Question: How many interactions between photons and atoms are theoretically possible?
About the article "Photoelectric effect", section "Cross section".

Candidate passages:
Interactions between atoms and molecules: 
There are three types of possible interactions between a system of atoms and light that are of interest: 
There is an entire literature concerning the "structure of matter", ranging from the "electrical structure" in the early 20th century, to the more recent "quark structure of matter", introduced today with the remark: "Understanding the quark structure of matter has been one of the most important advances in contemporary physics."
In this connection, physicists speak of "matter fields", and speak of particles as "quantum excitations of a mode of the matter field".
And here is a quote from de Sabbata and Gasperini: "With the word "matter" we denote, in this context, the sources of the interactions, that is spinor fields (like quarks and leptons), which are believed to be the fundamental components of matter, or scalar fields, like the Higgs particles, which are used to introduced mass in a gauge theory (and that, however, could be composed of more fundamental fermion fields).
"The modern conception of matter has been refined many times in history, in light of the improvement in knowledge of just "what" the basic building blocks are, and in how they interact.
In the late 19th century with the discovery of the electron, and in the early 20th century, with the discovery of the atomic nucleus, and the birth of particle physics, matter was seen as made up of electrons, protons and neutrons interacting to form atoms.
Today, we know that even protons and neutrons are not indivisible, they can be divided into quarks, while electrons are part of a particle family called leptons.
Both quarks and leptons are elementary particles, and are currently seen as being the fundamental constituents of matter.
These quarks and leptons interact through four fundamental forces: gravity, electromagnetism, weak interactions, and strong interactions.
The Standard Model of particle physics is currently the best explanation for all of physics, but despite decades of efforts, gravity cannot yet be accounted for at the quantum level; it is only described by classical physics (see quantum gravity and graviton).
Interactions between quarks and leptons are the result of an exchange of force-carrying particles (such as photons) between quarks and leptons.
The force-carrying particles are not themselves building blocks.
As one consequence, mass and energy (which cannot be created or destroyed) cannot always be related to matter (which can be created out of non-matter particles such as photons, or even out of pure energy, such as kinetic energy).
Force carriers are usually not considered matter: the carriers of the electric force (photons) possess energy (see Planck relation) and the carriers of the weak force (W and Z bosons) are massive, but neither are considered matter either.
However, while these particles are not considered matter, they do contribute to the total mass of atoms, subatomic particles, and all systems that contain them. 
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation.
Radiation pressure implies an interaction between electromagnetic radiation and bodies of various types, including clouds of particles or gases.
The interactions can be absorption, reflection, or some of both (the common case).
Bodies also emit radiation and thereby experience a resulting pressure.
The forces generated by radiation pressure are generally too small to be detected under everyday circumstances; however, they do play a crucial role in some settings, such as astronomy and astrodynamics.
For example, had the effects of the sun's radiation pressure on the spacecraft of the Viking program been ignored, the spacecraft would have missed Mars orbit by about 15,000 kilometers.
This article addresses the macroscopic aspects of radiation pressure.
Detailed quantum mechanical aspects of interactions are addressed in specialized articles on the subject.
The details of how photons of various wavelengths interact with atoms can be explored through links in the "See also" section. 
The vast majority of photons that come anywhere near a particular atom are almost completely unaffected by that atom.
The atom is almost completely transparent to most frequencies (colors) of photons.
A few photons happen to "resonate" with the atom, in a few very narrow bands of frequencies (a single color rather than a mixture like white light).
When one of those photons comes close to the atom, the atom typically absorbs that photon (absorption spectrum) for a brief period of time, then emits an identical photon (emission spectrum) in some random, unpredictable direction.
(Other sorts of interactions between atoms and photons exist, but are not relevant to this article.
)The popular idea that lasers increase the thermal energy of matter is not the case when examining individual atoms.
If a given atom is practically motionless (a "cold" atom), and the frequency of a laser focused upon it can be controlled, most frequencies do not affect the atom — it is invisible at those frequencies.
There are only a few points of electromagnetic frequency that have any effect on that atom.
At those frequencies, the atom can absorb a photon from the laser, while transitioning to an excited electronic state, and pick up the momentum of that photon.
Since the atom now has the photon's momentum, the atom must begin to drift in the direction the photon was traveling.
A short time later, the atom will spontaneously emit a photon in a random direction, as it relaxes to a lower electronic state.
If that photon is emitted in the direction of the original photon, the atom will give up its momentum to the photon and will become motionless again.
If the photon is emitted in the opposite direction, the atom will have to provide momentum in that opposite direction, which means the atom will pick up even more momentum in the direction of the original photon (to conserve momentum), with double its original velocity.
But usually the photon speeds away in some "other" direction, giving the atom at least some sideways thrust.
Another way of changing frequencies is to change the positioning of the laser.
For example, using a monochromatic (single-color) laser that has a frequency that is a little below one of the "resonant" frequencies of this atom (at which frequency the laser will not directly affect the atom's state).
If the laser were to be positioned so that it was moving "towards" the observed atoms, then the doppler effect would raise its frequency.
At one specific velocity, the frequency would be precisely correct for said atoms to begin absorbing photons.
Something very similar happens in a laser cooling apparatus, except such devices start with a warm cloud of atoms moving in numerous directions at variable velocity.
Starting with a laser frequency well below the resonant frequency, photons from any one laser pass right through the majority of atoms.
However, atoms moving rapidly "towards" a particular laser catch the photons for that laser, slowing those atoms down until they become transparent again.
(Atoms rapidly moving "away" from that laser are transparent to that laser's photons — but they are rapidly moving "towards" the laser directly opposite it).
This utilization of a specific velocity to induce absorption is also seen in Mössbauer spectroscopy.
On a graph of atom velocities (atoms moving rapidly to the right correspond with stationary dots far to the right, atoms moving rapidly to the left correspond with stationary dots far to the left), there is a narrow band on the left edge corresponding to the speed those atoms start absorbing photons from the left laser.
Atoms in that band are the only ones that interact with the left laser.
When a photon from the left laser slams into one of those atoms, it suddenly slows down an amount corresponding to the momentum of that photon (the dot would be redrawn some fixed "quantum" distance further to the right).
If the atom releases the photon directly to the right, then the dot is redrawn that same distance to the left, putting it back in the narrow band of interaction.
But usually the atom releases the photon in some other random direction, and the dot is redrawn that quantum distance in the opposite direction.
Such an apparatus would be constructed with many lasers, corresponding to many boundary lines that completely surround that cloud of dots.
As the laser frequency is increased, the boundary contracts, pushing all the dots on that graph towards zero velocity, the given definition of "cold".