How does an emission take place by radiation?

[hokhani]

When an electromagnetic wave is impinged on a semiconductor and we solve the perturbed Hamiltonian by taking into account the vector potential in the momentum term, two terms would add to the unperturbed Hamiltonian which one of them deals with absorption while the other with emission. I like to know whether the emission is made by the incident radiation itself, or first the radiation is absorbed then the excited electron would come back and radiate?

 

[Simon Bridge]

The usual picture is that a photon gets absorbed and then another photon is radiated.

Careful though, a model need not follow the physical process.
If you have not measured it, then there may be a range of things that could have happened.

 

[hokhani]

The usual picture is that a photon gets absorbed and then another photon is radiated.

If we have some electrons at higher levels before impinging, would they come down to the lower levels by emission after impinging? (Suppose that we have only two possible ways, absorption and emission by electrons)? Or first the photon is absorbed by an electron and then emission is only made by the excited electron when is coming back to lower levels?

 

[Simon Bridge]

Sure - if you have already excited electrons, the incoming radiation can prompt them to decay.
The process is called "stimulated emmission" ... you can look it up ;)