Using lasers to watch electrons in action
Canadian and U.S. scientists have used ultrafast lasers to take snapshots of electrons in action. The tools they've used work at a femtosecond time-scale -- a femtosecond lasts 10-15 seconds. But they want to observe the electron recollision process which occurs in the attosecond time-scale -- an attosecond is just 10-18 seconds. The researchers are approaching these somewhat incredible limits. As said one of the lead researcher, 'The Holy Grail in molecular sciences would be to be able to look at all aspects of a chemical reaction and to see how atoms are moving and how electrons are rearranging themselves as this happens. We're not there yet, but this is a big step toward that goal.' But read more...
You can see on the left how lasers are used to watch how electrons behave. "A laser beam first excites dinitrogen tetraoxide molecules, or N2O4, inducing large vibrations. A second laser beam then generates X-rays from the vibrating molecules." (Credit: CU-Boulder) Here is a link to the original version of this image.
This research project has been led by CU-Boulder physics professors and JILA fellows Margaret Murnane and Henry Kapteyn who manage the Kapteyn-Murnane Research Group and are president and vice-president of KMLabs Inc.. They also worked with colleagues in their lab and with scientist Albert Stolow of the Canadian National Research Council's Steacie Institute for Molecular Sciences.
The research team said that "understanding how electrons rearrange during chemical reactions could lead to breakthroughs in materials research and in fields like catalysis and alternative energy." And here is a short description of what they did. "they shot a molecule of dinitrogen tetraoxide, or N2O4, with a short burst of laser light to induce very large oscillations within the molecule. They then used a second laser to produce an X-ray, which was used to map the electron energy levels of the molecule, and most importantly, to understand how these electron energy levels rearrange as the molecule changes its shape."
Here are additional quotes from the researchers. "'This is a fundamentally new way of looking at molecules,' Kapteyn said. 'This process allowed us to freeze the motion of electrons in a system, and to capture their dizzying dance.' The researchers describe their process of stretching the N2O4 molecule as being similar to pulling on a Slinky toy and then letting it go and watching it vibrate. They used the N2O4 molecule because it vibrates more slowly compared to other molecules, allowing them to observe the physical processes under way. In many ways, molecules are like tiny masses connected by tiny springs of differing strengths, Murnane said.
This research work has been published online by Science Express under the title "Time-Resolved Dynamics in N2O4 Probed Using High Harmonic Generation" on October 30, 2008. Here is a link to the abstract. "The attosecond time-scale electron recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. Here, we show that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in N2O4, we show that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration, but populates the first excited state at the inner turning point. This state switching mechanism is manifested as bursts of high harmonic light emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals."
Sources: University of Colorado at Boulder news release, October 30, 2008; and various websites
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