Molecular Physics Lab	Current Research
	Department of Physics & Astronomy - Texas Christian University - Fort Worth, Texas

• Carbon Clusters -- Astrophysical molecules, combustion, and buckyballs
• Transition Metal Carbon Clusters -- Nanomaterials and carbon stars
• Semiconductor Molecules
• Silicon-carbon Clusters -- Chemical vapor deposition, stellar atmospheres
• Free Radicals -- Combustion, semiconductor fabrication

Carbon Clusters
Astrophysical molecules, combustion, and buckyballs

Detail of the molecular cloud in the Orion Nebula. (NASA)

Chains of carbon atoms form the backbones of many of the molecules found in interstellar space and in the atmospheres of stars. Carbon rings and chains are the building blocks for more complex molecular structures, including biomolecules such as the amino acids, which have been found in meteorites and very recently, in interstellar space.

Although the majority of astrophysical molecules have been detected in space by their radio frequency spectra, the advent of space-based infrared telescopes points to the increased importance of the infrared characterization of astrophysically relevant carbon molecules.

At a more practical level, transient carbon molecules have crucial roles in the combustion of fossil fuels. Fullerenes, a class of carbon molecules with characteristic spherical or tubular structures that are of growing importance in the development of new nanomaterials, start with the condensation of small cyclic carbon clusters or chains.

The infrared Spitzer Space Telescope launched in August 2003. (NASA/JPL)

At the most fundamental level the question is how the structures and optical properties of carbon molecules evolve as the cluster size increases.

Our Fourier transform infrared studies characterizing small carbon clusters formed by the laser vaporization of graphite and trapped at 10 K in inert solids have resulted in the first infrared measurements of vibrations of the C₄, C₅, C₆, C₇, C₉, and C₁₂ chains. We have also successfully synthesized one of the fundamental units of the fullerenes, cyclic C₆, as well as cyclic C₈. Current work using recently developed techniques for isotopic substitution studies is focused on the identification of larger C_n clusters (n > 12), which theory predicts should all have cyclic structures.

Transition Metal Carbon Clusters
Nanomaterials and carbon stars

Interferometry of the carbon star IRC +10216 by Weigelt et al. Astronomy and Astrophysics 333, L51-L54 (1998).

The interaction of transition metals with carbon is of interest in the formation of novel nanomaterials including metcars and the catalytic growth of carbon nanotubes.

The discovery of simple diatomic molecules containing transition metals in the circumstellar shells of carbon-rich late stars like +IRC 10216 in which C_n carbon chains have been identified, is another reason for interest in the characterization of the spectra and structures of transition metal-carbon clusters MC_n.

Fourier transform infrared and theoretical studies have been initiated on clusters of carbon with Cr, Mn, Ni, and other transition metal atoms formed by laser ablation and condensed at low temperature, with the objective of determining their structures and signature vibrational spectra.

Semiconductor Molecules

The effort to characterize the geometries and electronic structures of small elemental semiconductor clusters is fueled in part by their applications in the semiconductor industry, but these nanoscale systems also offer the possibility for exploring size dependent effects arising from changes in structure and electronic states.

We have been investigating the low temperature spectroscopy of pure and mixed clusters of germanium, silicon, and carbon. The structures and vibrational spectra have been determined for a new group of germanium-carbon clusters, including GeC₃Ge, GeC₇, and GeC₉. The first cluster containing all three of the elements, GeC₃Si, has also been identified and characterized.

Silicon-carbon Clusters
Chemical vapor deposition, stellar atmospheres

Evaporation chamber for the preparation of silicon/ carbon samples by laser ablation.

Mixed silicon-carbon clusters have roles in the manufacture of silicon carbide thin films by chemical vapor deposition and in the atmospheres of late-type carbon stars.

The variety of their possible geometries also provides an important testing ground for theoretical predictions and our understanding of the evolution of cluster structures with increasing size. Following the measurement of new spectra of the known triatomics SiC₂, and Si₂C, our lab has detected and characterized for the first time a variety of Si_nC_m clusters including the SiC₄, SiC₃Si, SiC₄Si, SiC₇, and SiC₉ chains, rhomboidal Si₂C₂ and Si₃C and planar Si₃C₂.

These studies are providing insights into the structural transition as C atoms are substituted by Si, from the planar or linear geometries of the more carbon-like clusters to the 3-dimensional structures of silicon-rich clusters.

Free Radicals
Combustion, semiconductor fabrication

Model of the H2SiC2H radical.

Transient molecular species play critical roles in various industrial applications, but they are also important in the interstellar medium and in stellar and planetary atmospheres.

Primary attention has been focused on small hydrocarbon radicals such as C₃H and HC₃O for their interest in understanding combustion processes, but studies have also included silicon-bearing hydrocarbon radicals such as SiCH, SiC₂H, and H₂SiC₂H because of their potential importance in semiconductor fabrication.