Understanding evolution energy charge onmolecules
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Understanding evolution of energy and charge on the molecules

1:41 AM, 9th January 2012
Understanding evolution of energy and charge on the molecules
An impressionistic look at photosynthesis: left, the oxygen-evolving complex in photosystem II (Yachandra/Yano lab); right, electronic energy transfer in photosystem II’s light harvesting complex as simulated by supercomputers at NERSC, Graham Fleming group.

CALIFORNIA, US: Ali Belkacem, Atomic, Molecular and Optical Sciences Program Leader, Berkeley Lab’s focus on creating better ways to track the evolution of energy and charge on the molecular level. For this purpose, he made use of ‘nonlinear multidimensional spectroscopy’. Belkacem points to the work of Graham Fleming, founder of Berkeley Lab’s Physical Biosciences Division. Fleming has tracked energy flow in photosynthesis, demonstrating the electronic coherence among structures in the photosynthetic reaction centres that transform sunlight energy into chemical energy.

“We want to do this same kind of chemistry with X-rays instead of visible light, because understanding photosynthesis and other complex systems means learning how electronic charge is transferred among specific atomic sites, in particular by grasping how valence states are correlated,” said Belkacem. X-rays and energetic ultraviolet light can probe the core electrons of atoms, which uniquely identify atomic species in a way that valence electrons can’t. Synchrotron light sources can produce coherent (laser-like) light in the desired soft X-ray region, but today’s light sources can’t achieve the enormous peak photon flux, the peak brightness needed to realize the full potential of nonlinear multidimensional spectroscopy.

At present, Belkacem’s team generates X-rays with a ‘table-top’ laser. They begin by focusing an intense laser beam on a cell containing gas under high pressure. The laser excites electrons in the gas and recaptures their energy, yielding very short pulses of high-frequency X-rays at multiples (harmonics) of the laser’s original energy. While high-harmonic generation with Belkacem’s laboratory-scale laser can generate high-energy pulses, beam intensity is low and the pulse repetition rate is limited to 50 hertz.

“X-rays excite an atom’s core electron, that’s both the strength and the technical challenge of multidimensional X-ray spectroscopy. Since the chemistry is in the valence electrons, somehow you have to use X-rays to involve the core electrons for atom specificity or the charge migration,” said Belkacem. To assess critical needs for future light sources, the team will first examine the electronic states of neon, which he calls ‘an ideal test bed’ to prove the feasibility of the approach. In the experiment, an X-ray photon tuned to the energy of the 2s shell will boost an electron into the neon atom’s previously unoccupied 3p shell. This needs energy on the order of 45 electron volts – a factor 20 to 30 times higher than typically used in the visible-light range.

“In our final experiment, we’ll use three beams. The first pair will excite the atoms we want to follow in time, and the third will let us track how the valence states at different sites in the compound are coupled as the process evolves in space,” said Belkacem. Belkacem’s laser studies have broke ground for a new way of doing chemical research in the lab. One-of-a-kind new technology on the laboratory scale will open a new path to crucial experiments at chemistry’s leading edge.

© Berkeley laboratory News



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