Explosive Experiment Explores Escaping Electrons

Nothing in the universe stands still. The study of particle motion in molecules allows physicists to probe the fundamental properties of molecules and how they work, which is crucial to understanding the driving forces behind chemistry, biology, and pharmaceutical development. Researchers from Berkeley Lab, Kansas State University, and institutions in Germany, Australia, and Spain used a pulsed beam of photons from the ALS to ionize the electrons in a deuterium molecule, causing it to fragment in a "Coulomb explosion." The team then used position and timing data to construct a three-dimensional "photograph" of the simultaneous motion of all the electrons and nuclei at the moment of fragmentation. In the process, they discovered that even simple molecular hydrogen is full of surprises.

Making Molecules Explode

How do you make a molecule explode, and why would you want to? Typically, when physicists want to know what's happening inside a microscopic object, they blow it up to see what happens. Molecules ionize and explode when subjected to energetic radiation or particle impact. Using a variety of projectiles, this destructive approach has revealed the structure of atoms, molecules, and nuclei.

Molecular fragmentation can be initiated by collision with a charged particle, by exposure to a strong laser pulse, or by the absorption of a single photon. When a molecule absorbs enough energy, bonds are broken and ions can form, splitting the molecule into component parts (electrons and nuclei) that can react with neighboring molecules. A natural example of this process occurs in Earth's upper atmosphere, where solar ultraviolet radiation prompts molecular ionization and fragmentation to create a dense layer of free electrons in the ionosphere.

In the case of deuterium (a "heavy" isotope of hydrogen), a Coulomb explosion occurs in the rare event when two electrons—the glue holding the molecule together—are released (double ionization) and the two remaining nuclei explode apart in opposite directions because they both have positive charges and repel each other. Synchrotrons such as the ALS, with photon beams that are 100% linearly polarized, can generate the necessary extreme-ultraviolet radiation to initiate Coulomb explosions.

A molecule's geometry, and how it binds with other molecules, is determined by the internal motion (momenta) of its electrons and nuclei. Quantum mechanical theory can make predictions about the location or momentum of an electron, but determining the momenta of many particles at the same time has not been possible until now.

Artist's rendition of the momentum spectrometer. The deuterium molecules, prepared in a supersonic jet going from the bottom to the top, intersect the pulsed photon beam in the middle of the copper rings, which define a homogenous electric field. The large coils are a Helmholtz pair that establish a uniform magnetic field parallel to the spectrometer axis. The charged nuclei (ions) are guided to a position-sensitive detector to the left while the electrons spiral to a similar detector on the right in the magnetic field. Unlike the light electrons, the motion of the heavy ions is not significantly influenced by the magnetic field.

To study the motion of multiple particles, the researchers needed a delicate way to fragment a molecule while preserving some of its internal motion. Rather than smashing open a molecule with charged particles, they decided to insert a single photon, which has no mass or charge. Without disturbing the momentum of the particles inside the molecule, the photon quietly transfers its energy, like a cold virus sneaking into a cell, to "kick-start" the fragmentation process.

Using a pulsed beam of linearly polarized photons from the ALS, the team ionized a jet of deuterium molecules. They used deuterium instead of ordinary hydrogen because it's heavier, providing a thicker target for the photon beam. (A hydrogen nucleus only contains one proton, while a deuterium nucleus contains a proton and a neutron.) A photon strike usually ejects only one of the molecule's electrons, but can sometimes eject both of them, driving the two remaining equally charged nuclei apart in a "Coulomb explosion."

After the explosion, electric and magnetic fields accelerated the electrons and nuclei away from each other, where they drifted onto microchannel plate detectors. Ultrafast timing techniques ensured that the pairs of detected electrons actually came from the same molecule. For each charged particle, the position of impact on the detector and the flight time from the explosion were measured. With the data from this "microscope for motion" (momentum spectrometer), the team was then able to calculate the initial, simultaneous momentum of all four particles, and build a three-dimensional image of the fragmentation.

Results of this experiment show electron behavior that is strongly influenced by the separation of the nuclei at the instant the photon is absorbed. The escape directions of the electrons do not solely depend on the polarization of the synchrotron's photon beam. The reason for this electron behavior is likely hidden in the entangled motion of the electron pair in their initial molecular binding state.

3d view of fragmentation

Three-dimensional view of photon-induced fragmentation of a deuterium molecule, showing the angular distribution of one ejected electron in the plane containing the molecular and light polarization axes. Another escaping electron of the same energy is emitted upwards out of the plane. The direction of the molecular axis is given by the exploding nuclei (in green).

Quantum mechanical theory predicts an electron emission pattern shaped like a dipole, but the researchers observed highly structured angles of escape. Although the forces, charges, and angular momenta are known, the team's observations of particles simultaneously moving together in a few-particle system do not match theoretical predictions. This experiment provides insight into the quantum dynamics of many-particle systems, leading to a better understanding of countless physical and chemical processes. In this case, the experimental physicists are ahead of the theorists.

Nothing in the universe stands still, including molecular theory.

Research conducted by Th. Weber (Universität Frankfurt, Berkeley Lab, and Kansas State University); A.O. Czasch, O. Jagutzki, A.K. Müller, V. Mergel, H. Schmidt-Böcking, and R. Dörner (Universität Frankfurt); A. Kheifets (Australian National University); E. Rotenberg, G. Meigs, M.H. Prior, and S. Daveau (Berkeley Lab); A. Landers (Auburn University); C.L. Cocke and T. Osipov (Kansas State University); and R. Díez Muiño (Donostia International Physics Center and Unidad de Fisica de Materiales, Spain).

Research funding: Deutsche Forschungs Gemeinschaft; Bundesministerium für Bildung und Forschung; and U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: Th. Weber, A.O. Czasch, O. Jagutzki, A.K. Müller, V. Mergel, A. Kheifets, E. Rotenberg, G. Meigs, M.H. Prior, S. Daveau, A. Landers, C.L. Cocke, T. Osipov, R. Díez Muiño, H. Schmidt-Böcking, and R. Dörner, "Complete Photo-Fragmentation of the Deuterium Molecule," Nature 431, 437 (2004).

ALSNews Vol. 247, November 24, 2004

Explosive Experiment Explores Escaping Electrons

Making Molecules Explode

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