Why does vibrational relaxation occur.

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The discussion centers on the interaction of a photon with a water molecule, leading to the excitation of a vibrational mode, specifically a symmetric stretch. As the excited state evolves, it decays into various modes of the surrounding water bath, including bending, rotation, and translation. This relaxation occurs due to the coupling between the excited mode and the bath, resulting in a mixture of eigenstates rather than a pure eigenstate. The conversation seeks to understand the physical mechanisms behind this decay process and questions whether it is possible to achieve a true eigenstate in such a coupled system. The Bixon-Jortner model is referenced as a valuable resource for further exploration of these concepts, along with a specific paper that provides additional insights into the topic.
DinosaurChemi
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Take the case of a bath/system of water. A photon of light is allowed to interact with one molecule of water giving it let's 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.
 
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