NRC Researchers Capture First Image of an Electron Orbital

Every holiday season the big Hollywood studios roll out their slate of major movie releases. Researchers from the NRC Steacie Institute of Molecular Sciences (NRC-SIMS) recently introduced their own action "film" and it may just get the attention of a number of viewers. The team published their results in the December 16^th issue of Nature.

The film represents a true landmark because it represents the first picture of an electron orbital or cloud, the area in which an electron moves. And, it's a true "action" film. Electrons move at speeds far faster than any car chase or even a speeding bullet and react with one another with unthinkable energy and violence. "We now have a method for filming the small, fast-moving and violent world of atoms and molecules, almost as we might film our own world with a conventional video camera," notes Dr. Paul Corkum, group leader for the NRC-SIMS femtosecond research group and one of the report's authors.

Femtosecond Science

How fast is a femtosecond?

Consider the following analogy. In a few months, many of us will adjust our clocks for Daylight Savings Time, in this case, moving clocks ahead by an hour. Since the minute hand of clocks makes one revolution per hour, moving clocks ahead can be done quickly with just quick turn of the dial. But, imagine if you had to move an imaginary "femtosecond hand" to bring the clock forward. An hour equals 360 million billion femtoseconds, meaning that you would have to turn the hand 360 million billion revolutions each spring. And, finally one makes the change just as fall arrives and then the "femtosecond hand" needs be moved 360 million billion times the other way!

Electrons, and changes that happen to them in relation to other molecules, are the basis of all chemical reactions. Creating a 3-D picture of an electron orbital is the first step towards creating images of how chemical bonds are broken and formed during reactions, an achievement with major implications for any industry where chemistry is involved, such as the design of new drugs. Dr. David Villeneuve of NRC-SIMS, also an author on the paper, comments that the ability to observe these changes "is to observe the essence of chemistry."

How did they do it?

To be able to catch up with these electrons, you have to use a special kind of extremely fast and intense laser which produces laser pulses measured in femtoseconds (see sidebar for definition of a femtosecond). Using a femtosecond laser, the team fired a pulse into a vacuum chamber filled with nitrogen gas. Just an instant earlier, another femtosecond laser had been fired to make sure all of the molecules in the gas were lined up in the same direction, in essence, making sure all the actors were lined up before proceeding with taking the picture.

The team deliberately targeted one of the outermost and loosely bound electrons which, with the help of the laser, was temporarily dislodged from the parent nitrogen molecule. Temporary in this case means about 1.3 fs, after which the electron came hurtling back towards the parent molecule. During the process, the electron gains a tremendous amount of energy from the laser and when it collides it creates an intense emission of light in the extreme ultraviolet range, which the researchers have termed high harmonics.

At first, the team was only interested in the unique qualities of these high harmonics, mainly the fact that the emission represents a powerful light that can be used as a research tool in the same way as femtosecond lasers are now being used.

Attention shifted when one member suggested that since the electron collision produced the emission, perhaps these high harmonics could give information about the shape of the orbital itself. In other words, this was more than just bright light and that, somehow, if analyzed correctly; this spectrum would reveal the actual underlying shadow of the molecular orbital.

Because of Heisenberg's Uncertainty Principle, an electron in a molecule does not occupy a single point in space. It is spread out in a cloud. Villeneuve explains it this way: "Imagine you have a car going around the race track and you take a picture every once in a while and you record the location of the car. Eventually you'll build up the shape the race track. So what we're really measuring is the race track," he notes. But measuring an object from one angle does not reveal the true shape of the object. The team used a widely-used technique in medical imaging, known as tomography. By rotating the molecules in the vacuum chamber, they were able to build up a three-dimensional picture of the molecular orbital.

He adds that, beyond the obvious significance of the project for chemistry, the project is incredibly interesting for its illustration of how science works. In the Nature paper, for example, the authors draw attention to the fact that their "seemingly unlikely" technique produced the result that it did. But, according to Villeneuve, this kind of scenario is played out again and again in science. "Would we have been able to produce these results if we knew this was where we wanted to go in the first place? I would say, highly unlikely. The results that you are seeing today come from the happy coincidence of several key breakthroughs, but it is not as if one logically followed from the next," he explains.