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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 16 February, 2000 Next Week's Question
Electrons emit photons in returning to ground state
Question
What causes the electron within an atom to emit photon packets? We were told to make a diagram of it. What causes the electron to skip down a level and emit photons?

Question
Electrons in an atom are constantly buzzing around their nucleus, following orbits that are predicted by quantum mechanics. These orbits are fuzzy (people often speak of an electron cloud), but you can associate with each a certain average distance from the nucleus. Also, to each average radius is associated a certain energy: this is the energy that you'd have to give the electron to free it from the nucleus.

A single electron would normally sit at the lowest energy state (also called the ground state) unless other electrons are already occupying it. In this state, the electron is closest to the nucleus. How does an electron get to a different state, corresponding to a different average radius and a different energy?

Well, if the electron is in the ground state it won't spontaneously jump to an excited state (it lacks the energy to do so). But if a photon with enough energy comes along, the electron will be happy to absorb it and get into the higher energy state. Once it's there, it can decay back to the ground state by emitting a photon just like the one it absorbed before. This happens because the system always tries to reach the lowest energy state, which is the most stable.

The first process is called absorption, the second spontaneous emission. But this isn't the end of the story... these processes are not the only ones that result in transitions between different states. It was Einstein who figured out that unless something else happens we'd end up with the electrons being in excited state most of the time, something that is not observed.

This 'missing' process is called stimulated emission. It happens when a photon collides with an atom in an excited state: the incoming photon 'forces' the electron to jump back to a lower energy state, and emits another photon. The end result is the emission of two identical photons and the return of the electron back to the lower energy state. This is the principle of operation of a laser (which stands for Light Amplification by Stimulated Emission of Radiation): one photon comes in, two photons come out.

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