Four-dimensional material maps: GAMERS method

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  • Published: May 10, 2017
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Four-dimensional material maps: GAMERS method

GAMERS system

Simulated 4-D GAMERS spectrum shown as a 3-D cut through one spectral axis. Two dimensions encode information on vibrational frequencies, while the remaining dimensions represent electronic transitions in the molecule. (Credit: Harel )

Subtle effects governing the chemical, physical and optical properties of systems can be extracted using the multidimensional coherent Raman spectroscopic method called "GAMERS".

Researchers at Northwestern University in the USA have developed a novel way to extract the static and dynamic structure of complex chemical systems, rather than the mere static three-dimensional structure of atomic positions and connections. Their method offers up the time-dependent quantum-mechanical degrees of freedom that dictate the optical, chemical and physical properties of the system. Just as a two-dimensional representation of a solid three-dimensional object loses a degree of information, so viewing a molecule with four-dimensions of information provides a more complete perspective and allows the team to uncover hidden features of molecular structure.

Writing in The Journal of Chemical Physics, chemist Elad Harel and colleagues describe details of the new 4D coherent spectroscopic method and explain how it directly correlates within and between electronic and vibrational degrees of freedom of complex molecular systems. The work builds on the theoretical description of a recent experimental method developed in the Harel lab, known as GRadient-Assisted Multi-dimensional Electronic Raman Spectroscopy, or GAMERS for short.

Multiple pulses

"Using multiple pulses of light, GAMERS probes how these different degrees of freedom are correlated with one another, creating a sort of spectral map that is unique to each molecule," explains Harel. "It demonstrates that subtle effects dictating the chemical, physical, and optical properties of a system, which are normally hidden in lower-order or lower-dimensionality methods, may be extracted by the GAMERS method." This view of a molecule's energy structure may open up a predictive capacity that is not available to other approaches.

"The shape of the potential surface, which is important for determining the kinetics and thermodynamics of a chemical reaction, may be directly measured," Harel adds. "The level of molecular detail afforded by using more pulses of light to interrogate the system was surprising."

Team players

The team suggests that a potential application of their GAMERS technique might be to pinpoint the physical mechanism of energy transfer during the earliest stages of photosynthesis, the process by which sunlight is used by green plants and certain microbes to convert carbon dioxide from the atmosphere into sugar molecules. This question has tantalised scientists for decades and a solution to the puzzles that remain and the resolution of various controversial mechanisms might open the way to artificial leaves that could be used to trap solar energy more efficiently than with any current system, plants excluded.

Until that time, the primary application of Harel's work is to enable insights into the physical mechanisms behind a host of quantum phenomena in a wide variety of chemical systems. Harel suggests that, "These include singlet fission processes, charge carrier generation and transport in hybrid perovskites, and energy transfer in pigment-protein complexes. Understanding these processes has important implications for developing next-generation solar cells."

Harel concedes that the GAMERS method is still in the early stages of development but iterative technical advances will make it widely applicable in the chemical physics community.

Related Links

J Chem Phys 2017, online: "Four-dimensional coherent electronic Raman spectroscopy"

Nature Commun 2017, online: "Quantum Coherence Selective 2D Raman 2D Electronic Spectroscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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