M. Balooch

The interaction of a collisionless beam of thermal C₆₀ nanoclusters with a silicon dioxide surface has been investigated with modulated molecular beam‐mass spectroscopic techniques. Analysis of the amplitude and phase lag of the desorbed C₆₀ shows the interaction mechanism to involve the elementary steps of sticking, desorption, and long‐range surface diffusion. Surface diffusion coefficients determined in this measurement indicate that surface C₆₀ nanoclusters approach two‐dimensional gas‐like behavior. The sticking probability of C₆₀ clusters on SiO₂ is determined to be unity. The best fit desorption rate constant k_d for C₆₀ from SiO₂ is 5×10¹⁰ exp(−23 kcal/mol/RT) in agreement with previous temperature programmed desorption experiments. The efficiencies of electron impact ionization of C₆₀ to C₆₀⁺ and C₆₀⁺⁺ is measured for electron energies from 10 to 105 eV for neutrals at 875 K.

Three duplex DNAs 22, 47, and 100 base-pairs in length have been imaged with the scanning tunneling microscope (STM) after deposition on highly oriented pyrolytic graphite (HOPG). Images of the 47 base-pair (bp) molecules are resolved sufficiently to identify the two phosphodiester strands, the direction of helical coiling (this molecule contains three turns of left-handed helix), and single-stranded ends. Length measurements indicate that all three DNA sequences have adopted an "A-like" conformation. DNA-protamine complexes were also prepared and imaged under similar conditions. Length measurements of the complexes demonstrate that the binding of bull protamine 1 to the 47-mer stabilizes the DNA in a B conformation and prevents the B to A transition that has been shown to occur as the DNA molecules dehydrate on the surface. Measurements of the diameter of the complex (3 nm) were also obtained and were found to be only slightly larger than the DNA molecule. This observation is consistent with the binding of the protamine molecule inside one of the grooves.

The scanning tunneling microscope has been used to obtain images of DNA that reveal its major and minor grooves and the direction of helical coiling, but sufficient resolution has not yet been achieved to identify its bases. To determine if this technology is capable of identifying individual DNA bases, we have examined the molecular arrangements of adenine and thymine attached to the basal plane of highly oriented pyrolytic graphite. Both molecules form highly organized lattices following deposition on heated graphite. Lattice dimensions, structural periodicities, and the epitaxy of adenine and thymine molecules with respect to the basal plane of graphite have been determined. Images of these molecules at atomic resolution reveal that the aromatic regions are strongly detected in both molecules while the various side-groups are not well-resolved. These studies provide the first evidence that tunneling microscopy can be used to discriminate between purines and pyramidines.

Thin films of β-SiC were grown on Si substrates by excimer laser pulse ablation of bulk SiC. The films were examined by Auger electron, x-ray, and photoelectron spectroscopies and laser ionization mass analysis techniques. The film was smooth as monitored by scanning electron microscopy. Scanning electron and scanning tunneling microscopy (STM) showed inclusions in the deposited SiC films, and laser ionization mass analysis detected SiC dimers in the vapor plume emitted from the target.