To excite an electron in the atom from lower state to higher you need an EM field of frequency that matches the energy difference between those levels. How does the near-resonance transition work then?
Thanks in advance.
If you use the drude model for an oscillator and include a damping term (modelled as a fictional force on the dynamical equation), you can allow for broadening and the peak of the resonant absorption frequency is not a sharp peak but rather a broad peak that is be centered around some maximum value. It is this broadening causes near-resonance transition.