The pulse of speed

A theory is taking shape which might enable us to observe the motion of electrons

Published: Saturday 30 September 1995

-- (Credit: Shri Krishan)IMAGINE a pulse of light which lasts for only a few hundred billionths of a billionth of a second (10-18 seconds, or an attosecond). This is about the time that light travelling at 300 million metres per second takes to move across an atom!

If the calculations of some theorists are correct, it would be possible to generate such a short-lived pulse. Currently the fastest pulse has a duration of about a ferntosecond, which is 1,000 times longer than the attosecond pulse. Shortening of the pulse time is a difficult task. The pulse has to be composed of many different frequencies, all of which need to be coherent or in phase. The pulse has to be strong enough to be of use and the consecutive pulses have to be separated by about a millionth of a second (Science, Vol 269).

The theoretical ideas of Paul Corkurn and his colleagues at the National Research Council at Ottawa, Canada, have the potential of meeting these challenges. These researchers plan to use a pulse from a titanium sapphire laser to ionize a gas of xenon or argon. The laser light will generate an oscillazing electric field which will force'the electrons in the ionized gas to move at extremely high energies. These electrons will collide with their parents to generate a high energy photon.

Limiting the number of gas atoms used and making use of special metallic filters, Corkurn plans to generate a pulse of ultraviolet'light lasting only about 400 attdseconds. These short-lived pulses could prove to be a wonderful tool to study natural phenomenon which occur at such short time scales. For instance, a laser firing attosecond pulses could be used as ,a high speed flash to "see" the motion.%)f electrons, the formation of chemical bonds or even the formation of molecules.

The researchers feel that though no group has all the equipment required to generate an attosecond pulse and mea- sure it, it is only a matter of a couple of years before it becomes a reality and we are able to actually see the electrons in motion around the nucleus.

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