Exploring the Energy Transfer of Light and Heat in Matter

[Tiwari]

I read that atoms transition from electronically excited to vibrationally excited. But how?

 

[Drakkith]

I read that atoms transition from electronically excited to vibrationally excited. But how?

I'm not sure that they 'transition' from one to the other. As far as I know both can happen at any time. To answer your question, when light is absorbed by a material, the energy is transferred from the incoming light wave to the molecule, exciting electrons into higher energy levels (electronic transition), causing the molecules to vibrate, or giving the molecules more kinetic energy (in the cases of liquids and gases). If the light excites an electron into a higher energy state, the electron then usually falls back down to a lower energy state shortly thereafter and gives up energy in the process. This energy usually ends up being converted into heat or thermal radiation.

 

[hilbert2]

Usually any interacting system will proceed to thermal equilibrium, where energy is equipartitioned to all the degrees of freedom (electronic, vibrational and rotational excitations) that are available. The only requirement for this is that these degrees of freedom have to be "coupled", which in this case means that the strength of a chemical bond with an excited electron is different from that in the ground state, affecting its vibration. Similarly, the vibrational and rotational motion of a molecule are coupled because a rotating molecule has the centrifugal effect stretching it, and the stretching of a chemical bond affects the moment of inertia in turn.

 

[PeterDonis]

I read that atoms transition from electronically excited to vibrationally excited.

Can you give some specific references for where you have read this? Knowing the specific context (for example, what kind of process is involved) will help in giving good responses.

 

[Khashishi]

Atoms can't vibrate, but molecules can.

Heat isn't just vibrations. Heat is energy that is randomly spread among all the objects and all the ways that each object can be excited. But, the minimum vibrational energy is smaller than the minimum electronic energy, so statistically, it is more likely for the energy to randomly be in vibrational modes, since the energy likes to be as spread out as possible among all the objects.

On Earth, we are typically working with things which are at relatively low temperatures. At these low temperatures, in thermal equilibrium (after everything has settled out), it is very unlikely for any molecule to have enough energy to be in an excited electron state, but it is not unlikely to be in an excited vibrational state, because these states have low energy. (For more, see Boltzmann distribution).