An Atomic (Alarm) Clock – Part 2

Doc Mike Finnley, Going Postal
Various minerals fluorescing under ultra-violet light. (From Wikipedia)

You might have been to a night-club or concert and seen your clothes light-up. That’s because the organisers of the said venues have had the stupid idea (in my opinion) of illuminating the place with UV-A lamps or ‘black lights’. What you are witnessing, in such cases, are the chemicals in the fabric of your clothes absorbing the short wavelength ultra-violet light and emitting a longer wavelength visible light.

This is pretty much the key to how a rubidium clock works. Except for one thing: we have to stop our clothes lighting-up when exposed to the UV.

Rubidium Metal, Absorption And Emission

Don’t let that heading put you off continuing to read. If I can (sort of) understand it you should have no problem.

Rubdium is a soft, silvery, alkali metal with a melting point of around forty degrees centigrade. It comes in two naturally occurring flavours: rubidium85 and rubidium87. Or, Rb85 and Rb87.

Rb85 is the most common at around 72% natural occurrence and Rb87 at 28%.

The ‘Rb85’ means that there are 37 protons and 48 neutrons. The ‘Rb87’ means that there are 37 protons and 50 neutrons. Both are know as isotopes of rubidium.

There are other interesting facts about rubidium but we don’t need to consider them to understand how an atomic clock works with the stuff.

First, let’s have a think about that tee-shirt lighting-up.

Doc Mike Finnley, Going Postal

In fig.1 (forgive the pencil work) we have a photon from the UV lamp – blue wiggly line – hitting an atom in the shirt. When it does so the atom releases a longer wavelength photon – red wiggly line. Why?

The process is called fluorescence and in the diagram on the right we can start to understand how this works.

I’ve drawn three horizontal lines labelled 1,2 and 3. These represent the orbits of an electron around its nucleus. The bigger the number the farther away from the nucleus the electron orbits. Like a planet around a star.

The little black dots represent electrons. When they are in position 1 they are said to be in their ground state. Chilling out – so to speak.

If a photon of the right energy hits one of these electrons it knocks it to a higher orbit, but it doesn’t stay there for long. If the electron were to be ‘knocked’ from A to B and then immediately return to A it would emit a photon of the same colour as the one that knocked it in the first place. Like an apple looks green or a tomato looks red. We have a basic absorption and emission of the energy. But that is not what is happening with our tee-shirt and energy conservation dictates that what goes in comes out!

When a photon (from the black-light) hits our tee-shirt it knocks it from position A to B. This is the interesting bit: instead of it falling back to A it slowly falls to C. When I say ‘slowly’ I mean a few nanoseconds – which is a long time atomically. By doing so it loses some of its energy as heat. It then falls rapidly from C to D and emits a longer wavelength photon – a different colour.

This process is called florescence. Okay, I’ll confess: you don’t really need to know about florescence to understand a rubidium clock, but you do need to think about absorption and emission.

In the photo below I’ve taken the ‘lid’ off of a X72 and powered it up so that you can see the rubidium lamp lit-up. Think of it as a UV black-light. It glows a soft pinkish colour. Inside the larger metal box on the right is a metaphorical tee-shirt.

Doc Mike Finnley, Going Postal

Referring back to fig.1 you can see that I’ve drawn in a magic switch that’s connected to the shirt.

Now imagine this: if I pushed that switch exactly once per second the photon from the black-light would no longer cause the tee-shirt to fluoresce. The electron simply jumps from A to B and back to A emitting the original black-light. If I push the switch a little faster or slower then the electron jumps from A to B to C and back to D and the tee-shirt fluoresces.

If we could do that we would have found a way to use an atom to measure time. An atomic-clock.

In part 3 we’ll look at how rubidium can do just that and explain a bit more about what’s going on in the above photo.

© Dr Mike Finnley 2018

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