Stimulating Emission - Explained

[luxiaolei]

Hi,all.

Can anyone help me understand. Why in the process of a stimulate emmission, the comming (perturbate) photon has the similar wavelength and direction as the released one?

Also, how can a photon make an atom fall to its lower state.

Thanks in advance

 

[Bombastic_]

Sorry if I'm wrong in not making my own thread for this, but I am also curious about the latter question.

how can a photon make an atom fall to its lower state.

I understand that if the electron is losing energy it must release it as a photon, but it is not obvious to me why this happens at all. When a photon interacts with the electron, why does this kick the electron down in energy rather than up? My intuition suggests that bombarding an electron with additional energy would bump the energy state of the electron up, but clearly this is not the case in stimulated emission.

Clearly the excited state, the (possible) further excited state, and the ground state are all solutions of the harmonic oscillator. I also understand that the electron would prefer to be in the lowest energy state, but it is still unclear why the photon-electron interaction is required to force the electron back into the ground state.

 

[Pete99]

I may be wrong, but my view is that since the states in the atom are orthogonal to each other, without any perturbation the atom would remain in the excited state forever.

The incoming photon perturbs the atom and couples the excited and ground states of the atom making the transition possible. According to Fermi's golden rule the probability of transition up or down are the same, so if the atom is excited there is the same probability to cause stimulated emission than absorption when the atom is in the ground state.

 

[Bombastic_]

I understand that the perturbation makes the transition possible, but it is unclear to me why the electron cannot move to a higher energy level. Does this also happen with equal probability, but the effect is ignored, or can this simply not happen for a reason I am missing?

 

[Cthugha]

Atoms have discrete energy levels. If a photon of energy E causes a transition from the ground state g of the atom to an excited state e, this does not automatically mean that there will also be some other excited state e' at energy 2E. In fact, this is almost never the case. So usually there is no matching higher energy level.

 

[Pete99]

Exactly, as Cthugha pointed out the energy levels in the atom are not equally spaced so if the energy of the photon matches the down transition it will probably not match the up transition.

 

[Edgardo]

Just a side note:
https://www.physicsforums.com/showthread.php?t=403356