Take the case of a bath/system of water. A photon of light is allowed to interact with one molecule of water giving it lets say 3200 1/cm exciting a "symmetric" stretch or some normal mode of the system similar to a symmetric stretch. The molecule will begin to vibrate but as time goes on this state which is excited will decay into either the bend, rotations or translations of the bath (i.e. the bath being everything but the excited stretch). Why does the relaxation occurs. I understand that due to coupling of the system (excited mode) with the bath I will be faced with a excitation of a linear combination of eigenstate for my system and not a true eigenstate since i have brought all these water molecules together. Is there even a way to excite a true eigenstate in such a situation? I guess I am looking for a physical understanding of the decay.