# What is Photon absorption: Definition and 49 Discussions

Two-photon absorption (TPA or 2PA) or two-photon excitation or non-linear absorption is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state (usually the ground state) to a higher energy, most commonly an excited electronic state. Absorption of two photons with different frequencies is called non-degenerate two-photon absorption. Since TPA depend on the simultaneous absorption of two photons, the probability of TPA is proportional to the square of the light intensity, thus it is a nonlinear optical process. The energy difference between the involved lower and upper states of the molecule is equal or smaller than the sum of the photon energies of the two photons absorbed. Two-photon absorption is a third-order process, with absorption cross section typically several orders of magnitude smaller than one-photon absorption cross section.
Two-photon excitation of a fluorophore (a fluorescent molecule) leads to two-photon-excited fluorescence where the excited state produced by TPA decays by spontaneous emission of a photon to a lower energy state.

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1. ### I How Does Inverse Bremsstralung Affect Photon Absorption?

There are lots of diagrams showing bremsstralung as a deflection of an electron by a nucleus but none of inverse bremsstralung. Does the inverse process I.e. photon absorption only depend on the direction of the deflection by the nucleus? If not ,what determines wether the electron will absorb...
2. ### I Can an atom absorb a photon, yet its total kinetic energy is decreased?

Let's assume an atom consists of the nucleus and electrons as point particles. Take the inertial frame to be that of the fixed laboratory. Its total energy consists of the total kinetic and potential energy of the system of particles.If an electron absorbs a photon of energy E, the total energy...
3. ### B Atom absorbing a photon and emitting a photon afterwards

If an atom absorbs a photon it can only do it if the incoming photon has precisely the energy of the required electron excitation (difference between 2 energy levels of the atom). The very basic question: Why can't an electron (bound in an atom) absorb the incoming photon with an energy...
4. ### I How does EM wave geometrical attenuation affect atomic absorption?

Let's say we have a point source of an EM wave in a vacuum of total energy E, and an absorber atom at some distance from this source, whose first excited state is at the energy B, with B < or = E. The energy of the wave is constant as a whole, but at each point around the source the energy...
5. ### A Jaynes-Cummings Hamiltonian: Where did the time dependence go?

Consider the interaction of a two level atom and an electric field (semiclassically, we treat the field as 'external' i.e. not influenced by the atom; the full quantum treats the change in the field as well) Electric field in semiclassical Hamiltonian: plane wave ##H_{int,~semiclassical}=-\mu...
6. ### B Why are things the color they are?

I have not noticed anything in common between all things that are the same color. It must be something on the atomic level but if I look at the periodic table I see no pattern in how things behave and what their color is. So how can the sun be the same color as a highlighter and plastic the same...
7. ### B Photon absorption -- What happens to the excess energy?

Wikipedia: "When a photon has about the right amount of energy to change the energy state of a system (usually an electron changing orbitals), the photon is absorbed." What happens if a somewhat higher energy photon arrives?
8. ### Energy transferred to nucleus in pair-production

In most textbooks, the recoil energy of the nucleus is ignored as it absorbs so little energy, and since its main role in the reaction is to absorb some of the photon's momentum without absorbing much energy. I'm tempted to say that the nucleus gets the maximum energy when the kinetic energy of...
9. ### I Photon absorption for an atomic electron

Quick question: let's say we have an atomic electron in the ground state which requires, say, one "unit" of energy* to jump up to the next orbital energy state. If a photon arrives with a bit more or less than this, say 1.00003 or 0.99997 units of energy, is there some finite, non-zero...
10. ### MIT Material 10x Blacker Than Any Other: 99.995% Light Absorption

Summary: MIT researchers have created a material that is 10x blacker than any other material ever recorded. The foil captures at least 99.995% of incoming light. Does this mean that there is practically no EM re-radiated?
11. ### I Excited electrons' behaviour, how do they work?

Mainly I want to know the following thing: electrons when excited they tend to want to go back to ground state, right? One way is by photons, but how does that work? Accelerating charges creates EM waves, but in this case there was no acceleration, right? Or is the term accelerating only a way...
12. ### I How electrons absorb or emit photons

Hi, I am aware that the reason why objects have color is an old subject. However I come across two claims which sound like they are in contrast to each other. On one hand I read that the photons with specific frequency in incoming light are absorbed (i.e. have enough energy to move an electron...
13. ### I Photon Absorption & Electrons: Mass, Energy & Releasing Photons

I'm not that familiar with the current theoretical standing on how electrons "absorb" photons, as in the sense that electrons in an atom absorb photons and move from lower to higher energy states. But during the absorption of a photon the electron, if you set units of c=1, gains energy and...
14. ### I Photoelectric effect questions: photon absorption and electron ejection....

Is a photon fully absorbed by a electron in metal? Ejected electron is a free electron or bonded one? Is it possible to have a zero work function? If not why? I want to study about this detaily ,can you prefer some reference papers?
15. ### How a solid body emits a lower frequency photon than absorbed

I’m trying to understand how a solid body changes the wavelength of radiation it re-radiates from that which it originally absorbed. I’m thinking in context to the way that the Earth absorbs higher frequency radiation from the sun, but when it re-emits the energy it’s at a much lower frequency...
16. ### I Compton effect: how can it take place?

English is not my native language. So, I hope to be understood. :-) The Compton effect is the dynamics in which high-energy incident photons (X or gamma) are scattered by electrons of certain materials, like graphite. The electrons are supposed to be free, as they are only weakly bounded by...
17. ### I Exploring Light Absorption: What Happens When a Photon Hits an Electron?

Hello, I have a presentation tomorrow and in a segment, I talk about light absorption. It's more conceptual than technical. I did quite a bit of research on the topic but because of simplifying information I may have butchered the facts and written something wrong. Could anyone please confirm/...
18. ### I Photon absorption and speed of wave function propagation....

Hi all, Whew, last question for a while: I think I already know the answer, but want to confirm (e..g, I think this thread basically answers the question, https://www.physicsforums.com/threads/propagation-of-wavefunction.152053/) As an example, let's say I have an electron (in free space or...
19. ### Where could I look for down-to-earth X-ray facts?

Hello, I'd like to know which book or papers I'd read to understand, possibly without too rigorous mathematics knowledge, how do photons like X-rays or gamma ones reflect off surfaces and attenuate through solids or generally speaking, facts about how do they interact with the real world. So...
20. ### I Two photon absorption or multi-photon absorption?

Hello Forum, What does the process of two-photon absorption photon entail? That, in general, a molecule or atom (composed of many electrons) that is illuminated by an electromagnetic field can only absorb one photon at a time except in those special cases where two photons can be absorbed...
21. ### I Can an Electron Absorb a Photon if the Next Energy Level is Full?

Can an electron absorb a photon and jump to a higher orbit if the next level is already full of electrons? please explain what happens and also link info that might help me with electron/ photon interaction. I have many questions. Thanks
22. ### Calculate photon flux at a distance from the source

Homework Statement A 100W sodium lamp(lambda=589nm) radiates energy uniformly in all directions. (A) At what distance from the lamp will a totally absorbing screen absorb photons at the rate of 1.00 photon/cm2.s? (B) What is the photon flux on a small screen 2m from the lamp? Homework...
23. ### Photon absorption by electrons at opposite k-points

Transition of an electron from the valence to conduction bands (direct transition at a k-point near the band edge) would change the momentum of electron because the sign of the group velocity in valence and conduction bands are opposite. Could one infer that the direction of radiation is a...
24. ### High and low energy photon absorption

Hi All, Kindly find here 2 questions, i was wondering about: 1) Will (helium cooled) Mono C-silicon be able to absorb a high intensity laser of about 900-1100 nm ? 2) How can Ultra-violet and visible light cause rise in material temperatures. Thanks.
25. ### Atomic excitation via photon absorption

Hi guys! I have come across a problem I can't seem to wrap my head around. I've learned that E.M. radiation can be propagated by discrete packets of energy , photons. Is the energy of each photon discrete, or can it have a continuous range of energies depending on its frequency? I would be...
26. ### Hydrogen excitation, ionisation and photon absorption

I need help with something that i seemed to remember was fairly easy, but just can't seem to find the simple formula for: I have an amount of hydrogen atoms at a temperature of 10000 Kelvin. What percentage of them are ionised and what percentage of them are in the n=2 excited state...
27. ### Mechanism behind photon absorption and photon emission

hi all, i learned that only certain orbits were allowed in the atom and that if the electrons occupied any of the orbits in between, that they would no longer be in a resonance orbit (i was taught that the allowed orbits were the electron probability wave in resonance and therefore no EM...
28. ### Photon absorption - Newton vs. Einstein

A note from Newton's Principles definition #4 in 1687: Impressed Force - This force conflicts in the action only; and remains no longer in the body when the action is over. In Einstein's second paper on relativity in 1905, he explicitly concludes: "Radiation carries inertia between...
29. ### Can Free Fundamental Particles Absorb Photons?

I recently learned that a free electron can't absorb a photon and derived it by showing it would be impossible to conserve both momentum and energy if that were the case. It seems like the same argument would extend to other fundamental particles. Is it true that no free fundamental particle can...
30. ### Evidence of single photon absorption?

Hey all, Does anyone know of a paper or experiment that verified that a single photon is absorbed in exchange for electron excitation? Thanks
31. ### Photon absorption and momentum recoil

Hi When talking about atoms absorbing a photon, a classical picture is utilized. The intuitive picture often presented is that when an atom absorbs a photon, a recoil of \hbar k is obtained by the atom. After a time \tau=1/\Gamma the atom emits the photon again. So the force in this picture is...
32. ### Photon absorption via changing EM field

Does it work like that.. that a changing EM field in-phase (actually, like.. 180 degre (spell?) out-of phase) with the incoming photon would absorb it? .. when sound waves of 180 degre.. (spell) out of phase with each other would cancel each other out. The same thing would happen with EM...
33. ### Information Loss in Photon Absorption?

How can I tell the difference between a free electron that's absorbed one photon at E = hf and a free electron that's absorbed two photons at E = .5hf? If a photon at frequency f represents a '0' and a photon at frequency .5f represents a '1', then I could send out '11', have an electron...
34. ### Photon Absorption wavelength(s) to aid ionization?

If I have a group of atoms in a high voltage field, and I want to use photon energy to aid the ionization of the atoms what wavelength(s) would best be used to have the highest ionization efficiency? The shortest wavelength available? Or the resonant wavelength for each excitation level?
35. ### Photon absorption and probability of atomic electron transition

I have only taken one very intro quantum mechanics class and the furthest we got was solving the Schrodinger equation for the Coulumb potential of the hydrogen atom. So we solved for the allowed energy levels and the wavefunction for the hydrogen atom. We also learned how using the...
36. ### Multiple Photon Absorption Question

Question: Is it safe to say that if two photons were to simultaneously come in contact with a single atom, the total energy of the combined photons would be absorbed and then emitted as a single higher energy photon with a direction that is opposite that of the average angle of incidence of the...
37. ### Photon Absorption: Can It Still Happen?

I've been wondering, if a photon is very far off any atomic resonances, can it still be absorbed by the atom? or will there be compton scattering? or will the photon pass through the atom? or is there something else I'm not considering... Thanks.
38. ### How does the photon absorption process work in atoms?

What is the process through which photons are absorbed/emitted from an atom? Also... One of the threads i visited first was this: https://www.physicsforums.com/showthread.php?t=76246" Post number 11, the one by Zoobyshoe explaining what Claude meant by the rope analogy using a billiard...
39. ### Time scale of photon absorption in electron

Does anyone know, when an electron in an atom is exited by absorbing a photon, how long does it take for the electron to absorb the entire energy of the photon? Does the time scale depend on the energy level? Thanks!
40. ### Extra Energy from Photon Absorption

Quick question: when an electron absorbs a photon with MORE THAN enough energy to jump one energy level but not two, what happens to the extra energy the electron doesn't absorb? Thanks.
41. ### Photon Absorption: Does Energy Matter?

If a photon does not have the required energy for changing the quantum state of a particle/atom/molecule, is it absorbed and instantaneously re-emitted with the same energy when it arrives at said particle/atom/molecule or is it not absorbed at all? Also, does it make any difference in...
42. ### Photon Absorption: Atom, Transition Level, and Energy

All the while, I thought that an atom only absorbs a photon that precisely corresponds to one of the transition level (resonance), but now I read that the atom will also absorb photon with wavelengths different from the transition level in question. So, my question is, suppose I have two energy...
43. ### Two photon absorption coefficient of silicon

Hi, is there a way to calculate the two-photon absorption coefficient of silicon? I am especially interested in the two-photon absorption coefficient of silicon for the wavelengths 1030nm, 515nm and 343nm. Thanks for any help!
44. ### Photon absorption by an accelerated atom

I'm wondering whether QM considers photon absorption by an accelerating atom as an instantaneous event, or the change in velocity (and proper time) of the atom affects the perceived distribution of the wave packet and the outcome of the process. In other words, is there a "Doppler gradient"...
45. ### Free/Bound Electrons and Photon absorption

I've heard that atomically bound electrons can potentially "absorb" an incoming photon, whereas a "free" electron has no such potential. Why is this? Also, can rapidly alternating un-bound electrons emit photons?
46. ### Two photon absorption in silicon

How does work the two-photon absorption ? Does it exist a Beer-Lambert law for this kind of process? If yes, does someone know where i could find the order of magnitude of two-photons absorption coefficient for silicon at 1500 nm ? Barth
47. ### Magnetic Precession and Photon Absorption

In my understanding, when a nuclei absorbes a photon with the right freq it changes its spin... now, if this atom is in a magnetic field B on the Z direction, its magnetization vector is precessing around the Z direction, and after absorbing a photon it changes its angle relative to the Z axis...
48. ### Photon Absorption & Excitation of Electrons in Atoms

When a photon hits an atom it gets absorbed and an electron moves one or more shells up depending on the energy of the photon. When a bigger, many-electron atom gets hit by a photon, can the electrons in the lower shells get excited? I mean when they move up they have to remove the electrons in...
49. ### Photon Absorption by Electrons: Physical or Mathematical?

Hi everyone, I was wondering (while in the shower of all places) whether photon absorption by an electron is a physical process or a mathematical consequence. That is, does an electron literally "eat up" the photon at the quantum level or is it just some consequence of the math that we give...