[Winga]

I want to know that if the molecule(e.g.H2O) hasn't absorb the IR radiation yet, does the O-H bond also vibrate?

If after absorbing a particular IR radiation, it jumps to the next higher vibrational state. Is it increased the amplitude of the vibration? And this is called resonance?

In vibrational states, there are many energy gaps. Assume the energy difference between each state and to its next state is the same, if the IR radiation is emitted continuously to the molecule, will it jump from one to one...? And the amplitude increases and increases?

[Mitch]

Yes it will still vibrate, re-study your quantum mechanics.

Yes the amplitude is increased.