This is a real shot of the atoms in a Nitrogen molecule (N2).

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COLUMBUS, Ohio – Using a new ultrafast camera, researchers have recorded the first real-time image of two atoms vibrating in a molecule.

Key to the experiment, which appears in this week’s issue of the journal Nature, is the researchers’ use of the energy of a molecule’s own electron as a kind of “flash bulb” to illuminate the molecular motion.

The team used ultrafast laser pulses to knock one electron out of its natural orbit in a molecule. The electron then fell back toward the molecule scattered off of it, analogous to the way a flash of light scatters around an object, or a water ripple scatters in a pond.

Principal investigator Louis DiMauro of Ohio State University said that the feat marks a first step toward not only observing chemical reactions, but also controlling them on an atomic scale.

“Through these experiments, we realized that we can control the quantum trajectory of the electron when it comes back to the molecule, by adjusting the laser that launches it,” said DiMauro, who is a professor of physics at Ohio State. “The next step will be to see if we can steer the electron in just the right way to actually control a chemical reaction.”

A standard technique for imaging a still object involves shooting the object with an electron beam – bombarding it with millions of electrons per second. The researchers’ new single-electron quantum approach allowed them to image rapid molecular motion, based on theoretical developments by the paper’s coauthors at Kansas State University.

A technique called laser induced electron diffraction (LIED) is commonly used in surface science to study solid materials. Here, the researchers used it to study the movement of atoms in a single molecule.

The molecules they chose to study were simple ones: nitrogen, or N2, and oxygen, or O2. N2 and O2 are common atmospheric gases, and scientists already know every detail of their structure, so these two very basic molecules made a good test case for the LIED method.

In each case, the researchers hit the molecule with laser light pulses of 50 femtoseconds, or quadrillionths of a second. They were able to knock a single electron out of the outer shell of the molecule and detect the scattered signal of the electron as it re-collided with the molecule.

DiMauro and Ohio State postdoctoral researcher Cosmin Blaga likened the scattered electron signal to the diffraction pattern that light forms when it passes through slits. Given only the diffraction pattern, scientists can reconstruct the size and shape of the slits. In this case, given the diffraction pattern of the electron, the physicists reconstructed the size and shape of the molecule – that is, the locations of the constituent atoms’ nuclei.

The key, explained Blaga, is that during the brief span of time between when the electron is knocked out of the molecule and when it re-collides, the atoms in the molecules have moved. The LIED method can capture this movement, “similar to making a movie of the quantum world,” he added.

Beyond its potential for controlling chemical reactions, the technique offers a new tool to study the structure and dynamics of matter, he said. “Ultimately, we want to really understand how chemical reactions take place. So, long-term, there would be applications in materials science and even chemical manufacturing.”

“You could use this to study individual atoms,” DiMauro added, “but the greater impact to science will come when we can study reactions between more complex molecules. Looking at two atoms – that’s a long way from studying a more interesting molecule like a protein.”

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Coauthors on the paper included Anthony DiChiara, Emily Sistrunk, Kaikai Zhang, Pierre Agostini, and Terry A. Miller of Ohio State; and C.D. Lin of Kansas State. Coauthor Junliang Xu pursued the theoretical side of this research to earn his doctorate at Kansas State, and will soon join DiMauro’s lab as a postdoctoral researcher.

Funding came from the U.S. Department of Energy Basic Energy Sciences Program.

The system relies on a recent technology called a streak camera, deployed in a totally unexpected way. The aperture of the streak camera is a narrow slit. Particles of light — photons — enter the camera through the slit and pass through an electric field that deflects them in a direction perpendicular to the slit. Because the electric field is changing very rapidly, it deflects late-arriving photons more than it does early-arriving ones.

The image produced by the camera is thus two-dimensional, but only one of the dimensions — the one corresponding to the direction of the slit — is spatial. The other dimension, corresponding to the degree of deflection, is time. The image thus represents the time of arrival of photons passing through a one-dimensional slice of space.

— MIT.edu | Read The Full Article

The original image of the illusion.
The squares marked A and B are the same shade of gray, yet they appear different.

The original image plus two stripes.
By joining the squares marked A and B with two vertical stripes of the same shade of gray, it becomes apparent that both squares are the same.

Why does the illusion work?

The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface.

The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.

As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.

This can be proven using the following methods:

• Open the illusion in a image editing program and use the eyedropper tool – both A and B will register an RGB value of 120-120-120.
• Cut out a paper mask –- by viewing the areas of the image in question without the surrounding context, the effect of the illusion is dispelled.
• Print the image and cut out the areas labelled A and B –- once again, viewing them out of context removes all doubt.
• Use a photometer.

The experiment itself was simple: Nicholas Christenfeld and Jonathan Leavitt of UC San Diego gave several dozen undergraduates 12 different short stories. The stories came in three different flavors: ironic twist stories (such as Chekhov’s “The Bet”), straight up mysteries (“A Chess Problem” by Agatha Christie) and so-called “literary stories” by writers like Updike and Carver. Some subjects read the story as is, without a spoiler. Some read the story with a spoiler carefully embedded in the actual text, as if Chekhov himself had given away the end. And some read the story with a spoiler disclaimer in the preface.

Wired | Read the Full Article