Stimulated emission process in lasers, Is the atom making transition a correct statement?

Question

I was studying the Lasers, there is a section on absorption and the author is giving the statement as

"An atom in lower energy state E1 may absorb the incident photon and may be excited to E2 . This transition is known as stimulated absorption corresponding to each transition made by an atom one photon disappears from the incident beam."

How can an atom make a transition, because only an electron can make a transition? Does the author mean to say that the atom in the crystal or gas is raised to a higher state with respect to the other atoms in the group?

I am confused as the college professor also said that it does not make any difference.

Answer 1

When teaching students about the hydrogen atom we have a tendency to assume the proton is fixed and then just ignore it. Then we just have a single particle, the electron, moving in a $1/r$ potential. This approach is so common it even has a name - it's called the Born-Oppenheimer approximation. Viewed this way it makes sense to talk about exciting the electron because, well, the system we are considering only has an electron.

However the Born-Oppenheimer approximation is indeed only an approximation. Even though in an atoms the nucleus is far heavier than the electrons it too will move due to the interaction with the electrons surrounding it. So in reality you can't separate the energy levels of the electrons from the nucleus. You have a composite system of electrons and nucleus and you can only talk about the energy levels of the whole atom.

So when you add a quantum of energy to an atom you are not just changing the energy of the electrons, you are changing the energy of the nucleus as well. That's why is it more correct to talk about a transition between energy states of the whole atom.

having said this, it's usually an excellent approximation to treat the nucleus as fixed and just talk about electron transitions. I would guess most of us have used this sort of phrasing.

If you're interested in learning more about the energy levels of atoms see my answer to Why do non-hydrogen atomic orbitals have the same degeneracy structure as hydrogen orbitals?.

Answer 2

I think your professor is right, but I agree that this is ambiguous. Whatever you were reading I think was trying to be as general as possible when describing stimulated absorption. In one case, an electron in an atom can be excited and make a transition, but there are also other things that can happen as well. An atom in a crystal could be excited to a higher vibration state for instance. Molecules that are excited by photons could be excited in electronic, vibration, or rotational states.

So, while the electron does make the transition as you are thinking of it, the electronic state of the atom itself can be thought of as being excited, and the general use of the term atom here I think is to cover more than one type of transition.