Researchers Develop New Technique for Extracting 4D Molecular Structure

This is a 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.

A new technique for extracting the dynamic and static structure of complex chemical systems has been developed by scientists at the Northwestern University. According to this research, “structure” means not only the 3D arrangement of atoms that form a molecule - it means time-dependent quantum-mechanical degrees of freedom governing the physical, chemical, and optical characteristics of the system.

We perceive the world to be three dimensions in space and one dimension in time, that is, space-time. When any one of the dimensions is removed, the view turns out to be incomplete and more chaotic. That is why the new technique adopts four spectral dimensions to resolve a structure to unveil the unrevealed characteristics of molecular structure.

An innovative 4D coherent spectroscopic technique with the potential to directly correlate between, and within, vibrational and electronic degrees of freedom of complex molecular systems has been reported by assistant professor Elad Harel and professor Irving M. Klotz, from the Department of Chemistry at Northwestern University. The research appeared in The Journal of Chemical Physics, published by AIP Publishing.

Harel’s study includes a theoretical illustration of a recent experimental technique known as Gradient-Assisted Multi-dimensional Electronic Raman Spectroscopy (GAMERS) created in his lab. The technique is a multidimensional coherent spectroscopic technique, where the dimensions are the vibrational and electronic degrees of freedom of the system.

Using multiple pulses of light, GAMERS probes how these different degrees of freedom are correlated to one another, creating a sort of spectral map that is unique to each molecule. 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.

Assistant Professor Elad Harel

In contrast to other techniques, the new technique provides an in-depth knowledge of the molecules’ energy structure in a manner that might provide predictive value.

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

Assistant Professor Elad Harel

Harel said that one main usage of GAMERS might be to indicate the physical mechanism behind the transfer of energy during the preliminary phases of photosynthesis—a matter that is still contentious among researchers.

At present, the main implementation of the study is “to enable insights into the physical mechanisms behind a host of quantum phenomena in a wide variety of chemical systems,” as stated by Harel.

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.

Assistant Professor Elad Harel

Harel said that the GAMERS technique is only in its early stage of development, but the researchers believe the technique will prove significant in future applications.

We believe technical advances could make such analysis far more widespread within the chemical physics community.

Assistant Professor Elad Harel

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