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Flow of Energy through Molecules

J. Mathias Weber is intrigued by the flow of energy through molecules during chemical reactions. Such energy flow is the molecular analog of heat conduction in materials. In this context, energy flow through nanoscale systems is of particular interest since technology using progressively smaller electronic devices can encounter the destruction limit of energy density.

On the quantum mechanical level, energy flow occurs via anharmonic coupling of individual vibrational modes of molecules. As a result, intramolecular vibrational relaxation has long been a field of study. Until now, most experiments in this field have dealt with the question of how long it takes for energy to drain out of an excited vibrational state. The Weber group uses a different experimental approach to monitor the flow of energy as it drains out of a certain vibrational mode and arrives in a well-defined region in a molecule. The group uses nitroalkanes as model systems for energy-flow studies because their electron affinities are well below the excitation energies for CH-stretching modes, and any excess charge is localized on the nitro group.

The group recently demonstrated that vibrational auto detachment from nitro-hydrocarbon anions occurs when the fundamental CH-stretching modes are excited. This experiment has opened the door to depositing vibrational energy into a specific CH-stretching mode from which it will travel to the nitro group, which is a well-defined region of the molecule. The group can "see" the arrival of energy in the nitro group when it triggers vibrational auto detachment via the excitation of NO2 vibrational modes. The Weber group is currently using photoelectron imaging spectroscopy to study the quantum mechanical details of the coupling of individual oscillations.