Originally Posted By: paul
Quote:
>>> THE ENERGY LEFT THE ATOM IT WENT INTO THE PHOTON <<<.


then why does this character say that the atom absorbed the kinetic energy of the photon?


Ok this is a QM thing and is reasonably complex I will try the layman simple explaination from wiki because I am not sure I can translate correctly in english down to this level.

Laser cooling is a form of Doppler cooling

Wiki link => http://en.wikipedia.org/wiki/Doppler_cooling

Quote:

Doppler cooling involves light whose frequency is tuned slightly below an electronic transition in an atom. Because the light is detuned to the "red" (i.e. at lower frequency) of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one applies light from two opposite directions, the atoms will always absorb more photons from the laser beam pointing opposite to their direction of motion. In each absorption event, the atom loses a momentum equal to the momentum of the photon. If the atom, which is now in the excited state, emits a photon spontaneously, it will be kicked by the same amount of momentum but in a random direction. The result of the absorption and emission process is a reduced speed of the atom, provided its initial speed is larger than the recoil velocity from scattering a single photon. If the absorption and emission are repeated many times, the mean velocity, and therefore the kinetic energy of the atom will be reduced. Since the temperature of an ensemble of atoms is a measure of the random internal kinetic energy, this is equivalent to cooling the atoms.

The Doppler cooling limit is the minimum temperature achievable with Doppler cooling.


So answering your question the character correctly says the atom absorbs the photon which is the red slightly lower energy photon.

Go along a little bit more and he will talk about the emission of a photon which is how the atom loses energy because the emitted photon because of QM laws has to be a slightly higher energy so the atom makes up this small deficit by giving up heat vibration energy and quantum vibration energy.

So the whole process requires absorbtion of a slightly lower energy photon and then the emission of a slightly higher energy photon thereby taking energy away from the atom.

You will also note there is a limit to this cooling even under QM laws (http://en.wikipedia.org/wiki/Doppler_cooling_limit)

The photon emission creates a vibration itself which in our big macro world we would call a recoil (like a gun) so again even using this we can't actually get to absolute zero just down very close to it.

Last edited by Orac; 08/23/12 06:18 PM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.