Do Excited Molecules Emit the Same Frequency They Absorb?

[caybo]

1) When you excite amolecue and it releases photons off a given frequency, is that frequency the the very frequency which would best excite an identical molecue?

2) or are the release frequency(ies) and absorption frequency(ies) different?

3) If you excite a hydrogen atom with a certain frequency and it emits it's photons, will a different frequency make it it release a different freqency of photon back at us?

 

[Zizy]

Yes, No, Depends.

1 and 2) Electrons in atoms (or molecules, or bonds in molecules etc...) have discrete energy levels. If an electron jumps from one state to another it emits a photon with the same energy that is between those states. Absorbtion is mostly the same but in the opposite direction.
3) Depends on energy difference - if you put enough energy in the atom, emitted photon might have more energy. For smaller energy differences (you excite the atom with both frequencies to the same state), there is no difference in released photon.