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Chemistry gets a new set of eyes

14.09.2009
Imaging electron movements that cause chemical reactions is now possible by using high-speed lasers

Electrons can initiate chemical reactions at such breakneck speeds—often within a trillionth of a second—that the details of their movements remain largely inscrutable. But now, RIKEN researchers have developed a laser spectroscopy technique with unprecedented time resolution that can trace the paths of electrons during a photochemical reaction.

According to Toshinori Suzuki from the RIKEN Advanced Science Institute in Wako, the principal author of the study, this new method promises chemists a clearer view of chemistry and molecular functionality than ever before.

“Chemical reactions are after all just the motion of nuclei pushed by electrons,” says Suzuki. “Therefore, the key is to know why and how electrons push them.”

A need for speed

In their latest work, Suzuki and his team tackled a fundamental chemical problem: how do electrons move in a molecule after coming into contact with light? Answering this question can provide insight into many molecular reactions, including the stability of DNA exposed to ultraviolet light.

When DNA absorbs ultraviolet radiation, it must quickly disperse this excess energy through internal conversion, a process where electrons jump from higher to lower energy states, to prevent formation of free radicals that can split this vital polymer apart.

After undergoing internal conversion, electrons in DNA stimulate the nuclei to move, which releases the excess energy. These nuclear movements take place through what is known as a conical intersection —a quantum mechanical version of a funnel—that ensures DNA rapidly returns to its original state without free radicals.

For the past ten years, Suzuki has wanted to study internal conversion through conical intersections, but was hampered by technical limitations. “This is one of the fastest electronic deactivation processes,” states Suzuki. “To observe it in real-time, we needed lasers with as short a pulse duration as possible.”

The right tools for the job

To visualize electron movements, Suzuki and colleagues pioneered the development of time-resolved photoelectron imaging2. This technique uses two lasers that emit light in quadrillionth of a second, or femtosecond, bursts—faster than atoms can move—to track the distribution of electrons in a molecule during a photo-reaction.

The first laser burst, called the pump, excites an electron in a molecule from a lower to a higher energy state. This photo-excitation initiates internal conversion in the molecule. Then, the second laser burst, called the probe, ejects an excited electron from the molecule, creating an easily detectable photoelectron.

The signal of photoelectrons is strongest immediately after the pump burst: this is when the most electrons are moving in the molecule. Capturing this short-lived signal requires an exceptionally brief time delay—on the order of 20 femtoseconds—between the probe and pump lasers.

Depending on the pump–probe time delay, photoelectrons are generated from different molecular orbitals—finite regions of space with distinct energies and symmetries. Suzuki developed a two-dimensional detector that records the photoelectron angular distributions (PADs)—the ejection angles from molecular orbitals—and kinetic energies to disentangle the measured signal into separate orbital pathways.

“The PADs carry information about the three-dimensional distribution of electrons in a molecule,” says Suzuki. “They give us direct knowledge about how electrons are moving around.”

Seeing pyrazine in a new light

Using these high-speed tools, the researchers mapped out how electrons in a molecule called pyrazine, a hexagonal ring containing four carbon and two nitrogen atoms, initiate movement through a conical intersection when excited by ultraviolet light.

According to Suzuki, there were two main types of molecular orbitals in pyrazine involved in the photochemical reaction. One, called the pi orbital, was delocalized around the 6-membered pyrazine ring. The other, called a non-bonding orbital, was localized on the nitrogen atoms.

The researchers used the pump laser to excite one of the pi orbital electrons into a higher energy state. This created a vacancy in the pi orbital, which, within 24 femtoseconds, was filled by an electron from the nitrogen non-bonding orbital.

Because the non-bonding electron is energetically higher than the pi electron, it carries an excess of energy when filling the vacancy. The excess energy dissipates through molecular vibrations that funnel the nuclei through the conical intersection.

Picture-perfect detection

Suzuki says that because the kinetic energy distribution of photoelectrons did not change greatly during the experiment, only the PADs could provide unique evidence of internal conversion and fingerprint the participating electronic states.

A two-dimensional map of electron movements was created by plotting how the photoelectron asymmetry parameter—the ratio of signal intensity at any two ejection angles—changed with pump–probe time delay and kinetic energy.

The initial fast movement of an electron from non-bonding to pi orbital in pyrazine was identified on the map as a change in photoelectron asymmetry parallel to the laser light polarization—indicating an excited electron created by internal conversion from a higher to a lower energy state.

Processes where excited electrons were ejected from a higher energy, without internal conversion, produced asymmetry regions on the map oriented perpendicular to the probe laser. These graphic illustrations of polarization gave the scientists a real-time view of electron distributions.

Eyeing success

Now that chemists have a new set of eyes to observe fast electron movements, Suzuki expects a more complete picture of complicated chemical reactions to emerge.

“It is not rare that electronic states change several times during single chemical reactions,” states Suzuki. “Our method allows us to see such changes more clearly than ever.”

“And,” he says, “such important concepts come along not only with sweat and tears, but with beautiful images as well.”

The corresponding author for this highlight is based at the Chemical Dynamics Laboratory, RIKEN Advanced Science Institute.

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1. Horio, T., Fuji, T., Suzuki, Y.-I. & Suzuki, T. Probing ultrafast internal conversion through conical intersection via time-energy map of photoelectron angular anisotropy. Journal of the American Chemical Society 131, 10392–10393 (2009). | article |

2. Suzuki, T. Femtosecond time-resolved photoelectron imaging. Annual Review of Physical Chemistry 57, 555–592 (2006).

About the Researcher

Toshinori Suzuki

Toshinori Suzuki was born in Yamagata, Japan, in 1961. He graduated from the Department of Chemistry, Faculty of Science, Tohoku University, in 1984, and obtained his PhD degree in 1988 from the same university. He then became a research associate at the Institute for Molecular Science (IMS) in Okazaki, between 1988 and 1990, and a JSPS fellow for research abroad to carry out research on molecular beam scattering at Cornell University and the University of California, Berkeley, between 1990 and 1992. He returned to Japan as an associate professor at IMS, where he started his independent research group on chemical reaction dynamics in 1992. He moved to RIKEN to become a chief scientist and the director of the Chemical Dynamics Laboratory in 2002. He has received the Broida Award from the international symposium on free radicals, the JSPS Award, and the IBM Science Award, the Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology for his achievements in chemical reaction dynamics.

Saeko Okada | Research asia research news
Further information:
http://www.rikenresearch.riken.jp/eng/hom/6042
http://www.researchsea.com

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