Shaffer Research Group

Atomic, Molecular, and Optical Physics at the University of Oklahoma
 

"Spinning Top" Analogy to the Stark Effect

Think of a spinning top, spinning on the table. That is your molecule. Now remove the table and gravity and imagine it spinning while it is floating in the air.

Furthermore, imagine it having a positive charge on its one end, symbolized by a red dot somewhere on it and a negative charge on its other end, a black dot. As it spins, the charges, the red and black dots on it, spin together with it.

Now you apply a huge electric field to the top. The electric field might be generated by a large positively charged metal plate above and a large negative one below it. So, say one positively charged huge red metallic plate is now floating somewhere above the spinning top and one huge negatively charged black metallic plate is floating below the top. They are powered by a high voltage power supply.

The upper positive plate will attract the black dot on the spinning top (+ and - attract) and the lower will attract the red dot. But because the top is spinning it will not simply turn around to have the black dot as close to the upper plate as possible but will rather go into a kind of tumbling motion, rotating wildly in all kinds of directions. Now as we change the electric field, this motion will change. In the end, as the field gets very large, the spinning motion does not matter much anymore and the top simply succumbs and aligns itself so that the red dot is pointing down and the black dot pointing up.

The energy of the top inside the field depends on how fast it rotates and where on average the red spot is compared to the electric field direction (here: down). The quantum mechanical version of that energy dependence on the electric field (very roughly) is called "Stark" effect and can be used to slow down molecules. Think of changing the electric field to slow down a moving spinning top by using the forces on the red and black dots.

The reason why spinning tops (and especially asymmetric molecules) behave weird when outside forces are applied to them is because of the way torques work and change angular momentum. You have seen this when playing with a spinning top and it starts to tumble more and more weirdly as it gets slower and slower (precession). Or if you bang it while it is spinning fast and it also starts tumbling because of that (nutation).
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