medium is composed of mostly hydrogen, with densities from 1-10 atoms/cm³ (diffuse clouds) to 10⁴ atoms/cm³ (dense clouds). Average temperature of dust grains is 10-20 K and their size is of the order of 0.1 micron.

A radiofrequency hydrogen source is used to dissociate hydrogen; hydrogen atoms are then cooled down to 150-200 K and sent in the form of a beam to a cold surface (5-15 K) where they scatter elastically, inelastically (loosing some energy) or they stick. The ones which stick diffuse rapidly on the surface and eventually they recombine and desorb. A mass spectrometer is used to determine the amount of atomic and molecular hydrogen coming off the surface.

The processes leading to the recombination can be studied as a function of surface morphology and chemical composition.

• For a description of the apparatus click on "Facilities" in the menu above .
• For a list of publications related to this topic, follow this link.

Surface Physics: Static and Dynamic Characterization of Ultra-Thin Films Grown In-Situ

The excitement that has characterized research in condensed matter physics in recent years is due to discoveries of phenomena occurring at surfaces. Experimental techniques are available in our laboratory to prepare and characterize the growth of solid films, from submonolayer coverage up to hundreds of layers.

Nowadays there is a keen interest in the preparation of films under far-from-equilibrium conditions, since kinetically determined structures exhibit novel static and dynamic properties which can be exploited in a variety of ways.

In the last few years we have looked at two systems, adsorption of Hg on Cu(001) and of Pb on Cu(001). We chose to work on these systems because they have the following characteristics:

1. Hg/Cu(001):

a) Large lattice mismatch: since Hg is much bigger than Cu, there is the possibility of observing different structures at sub- to monolayer coverage in which Hg atoms try to arrange themselves on the Cu lattice in order to minimize the overall energy.

b) Weak chemisorption (0.5 eV): it makes possible to measure the binding energy of Hg on Cu by performing reversible adsorption measurements (adsorption isobars).

c) Di-valent metal: there are theoretical predictions about the change of electric conduction (from metal to insulator) depending on the structure of the Hg layer.

For a list of relevant publications, click here

2. Pb/Cu(001):

   This system has an even larger lattice mismatch than the previous one (Hg/Cu(001)) but a stronger binding energy. By comparing results from the two systems, we have the opportunity to study the competition of the adsorbate-adsorbate vs. the adsorbate-substrate interaction energies.

a) Study of submonolayer phases

Study of submonolayer phases of Pb on Cu(001) Using He-beam scattering and LEED (Low Energy Electron Spectroscopy) we have determined the submonolayer phases of Pb on Cu(001). Some remarkable results were obtained. For example, low temperature phases are rotated high-order commensurate phases.

b) Manipulation of Growth Modes

Besides studying the structure of Pb overlayers, we characterized the growth of Pb on Cu(001). At high substrate temperature, the system grows in the Stranski-Krastanov mode, that is, a layer of Pb is formed and this is followed by the growth of three dimensional clusters. But as the substrate temperature is lowered, more flat layers can be grown. Thus. one can change the growth mode of a thin film by changing some parameters, such as substrate temperature.

c) Characterization of Growth Kinetics

At low substrate temperature (150 K) and high Pb flux (a few layers per minute) the Pb film is seen to roughen. The interface width and the lateral correlation length can be measured and shown to give useful information not only on the characteristics of the morphology, but also on the mechanisms which are responsible for the observed non-equilibrium structures.

It is worth noting that these studies are done in situ (i.e., in ultra-high vacuum conditions) and their growth is monitored in real time. A time-of-flight addition to the present apparatus is currently being built. This facility will allow us to study the dynamics of surfaces and adsorbed atoms or molecules.