Monday, June 25, 2007

Visualization of Single Electrons


Yes, you can believe your eyes. The above image has captured the movement of a single electron through liquid helium, with a velocity of 6 cm/s. Researchers at Brown University have captured these traveling electrons on film in a recent Journal of Low Temperature Physics article. (You can also watch a movie on the Maris group website).

How does this work? Well, the Pauli Exclusion Principle tells us that a free electron injected into liquid helium would experience some repulsion from the atom's own bound electrons. Thus, the electrons are encapsulated by so-called "electron bubbles." Electron bubbles can't be seen under normal pressure because of their tiny size (on the order of 10-23 cm2, but utilizing soundwaves to explode and expand the electron bubbles, Guo and Maris were able to actually see the movement of electrons through a 6.8cm long helium cell that was kept at an extremely low temperature (1.3o K which is approximately -457o F or -272o C). Although most of the observed bubbles traveled in a straight line, some of them followed a snakelike path, most likely following a superfluid vortex.

One question still remains--Where are these electrons coming from? As the authors point out, the liquid helium cell does not contain a source of electrons. One theory is cosmic rays might cause ionization of helium atoms, producing UV photons. As these photons hit the cell wall, it is possible that electrons could be ejected into the helium due to the photoelectric effect. A simpler explanation might be that some other kind of charged particles (such as muons) simply ionize helium atoms, producing positive and negative particles. While most of these charged particles would recombine, some might escape, allowing researchers to track their movement.

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