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.
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 of Pb on Cu(001) Uisng 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.