New Spectroscopy Technique Probes Photochemical Reactions

An experimental spectroscopic technique combines IR and electronic technologies to probe the relationship between electrons and atomic nuclei within molecular systems excited by light.

The new technique, developed by researchers at Lawrence Berkeley National Lab and the University of California, Berkeley, is called 2-D electronic-vibrational spectroscopy (2-D EV). It is the first that allows simultaneous monitoring of electronic and molecular dynamics on a femtosecond time scale.

Photochemical reactions have become key players in high technology, namely in the development of nanomaterials and solar energy systems. Better understanding of these light-triggered processes could lead to more efficient use.

“We think that 2-D EV, by providing unprecedented details about photochemical reaction dynamics, has the potential to answer many currently inaccessible questions about photochemical and photobiological systems,” said lead researcher Dr. Graham Fleming, vice chancellor for research at UC Berkeley and a faculty senior scientist with Berkeley Lab's Physical Biosciences Division.

With 2-D EV, three femtosecond laser pulses are used to image a sample. Two of the pulses are in the visible spectrum and generate excited electronic states within the sample; the third pulse is in the mid-infrared spectrum and analyzes the vibrational quantum state of the excited system.

This combination of visible excitation and mid-IR probe pulses demonstrates the correlation between the initial electronic absorption of light and the subsequent evolution of nuclear vibrations. This link has not been investigated previously, Fleming said.

2-D EV spectral data offers a better understanding of photochemical reactions and how such processes can benefit various applications. Courtesy of the Graham Fleming research group (Berkeley Lab/UC Berkeley).

“Combining these two techniques into 2-D EV tells us how photoexcitation affects the coupling of electronic and vibrational degrees of freedom,” said Dr. Thomas Oliver, a chemist and researcher in Fleming’s research group. “This coupling is essential to understanding how all molecules, molecular systems and nanomaterials function.”

In particular, 2-D EV could be useful in studying rhodopsin, the primary light detector in the retina, as well as carotenoids, the set of pigment proteins (such as chlorophyll) that exist in green plants and phototrophic bacteria.

The new technique could also assist in the study and development of nanomaterials, the researchers said, particularly in exploring how the coupling of phonons with electrons impacts the properties of carbon nanotubes and other nanosystems.

“We are continuing to refine the 2-D EV technology and make it more widely applicable so that it can be used to study lower frequency motions that are of great scientific interest,” Oliver said.

The work was funded by the U.S. Department of Energy’s Office of Science and the National Science Foundation. The research was published in Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1409207111).

For more information, visit www.lbl.gov.