IR Exposes Gas-Phase Metal Carbides

Daniel S. Burgess

Researchers have produced the first direct infrared spectra of the gas-phase metal carbide clusters Ti₈C₁₂ and Ti₁₄C₁₃. The vibrational spectra, which reflect the atomic forces that give a molecule its structure, support earlier conceptions of the clusters and may advance their use in catalysis.

The group, from the FOM Institute for Plasma Physics Rijnhuizen, the University of Nijmegen in the Netherlands and the University of Georgia in Athens, employed infrared resonance-enhanced multiphoton ionization spectroscopy. This technique, which is more sensitive than traditional infrared absorption spectroscopy by orders of magnitude, can measure the spectra of a sample that contains only a few thousand molecules.

The team, which detailed the study in the Dec. 13, 1999, issue of Physical Review Letters, vaporized titanium in a CH₄-seeded argon expansion. The researchers extracted the desired cluster species by ionizing the molecular beam, and a 5-µs "macropulse" (actually a series of 1-ps pulses with 1-ns spacing) from the institute's free-electron laser excited the clusters with 20 to 30 mJ of 6- to 25-µm light. A time-of-flight mass spectrometer measured the molecules' response.

Researchers used a free-electron laser to produce infrared spectra of the gas-phase metal carbide clusters Ti₈C₁₂ and Ti₁₄C₁₃. The former is believed to be tetrahedral (left), the latter cubic (right).

"The free-electron laser -- Felix -- is essential for the experiments that we have done," said Gert von Helden, a researcher at the institute. "It combines several features that, in their combination, are not available from other laser systems." Specifically, the structure of the tunable laser's macropulse allows a sample to absorb, redistribute and absorb more photons, and so remain in resonance with the excitation source.

Avoiding anharmonics in the vibrational mode of the samples was particularly important. A difficulty in studying exotic clusters such as titanium carbide is that relatively few of them are produced in the ablation process: Only about 10,000 to 1 million molecules find their way into the sampling beams. "So many photons have to be absorbed that it was a priori not clear if that is possible," said von Helden. "Maybe the biggest surprise was and is that the experiment works at all."

The spectra showed that Ti₈C₁₂ displays carbon-to-carbon bonding while Ti₁₄C₁₃ does not. This suggests that the former has a tetrahedral structure and that the latter is cubic, with only metal-to-carbon bonding.

A better understanding of the structure of the metal clusters may have implications for catalysis and for other aspects of chemistry and materials science. "Clusters can have a profoundly different chemical reactivity than the bulk or the atom," said von Helden, "and that can be and is put to use in the chemical industry."

The researchers plan to continue their investigation of metal carbide clusters and to extend it to include metal oxides and nitrides. "Some of these might have some relevance in astrophysics, especially for the formation of cosmic dust," he said. They already have used their data to identify TiC nanocrystals surrounding several stars.