Attosecond spectroscopy promises deeper understanding of chemical reactions

“The discipline, nonetheless, is exacting: everything that can be observed should be observed, even if it is only recalled as the bland background from which the intriguing bits pop out like Venus in the evening sky. The goal is always finding something new, hopefully unimagined and, better still, hitherto unimaginable.”
-K. Barry Sharpless, Nobel Lecture, 2001

Chemistry is the science of understanding reactions by stringing together fundamental steps into complicated transformations. As the science has advanced, the ability to parse out finer and finer details of reactivity has unveiled new horizons of understanding. Many physical chemists believe that the future of this finer understanding will be found in a technique called “attosecond spectroscopy.” “Attosecond” refers to the lifetime of the shortest pulses of light ever generated; creation of these light pulses by research groups at UC Berkeley allows scientists here to probe the workings of chemical reactions as they occur.

Sketch of Molecule

Fig. 1: Sketch of a molecule.

Chemical reactions consist of changes to molecules; these molecules are made up of atomic nuclei that are held together by a sea of electrons (Fig. 1). Physicists and chemists have become accomplished at understanding how these nuclei move during a chemical reaction, and have a variety of techniques to interrogate their positions.

Fundamentally, however, chemistry is all about the interactions of electrons; they bind nuclei together, and what we call a “chemical reaction” is really a set of motions of these electrons. Chemists use nuclei as tea leaves to try to understand what electrons are doing, but the nuclei are heavy and slow. The picture chemists intuit from the motions of nuclei, then, is based upon the time average of what those electrons have been doing. But what if they could observe electrons on their own terms, at their own timescale? Those experiments would reveal dramatically more about the nature of the matter around us.

In order to determine the motions of electrons, scientists need to be able to generate pulses of light that are short enough to avoid time averaging. That pulse length is called an attosecond. (“Atto” is the prefix for 10-18.) To put that time in context, consider this: the age of the universe is around 13.7 billion years, or 4.32 x 1017 seconds in scientific notation. Thus the difference in orders of magnitude between a second and the age of the universe is the same as the difference between an attosecond and a second.

HHG

Fig. 2: High harmonic generation in a gas cell.

The method used to generate attosecond light pulses is a technique called high harmonic generation, and it is currently being applied by the Neumark and Leone groups in Cal’s Department of Chemistry. Making short light pulses requires a photons to be stacked together like layer cake. Around 60 photons are combined to make one new, much shorter pulse. The medium in which photon stacking occurs is called a gas cell (Fig. 2).

To make a measurement, two attosecond pulses in quick succession are required: the first pulse starts the chemical transformation, and the second one measures what the system is doing at some time later. Varying the length of time between the first and second pulses time allows scientists to create movies that show the progression of an electron’s position around nuclei.

The future of this technique is incredibly bright. UCB’s chemists are forging ahead, and new installations like the Next Generation Light Source at Lawrence Berkeley National Laboratory promise to make attosecond spectroscopy more widely available. Led by attosecond spectroscopy, the coming decades promise enormous growth in our understanding of chemical reactions.

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