# Photon absorption by a hydrogen atom : [duplicate]

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?