1. A photon hits an atom perpendicularly to its speed v and it is absorbed as is known immediately. So it can not act after the time when it is not perpendicular (e.g. the force is 0 after that). Certainly all its energy goes to the atom when it is perpendicular to v.
2. But the impact (absorption) applies a force on the atom and it is postulated that a perpendicular force can not do work. So work has not been done on the atom. Consequently its energy can not change. Seems like contradiction?
Dear Ilian Peruhov ,
hope you are well!
The photon absorption by electrons in the atoms depends on the quantum energies of the electrons inside the atom. These energies are determined by the wave mechanics rather than the classical mechanics. The photons can only absorbed if electrons can be transferred from a lower energy level to a higher energy level with an energy difference equal to that of the photon energy Eph.
This is well known in the atomic physics and the interaction between the photons
and the atoms.
Best wishes
Photon absorption by an atomic electron occurs in the photoelectric effect process, in which the photon loses its entire energy to an atomic electron which is in turn liberated from the atom. This process requires the incident photon to have an energy greater than the binding energy of an orbital electron. For X- and γ-ray photons of sufficient energy, photoelectric absorption is most likely to be caused by the most tightly bound electrons, i.e. those of the K shell, because the concentration of electrons is highest in this shell. Note that the photoelectric effect cannot occur if the electron is unbound, as it will not be possible to conserve both energy and momentum. The reaction is also favored at low photon energy and for atoms with a large atomic number, where there are many electron shells for the incoming photon to interact with, and to match in energy. The incident photon is completely absorbed in this process and the electron is released carrying a kinetic energy equal to that of the incident photon minus its binding energy. Although, the atom recoils in this process to conserve momentum, the kinetic energy it carries is negligible. The released electron leaves behind a vacancy in the inner orbit it occupied. This vacancy is subsequently filled by an electron from a shell with a lower binding energy (an outer orbit, e.g. from an L to a K shell). The difference between the two binding energies of the electron filling the vacancy is released in the form of an X-ray photon, known as fluorescent radiation. Alternatively, the energy may lead to the ejection from the atom of another orbital electron, called the Auger electron. This electron in turn leaves a vacancy and the process of emission of X-ray photons or Auger electrons is repeated, and so on. In atoms with high atomic number, X-ray emission is usually favored over the releases of Auger electrons, and the opposite is true for light elements. In essence, while in the photoelectric effect the original photon is absorbed, other photons (X-rays) and electrons (the first liberated electron and any Auger electrons) are released.View chapterPurchase book
Thank you Zekry and Alsaad,
my concerns are not about the mechanisms of absorption which I know well.
My concerns are about the paradox about absorption (which you discribe) and the amount of work, which I show to be zero.
due to photoelectric effect process the photon will absorption by an electron in atom .It occurs when the photon loses energy to an electron in atom which is in turn liberated from the atom.This photon must be has an energy greater than the binding energy of an orbital electron. The photoelectric effect occurs when the electron is bound.For atoms have large Z the photons must be low photon energy .
Dear Ilian Peruhov ,
welcome!
As for the energy and momentum conservation it is always valid.
It is so that there is no mechanical work done in the direction of the motion of the atom but there will be work done in the normal direction which is the direction of the incident electron.
An example of this motion is a large mass moving in the x direction is subjected to a knock of a small mass in the y-direction. The large mass will be slightly deflected from its original path in the y-direction. The large mass will absorb energy and momentum from the small mass.
Best wishes
Dear Abdelhalim abdelnaby Zekry,
You write "but there will be work done in the normal direction which is the direction of the incident electron.'
I suppose you mean the incident photon. So you mean:
but there will be work done in the normal direction which is the direction of the incident photon.
But this is contradiction. Work can not be done if the force acts perpendicular to the velocity, e.g. in the normal direction to the direction of motion of the atom.
A=F.v where . is a scalar product. Work can be done only from that part of F which is along v by the definition of work.
Best wishes.
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