Apart from saying that electron orbitals have specific energy levels and can only absorb light of that energy/wavelength, what actually causes electrons to absorb the energy?
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$\begingroup$ The mechanism you can find here: physics.stackexchange.com/questions/281660/… $\endgroup$– UKHCommented Jan 20, 2017 at 15:33
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3 Answers
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Apart from saying that electron orbitals have specific energy levels and can only absorb light of that energy/wavelength,
Please note that we call them electron orbitals because we assume a framework where the nucleus is at rest. They are actually "atomic orbitals" It is the whole atom that changes in energy when a photon is absorbed.
what actually causes electrons to absorb the energy?
Electrons do not absorb energy. The atom does, and the electron is raised into a higher energy orbital. The cause is the impinging of a photon that carries the energy packet that separates the two orbitals.
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$\begingroup$ Off topic, but from my understanding, electricity works by movement of electrons and energy transferring through those electrons. How does that work if electrons don't absorb energy? $\endgroup$– IterCommented Jan 20, 2017 at 18:12
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2$\begingroup$ @Iter In this case, a free electron acquires kinetic energy in interactions with photons, compton scattering for example. en.wikipedia.org/wiki/Compton_scattering $\endgroup$– anna vCommented Jan 20, 2017 at 18:41
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$\begingroup$ @Anna. +1 good and simple answer and comment. Curious follow up: if the photon energy is higher that say the ionization energy, or just higher that the energy for a jump from one level to another, is there a non zero cross section for jumps lower than the total photon energy, with another lower photon energy emitted? I'm just thinking that in less than ionizing photons, it's pretty unlikely the photon energy is a perfect match to the orbital energy difference. I'm not even sure if that kind of non-elastic scattering is possible, and probably something I never ran into. $\endgroup$– Bob BeeCommented Jan 21, 2017 at 6:58
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$\begingroup$ @BobBee To start with all energy levels have a width, then the atoms itself has a statistical motion ( in a gas, or ...)which doppler shifting will allow more frequencies than just the width. Now there exists things like en.wikipedia.org/wiki/Quantum_cascade_laser , where the action happens in bands, so in a sense it would be possible in a material with electron bands to have a compton scatter and then the lower energy photon happen to be within an energy level width, but I am no expert. $\endgroup$– anna vCommented Jan 21, 2017 at 11:15
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$\begingroup$ Thanks @Anna. Yes, that makes sense, always wondered about that but never enough to try to find out. I did figure there were bandwidths and Doppler, but intuitively it seemed to me it would not be enough because when we see atomic spectra the lines, even with some width, as fairly separated at least in the lower energy levels. Thanks for your reference also. I'll try to find out more to see if there's more to the story $\endgroup$– Bob BeeCommented Jan 21, 2017 at 20:46
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Fundamentally, the process responsible for the absorption / emission of a photon is expressed in the Hamiltonian coupling the field to the atom, that is $$ H=-\bf{d.E} $$ so it is the the field-dipole interaction which is at stake here, and it is this interaction that dictates the selection rules for the transitions.
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I think the electrons requires energy to transit into higher energy level... Because energy changes when we move from one orbit to another