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How does the photon absorption takes place in a hydrogen?

The classical mechanics shows the absorption of photonic energy resulting in the excitation of atom. Intuitively, a photon with frequency higher than that required for excitation of electron brings about the excitation process. But something restricts the excitation of electrons by photons of higher energy.

Why the match of energy of electron and the photon is an essential condition for excitation.

@Urgje said,

The probability of atomic excitation by photon absorption is given in terms of the "cross- section" for single photon absorption. This quantity shows a sharp peak if the photon energy and the atomic energy level difference match (the coupling between the atom and electromagnetic field causes the atomic states to broaden, the natural line width). Thus excitation by photons of higher energy is possible but its probability is very small.

What is meant by the cross section for single photon adsorption? Is there a electron excitation probability vs. Frequency of the photon plot?

What are the direct factors that affects there probability of the electron excitation?

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marked as duplicate by John Rennie, ACuriousMind, Kyle Kanos, Jim, Brandon Enright Nov 17 '14 at 16:21

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ possible duplicate of How do photons know they can or can't excite electrons? $\endgroup$ – John Rennie Nov 17 '14 at 14:07
  • $\begingroup$ Photons with higher energy than the ionization energy may ionize an atom, and the remaining energy is carried partly by the kinetic energy of the electron, and partly by the photon, whose energy remains smaller - Compton effect. You can see such an example in Wikipedia, the topic "Gamma ray", section "matter interaction". $\endgroup$ – Sofia Nov 17 '14 at 15:22