Source of the damping force in Lorentz model

[asphy]

TL;DR Summary

Lorentz oscillator model

Hi. I have got question about the damper term in lorentz oscillator model. I understand that "spring" term comes from interaction between nucleus and electron, driving term form external electric field acting on molecule. But I don't understand damping term. So my question is where does it come from? For me, damping phenomen must be associated with something (some matter) that slows down a given object. But how it can be possible inside atom which is empty? If this phenomen (term) cannot be explained by classical physics I would appreciate the reference to some quantum mechanical articles or books. Or maybe someone knows the answer to my question. I'd really appreciate if could tell me.

 

[Lord Jestocost]

Damping is the removal of energy from a vibratory system. The energy lost is either transmitted away from the system by some mechanism of radiation or dissipated within the system.

From "The role of damping in vibration theory" by S.H. Crandall (Journal of Sound and Vibration, Volume 11, Issue 1, January 1970, Pages 3-18)