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## Main Question or Discussion Point

i was learning about laser cooling in thermodynamics, and my professor said that as the photon gets absorbed by the electron, the electron gets excited and re-emits the energy in the form of a photon, and that this process was continuous since it's getting hit by a continuous laser beam and that the emission process emitted the photons in no preferred direction so the average reaction momentum (the kickback momentum from emitting the photon one way) is zero. He said that the atom moved when absorbing the photon since the photon has momentum and it is this momentum that is being absorbed that moves the atom. (we were just talking about what happened for a single atom in a vacuum being hit by a continuous laser beam of the appropriate wavelength)

we have went back and forth various times now via email and i am still completely lost. The photon has energy hf. This gets absorbed by the electron so this particular quantity went into exciting the electron into a higher potential orbital. But now there is a momentum component that gets transferred.

What???

i thought the energy of a photon was hf. and all of this went into exciting the photon. if the acceleration/propelling of the atom came from the photon, the photon MUST have an additional energy component, because it takes energy to do so. I'm trying to avoid using momentum and i'm trying to put everything in terms of energy; it's just easier for me to keep track of everything when all the stuff i'm dealing with are in the same forms (energy) (yes i understand i'm not dealing with just a single energy form, i was just referring to the fact that i don't have a mix of an energy term, amomentum term, a torque term, a moment of inertia term, etc etc etc...). So my professor said that there is a momentum component that gives rise to the acceleration of the atom. I tried converting this 'momentum piece' in terms of energy and i asked him if then, the TRUE energy of a photon is hf+(1/2)pc

and he replied that it was not. I am still at a loss... where is the energy to accelerate the atom coming from? If all of the hf did go into exciting the electron, then there must be another/more energy term/s that i am not aware of. What are they?

thanks

we have went back and forth various times now via email and i am still completely lost. The photon has energy hf. This gets absorbed by the electron so this particular quantity went into exciting the electron into a higher potential orbital. But now there is a momentum component that gets transferred.

What???

i thought the energy of a photon was hf. and all of this went into exciting the photon. if the acceleration/propelling of the atom came from the photon, the photon MUST have an additional energy component, because it takes energy to do so. I'm trying to avoid using momentum and i'm trying to put everything in terms of energy; it's just easier for me to keep track of everything when all the stuff i'm dealing with are in the same forms (energy) (yes i understand i'm not dealing with just a single energy form, i was just referring to the fact that i don't have a mix of an energy term, amomentum term, a torque term, a moment of inertia term, etc etc etc...). So my professor said that there is a momentum component that gives rise to the acceleration of the atom. I tried converting this 'momentum piece' in terms of energy and i asked him if then, the TRUE energy of a photon is hf+(1/2)pc

and he replied that it was not. I am still at a loss... where is the energy to accelerate the atom coming from? If all of the hf did go into exciting the electron, then there must be another/more energy term/s that i am not aware of. What are they?

thanks