Question: What happens if the photon energy i s absorbed?
About the article "Photoelectric effect", section "Emission mechanism".

Candidate passages:
The photons of a light beam have a characteristic energy proportional to the frequency of the light.
In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function (the electron binding energy) of the material, it is ejected.
If the photon energy is too low, the electron is unable to escape the material.
Since an increase in the intensity of low-frequency light will only increase the number of low-energy photons sent over a given interval of time, this change in intensity will not create any single photon with enough energy to dislodge an electron.
Thus, the energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy (equivalently frequency) of the individual photons.
It is an interaction between the incident photon and the outermost electrons.
Electrons can absorb energy from photons when irradiated, but they usually follow an "all or nothing" principle.
All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or else the energy is re-emitted.
If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron's kinetic energy as a free particle.
When a photon hits a piece of silicon, one of three things can happen:  the photon can pass straight through the silicon — this (generally) happens for lower energy photons,  the photon can reflect off the surface,  the photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap value.
This generates an electron-hole pair and sometimes heat, depending on the band structure.
When a photon is absorbed, its energy is given to an electron in the crystal lattice.
Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far.
The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor.
The covalent bond that the electron was previously a part of now has one fewer electron — this is known as a hole.
The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice.
Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.
A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band.
However, the solar frequency spectrum approximates a black body spectrum at about 5,800 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon.
These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations — called phonons) rather than into usable electrical energy.
The photovoltaic effect can also occur when two photons are absorbed simultaneously in a process called two-photon photovoltaic effect.
However, high optical intensities are required for this nonlinear process. 
In electronic absorption, the frequency of the incoming light wave is at or near the energy levels of the electrons within the atoms which compose the substance.
In this case, the electrons will absorb the energy of the light wave and increase their energy state, often moving outward from the nucleus of the atom into an outer shell or orbital.
The atoms that bind together to make the molecules of any particular substance contain a number of electrons (given by the atomic number Z in the periodic chart).
Recall that all light waves are electromagnetic in origin.
Thus they are affected strongly when coming into contact with negatively charged electrons in matter.
When photons (individual packets of light energy) come in contact with the valence electrons of atom, one of several things can and will occur: An electron absorbs all of the energy of the photon and re-emits it with different color.
This gives rise to luminescence, fluorescence and phosphorescence.
An electron absorbs the energy of the photon and sends it back out the way it came in.
This results in reflection or scattering.
An electron cannot absorb the energy of the photon and the photon continues on its path.
This results in transmission (provided no other absorption mechanisms are active).
An electron selectively absorbs a portion of the photon, and the remaining frequencies are transmitted in the form of spectral color.
Most of the time, it is a combination of the above that happens to the light that hits an object.
The electrons in different materials vary in the range of energy that they can absorb.
Most glasses, for example, block ultraviolet (UV) light.
What happens is the electrons in the glass absorb the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum.
Thus, when a material is illuminated, individual photons of light can make the valence electrons of an atom transition to a higher electronic energy level.
The photon is destroyed in the process and the absorbed radiant energy is transformed to electric potential energy.
Several things can happen then to the absorbed energy: it may be re-emitted by the electron as radiant energy (in this case the overall effect is in fact a scattering of light), dissipated to the rest of the material (i.e.
transformed into heat), or the electron can be freed from the atom (as in the photoelectric and Compton effects). 
It typically has a very strong maxima at the minimum number of photons to ionize the system, with successive peaks (known as ATI peaks) separated by the photon energy and thus corresponding to higher numbers of photons being absorbed.
In the non-perturbative regime the bound states are dressed with the electric field, shifting the ionization energy.
If the ponderomotive energy of the field is greater than the photon energy formula_5 then the first peak disappears. 
If an atom is already in the excited state, it may be perturbed by the passage of a photon that has a frequency ν21 corresponding to the energy gap Δ"E" of the excited state to ground state transition.
In this case, the excited atom relaxes to the ground state, and is induced to produce a second photon of frequency ν21.
The original photon is not absorbed by the atom, and so the result is two photons of the same frequency.
This process is known as "stimulated emission".
Specifically, an excited atom will act like a small electric dipole which will oscillate with the external field provided.
One of the consequences of this oscillation is that it encourages electrons to decay to the lowest energy state.
When this happens due to the presence of the electromagnetic field from a photon, a photon is released in the same phase and direction as the "stimulating" photon, and is called stimulated emission.
The rate at which stimulated emission occurs is proportional to the number of atoms "N"2 in the excited state, and the radiation density of the light.
The base probability of a photon causing stimulated emission in a single excited atom was shown by Albert Einstein to be exactly equal to the probability of a photon being absorbed by an atom in the ground state.
Therefore, when the numbers of atoms in the ground and excited states are equal, the rate of stimulated emission is equal to the rate of absorption for a given radiation density.
The critical detail of stimulated emission is that the induced photon has the same frequency and phase as the incident photon.
In other words, the two photons are coherent.
It is this property that allows optical amplification, and the production of a laser system.
During the operation of a laser, all three light-matter interactions described above are taking place.
Initially, atoms are energized from the ground state to the excited state by a process called "pumping", described below.
Some of these atoms decay via spontaneous emission, releasing incoherent light as photons of frequency, ν.
These photons are fed back into the laser medium, usually by an optical resonator.
Some of these photons are absorbed by the atoms in the ground state, and the photons are lost to the laser process.
However, some photons cause stimulated emission in excited-state atoms, releasing another coherent photon.
In effect, this results in "optical amplification".
If the number of photons being amplified per unit time is greater than the number of photons being absorbed, then the net result is a continuously increasing number of photons being produced; the laser medium is said to have a gain of greater than unity.
Recall from the descriptions of absorption and stimulated emission above that the rates of these two processes are proportional to the number of atoms in the ground and excited states, "N"1 and "N"2, respectively.
If the ground state has a higher population than the excited state ("N"1 > "N"2), the process of absorption dominates and there is a net attenuation of photons.
If the populations of the two states are the same ("N"1 = "N"2), the rate of absorption of light exactly balances the rate of emission; the medium is then said to be "optically transparent".
If the higher energy state has a greater population than the lower energy state ("N"1 < "N"2), then the emission process dominates, and light in the system undergoes a net increase in intensity.
It is thus clear that to produce a faster rate of stimulated emissions than absorptions, it is required that the ratio of the populations of the two states is such that"N"2/"N"1 > 1; In other words, a population inversion is required for laser operation.