There are three basic reasons which motivate the use of molecular beams in spectroscopy:
A molecular beam is crossed at right angles with the output of a frequency
stabilized laser. The energy deposited by the laser in the beam is
monotored downstream by a small microcalorimeter (bolometer) on which the
molecules impinge and (normally) condense. This optothermal detection
method (in contrast to absorption) does not require very high laser ampliture
stability. However, (like its bulk analog, optoacoustic detection)
it requires lasers with a fair amount of power. This is why, at the
beam-laser crossing point, we use multipass cells or power buildup cavities.
Using this technique, we have been able to arrive at an impoved understanding
of internal energy flow in isolated molecules, aided in this by the incisive
theoretical work recently published in the Journal of Chemical Physics by
Stuchebrukhov and Marcus. Finally, by carrying out IR-IR double resonance
experiments, we have been able to prepare almost saturated beams of v=3
molecules and compare the relaxation of molecules in which the energy is
localized in a single mode with those in which the energy is distributed
over two or more different modes.
Using a TiAl2O3 laser in new IR-near IR double
resonance experiments, we hope to access the "chemically relevant"
region around 2 eV while preserving the high resolution (a few MHz) obtained
so far. This should allow us to monitor the flow of internal energy
in molecules where chemical phenomena such as isomerization and ring opening
are occurring. The basic question which we are trying to answer is:
is the behaviour of molecules in transtion states purely random or is some
form of periodicity preserved? The relevance of these issues to bond
selective chemistry is quite clear.
The dynamics of the accommodation and sticking of a molecule to a chemically
active surface are only partially understood. For example, the change
upon vibrational excitation of the sticking (or reactivity) of a molecule
to (or with) a bare (or gas covered) crystal surface are largely not known. New,
molecular state selective, experiments in this area of surface science are
badly needed.
Using the techniques described above, we plan to pump to saturation a
beam of NH3 molecules from the ground vibration state to the
first overtone (v=2) state of the NH stretching vibration. The excited
beam will be made to impinge on the well-characterized surface of a Ni or
Fe crystal. The amount of chemisorbed NH3 will be measured
(in the presence or absence of excitation) by measuring the reflectivity
of the crystal surface for a beam of low energy helium atoms. A minimum
coverage of 0.0001 should be detectable in this way.
The UHV apparatus has been assembled and tested by measuring the adsorption
enthalpies and sticking coefficients of alkanes, alkenes and alkane thiols
on Au (111) as a function of the chain length. We have discovered that
the chemisorption energy of the thiol functionality on gold decreases by
increasing the size of the hydrocarbon residue attached to the sulphur atom
likely because of the repulsion between the metal electrons and the hydrocarbon
residue.
The sticking coefficient as a function of the vibrational state for NH3, C2H2 and other molecules should start soon.
Organic monolayers adsorbed on crystal surfaces are interesting for both
chemical and physical reasons. Most surface chemistry happens in or
on monolayers. Moreover, monolayers are two-dimensional systems with
peculiar physical properties which are especially interesting from the point
of view of phase transitions and lattice dynamics. Finally, monolayers
of long chain compunds provide excellent models for membranes and have important
applications in the field of corrosion protection and chemical sensors.
We study organic monolayers by means of low energy atomic beam diffration
and grazing incidence x-ray diffraction. A monoenergetic beam of atoms
or synchrotron radiation is produces and made to impinge on the surface
to be studied. The angular distribution of the diffracted beams is
measured and information on the surface structure and vibrations is extracted
very much as it is done with, say, electron diffraction, with the added
advantage of extreme surface specificity (zero sample penetration in the
case of atomic diffraction).
Recently we have, for the first time, succeeded in showing that the surface
of certain organic overlayers (made by long chain alkyl-thiols self-assembled
on gold (111) surfaces) is as organized as the surface of a single crystal. We
have also studied the dependence of the order at the surface on the length
of the hydrocarbon chain and the symmetry of the crystalline substrate. Finally,
we have studied the growth kinetics of these monolayers, discovering that
the formation process involves two steps in which the formation of the equilibrium
membrane-like phase is preceded by the formation of at least one lower density
phase in which the molecules lay down flat on the surface.
We now want to explore whether or not the surface structures of chains
with an even or odd number of carbon atoms are different and whether or
not order is also present when the last CH3 group is substituted
by other functional groups. When ordered surfaces will be available
with different functional top groups (such as double bonds for instance),
we plan to study their stereoreactivity towards an incoming beam of oxygen
atoms of variable energy and inclination with respect to the surface. The
stereoreactivity information so obtained is extremely difficult to obtain
by any other means. Furthermore, we will explore the influence of the
presence on the surface of a "solvent" layer on the growth kinetics
of monolayers made of either flexible or rigid molecules. Preliminary
results indicate that this "solvent" layer facilitates the formation
of the membrane-like phase. A new line of work (which is presently
starting in the framework of the new NSF Materical Science and Engineering
Research Center awarded to Princeton) is the study, conducted in collaboration
with the Electrical Engineering department, of the growth modes of organic
semiconductor thin films which have important optoelectronic properties
and applications.