Hello! I am not sure how to visualize (ideally in a simple, classical way) the vibrational energy levels of a molecule. For the electronic levels (similar to atoms), I usually think of it as the atom having multiple energy levels (fixed in space, in a probability distribution sense), and when the atom gains energy, the electron goes from one shell to another (of course this is a simplified view, but it is useful to visualize what is going on). In the case of vibrational energy, I imagined that when the molecule gains energy, the 2 atoms (say it is a diatomic molecule) start to vibrate like a spring, and the energy levels are like these of a harmonic oscillator. But from what I read about this topic, I have noticed that everywhere people talk about the electron transition between two vibrational states and the detected photon corresponds to the energy difference between these 2 states. So initially I thought that this vibrational energy goes into the nuclei, making them move around an equilibrium point. But it seems that the energy goes to the electron not to the nuclei. And I am not sure what is vibrating anymore. Of course quantum mechanically we have a many-body system, containing both the nuclei and electrons and one in principle solves the Schrodinger equation for everything at once (or within the Born-Oppenheimer approximation). But I was wondering if there is a simple way of seeing what is going on and what is vibrating, the nuclei or the electron. And if it is the electron what does it vibrate around? Also, are the nuclei moving at all? And if so, where is that energy i.e. why are we detecting only the transition energy of the electron? Can someone clarify this for me please? Thank you!