3. Molecular Beam Spectroscopy

Why:

There are three basic reasons which motivate the use of molecular beams in spectroscopy:

  1. By crossing the molecular beam at right angles with a laser, Doppler broadening is largely eliminated, so that, at the high level of resolution achievable in this way, it becomes possible to study the spectra of large polyatomic molecules and their perturbations.
  2. Beams are prepared from jet expansions in which molecules cool down to very low temperatures with consequent loss of rotation congestion and great simplification of the spectra.
  3. Due to the lack of collisions, the molecules are not perturbed, and large concentrations of unstable species (clusters, radicals) can be prepared and used in spectroscopic experiments.

How:

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.
 

So Far:

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.
 

What Next:

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.
 

4. Chemisorption

Why:

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.
 

How:

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.
 

So Far:

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.
 

What Next:

The sticking coefficient as a function of the vibrational state for NH3, C2H2 and other molecules should start soon.

 

5. Monolayers

Why:

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.
 

How:

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).
 

So Far:

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.
 

What Next:

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.


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