Awards Database

Cottrell Scholar Awards - 2014

Joseph E. Subotnik

Assistant Professor of Chemistry, University of Pennsylvania

What Every Undergraduate Should Learn About Electronic Relaxation

Subotnik is investigating ways to relax one of the hardest-working bits of nature in the modern world, the tiny electron, a negatively charged particle normally associated with the atom.

Increasingly we rely on high-energy electrons in all sorts of innovative devices and processes -- artificial photosynthesis, photovoltaics, organic light-emitting diodes (LEDs), and photocatalysis (the use of light to trigger chemical reactions). Oddly, electrons have become such exemplary little workers that getting them to relax after a hard day at the office has become a major issue for science and technology.

“In all cases, for an efficient device, one must control the relaxation of electrons, forcing their energy to be released through the correct channel,” Subotnik said, noting there are several ways to soothe the excited particles:

“For photovoltaics, relaxation should occur through electron transfer (moving a free electron far away from its natural habitat), rather than through radiation (light emission) or vibrations (frictional heat). For LEDs, relaxation must be through radiation and not through electron transfer or heat. For photocatalysis, relaxation needs to be through a combination of vibrational motion (breaking old bonds, forming new ones) and electron transfer, but not through radiation.”

It turns out that getting electrons to relax is a messy business. For example, after an electron is excited by absorbing at photon (light particle), “there are so many avenues for relaxation that an exact calculation is often impractical,” Subotnik said. “As a result, undergraduate physics majors usually avoid the subject entirely and study only atomic emission of photons; chemistry majors learn that electrons can relax through heat, but the concept is usually vague and never quantitative.”

The focus of Subotnik’s research is to bridge the many gaps between chemists and physicists when it comes to thinking about electron relaxation.

To do so he is employing state-of-the-art techniques to model nonadiabatic (frictional) relaxation through the prism of surface hopping (a branch of physics predicting how electrons move among atoms). He will investigate how accurate trajectories can be computed to reveal the essence of electron relaxation and the role of decoherence (the process by which an electron's relaxation pathway becomes irreversible).

RCSA’s Cottrell Scholar Award also funds Subotnik’s goals as a teacher. It will further his efforts to incorporate computational modeling into his department’s undergraduate curriculum and build up a course on computational chemistry.

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