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Electronic Imaging & Signal Processing

Fortuitous 'Accident' Captures Images of Electron Flight

Eye on Technology - ATOMIC SNAPSHOT

From oemagazine March 2005
28 March 2005, SPIE Newsroom. DOI: 10.1117/2.5200503.0001
A tomographic reconstruction of an orbital shows the probability density of regions where an electron is most likely to be found, with red showing the most positive value of the wave function and blue the most negative.

A technique that creates images of the positions of electrons in a molecule could lead to better understanding of chemical reactions, researchers say.

Jiro Itatani and his colleagues at the Steacie Institute for Molecular Sciences, National Research Council of Canada (Ottawa, ONT, Canada), used femtosecond laser pulses and computed tomography to make images of the electron orbital of a nitrogen molecule.

To achieve the effect, researchers hit a gas of nitrogen molecules with a 60-fs pulse of 800-nm laser light with an intensity of 2 x 1014 W/cm-2. The electrical field of the pulse oscillates, first driving an electron away from its atom, then causing it to reverse direction and smash into the atom. When the electron hits, it gives off radiation with a wavelength of about 0.14 nm.

Every atom or molecule has a series of orbitals, energy states where there is a probability the electron will be found. "The radiation that is emitted as the electron comes back, comes in multiples of the fundamental wavelength of the laser," says Dirk Zeidler, a post-doctoral fellow who was part of the research team. These multiples, called high harmonics, interact with the orbital and cause an interference pattern. By looking at the effect on the high harmonics, the researchers can see the underlying structure of the orbital. They look at the orbital in two dimensions from a number of angles and then use a computer to make a 3-D image of the orbital's shape (the same way doctors use computed tomography and x-rays to make a 3-D image of a human organ).

"For the first time ever, we're able to look at molecular orbitals on a femtosecond time scale," Zeidler says. "There are a number of other methods you can use, like x-rays and electron diffraction, but to the best of my knowledge none of those has reached this time resolution yet."

Good Chemistry

The technique affects only the highest-energy state of the electron - the highest occupied molecular orbital. In chemical reactions, it is the electrons in these orbitals that form or break bonds between different atoms. "This is the orbital that makes chemistry," Zeidler says. That fact, combined with the time resolution, could provide the ability to watch chemical reactions on the subatomic level as they happen, perhaps leading to new insights about how those chemical reactions work.

"The technique is completely novel and very promising," says Igor Litvinyuk, assistant professor of chemical physics at Florida State University (Tallahassee, FL). "If the full potential of this method is realized, we will be able to observe the evolution of electronic molecular wave functions as a molecule undergoes a chemical reaction on a femtosecond timescale."

Litvinyuk says there are other methods to measure electrons, but various forms of femtosecond spectroscopy are indirect. In addition, time-resolved electron diffraction is limited to hundreds of femtoseconds and measures the total charge density of the molecule, not the orbital itself. To really be useful, the pulses will have to last only 6 to 8 fs, he says, but that should not be too difficult to implement.

The next step will be to apply the technique to more complex molecules than nitrogen. The team may also look at whether they can study activity in attoseconds, the scale at which nuclear reactions take place, Zeidler says.

As is often the case in science, the snapshot of the orbital captured by the research team was an accidental discovery. The team was looking at how harmonic intensity changes with the alignment of the molecule. They had been studying their data for a year before they came up with the idea of applying tomography to it. "We were not looking for this at all when we took that data," Zeidler says.