What is the internal and external quantum numbers of a atom

[Robert_G]

I am reading a book about the interaction between atom and photon. I don't understand the following statement:

"for the sake of simplicity, we assume the atom to be infinitely heavy and disregard the external quantum numbers"

Q: what is the external (or internal) quantum number of an atom.

 

[Zarqon]

Since it's just one sentence out of context I guess a bit, but I think what they mean by external quantum numbers is outside things/fields that affect the atom. For example, for atom trapped in an external potential, like the lattice of a solid or the trapping fields of an ion trap, there are additional external quantum numbers related to the trapping potential. The atom can for example vibrate in the potential and this vibration is quantized into phonons. I assume the sentence means they ignore those external effects.

Internal quantum numbers should then be everything related to the atom itself, i.e. principal, angular momentum, spin etc.