Why do photons stimulate emission in atoms?

[Somali_Physicist]

I can't get my head around stimulated emission.Why does a photon traveling towards an atom stimulate emission? The best way I could answer is that a photon has an electro magnetic field and this causes an excited atom to oscillate.This induces an excited electrons to fall down an orbital.

But why does oscillation encourage an excited atoms electrons to drop down to its ground state.

 

[blue_leaf77]

The key concept is that when an electron bound in an atom or molecule interacts with an incoming electromagnetic field, it may undergo transition to the other levels. If the level it ends up lies higher than its initial one, then some photons are absorbed to raise this electron's energy hence this process is called absorption. On the other hand if the new level lies lower in energy than the initial one, some energy must be given up. If it is released in the form of photon, it is what you call stimulated emission.

But why does oscillation encourage an excited atoms electrons to drop down to its ground state.

Because QM says so. Have you read some literature on this upper introductory subject of quantum physics?
By the way, stimulated emission does not necessarily bring down electron to the ground state, it depends on the so-called selection rule involving the initial and the ground state. What makes excited electrons eventually drop down to ground state is another aspect of quantum theory, called quantum electrodynamics.

 

[Somali_Physicist]

The key concept is that when an electron bound in an atom or molecule interacts with an incoming electromagnetic field, it may undergo transition to the other levels. If the level it ends up lies higher than its initial one, then some photons are absorbed to raise this electron's energy hence this process is called absorption. On the other hand if the new level lies lower in energy than the initial one, some energy must be given up. If it is released in the form of photon, it is what you call stimulated emission.

Because QM says so. Have you read some literature on this upper introductory subject of quantum physics?
By the way, stimulated emission does not necessarily bring down electron to the ground state, it depends on the so-called selection rule involving the initial and the ground state. What makes excited electrons eventually drop down to ground state is another aspect of quantum theory, called quantum electrodynamics.

"Because QM says so"

hmm, that doesn't help unfortunently.Im just interested in lasers and want to understand how stimulated emission works.Any links to help explain the question I asked?

 

[blue_leaf77]

that doesn't help unfortunently.

A more elaborate explanation is provided in the first part of my previous post, although that does not really answer your "why" question. Why stimulated emission occurs, as well as many other non-classical phenomena, it is fundamentally due to how quantum world works.

m just interested in lasers and want to understand how stimulated emission works.Any links to help explain the question I asked?

If you are only interested in links, a google search of "stimulated emission" should do the job. But if you plan to be more into it, nothing can beat the orthodox way of reading textbooks. For this one, I recommend a chapter on time-dependent perturbation theory in Introduction to Quantum Mechanics by D. J. Griffiths. If you want a more application oriented text, I used to read Principle of Lasers by O. Svelto.

 

[Somali_Physicist]

A more elaborate explanation is provided in the first part of my previous post, although that does not really answer your "why" question. Why stimulated emission occurs, as well as many other non-classical phenomena, it is fundamentally due to how quantum world works.

If you are only interested in links, a google search of "stimulated emission" should do the job. But if you plan to be more into it, nothing can beat the orthodox way of reading textbooks. For this one, I recommend a chapter on time-dependent perturbation theory in Introduction to Quantum Mechanics by D. J. Griffiths. If you want a more application oriented text, I used to read Principle of Lasers by O. Svelto.

Thank you, since you sound educated in the topic.I was interested in the formation of high frequency lasers and why certain materials have a meta stable state.

 

[blue_leaf77]

Thank you, since you sound educated in the topic.I was interested in the formation of high frequency lasers and why certain materials have a meta stable state.

I used to be educated in this field but I work on a different subject now.
Metastable level usually refers to electric-dipole-forbidden transition from the metastable state to the lower lasing level sometimes the ground state. It is called metastable because it is almost stable, due to electrons at this level being forbidden to undergo dipole moment transition to the lower lasing level the lifetime of metastable level is much longer than the other excited levels. A simple example of metastable level is the 2s level of hydrogen atom. Unless it interacts with an intense EM field, electron in 2s level will spend very long time until it relaxes to 1s. In laser material metastable states serve as the upper lasing level because its long lifetime helps achieving population inversion. See https://www.olympus-lifescience.com/en/microscope-resource/primer/java/lasers/stimulatedemission/

 

[Spinnor]

This is probably as basic as it will get, https://www.eng.famu.fsu.edu/~dommelen/quantum/style_a/ase.html

From, https://www.google.com/search?safe=....64.psy-ab..0.19.733...0j0i67k1.0.0JgKCCsqcoM

Basic enough?

 

[Somali_Physicist]

I used to be educated in this field but I work on a different subject now.
Metastable level usually refers to electric-dipole-forbidden transition from the metastable state to the lower lasing level sometimes the ground state. It is called metastable because it is almost stable, due to electrons at this level being forbidden to undergo dipole moment transition to the lower lasing level the lifetime of metastable level is much longer than the other excited levels. A simple example of metastable level is the 2s level of hydrogen atom. Unless it interacts with an intense EM field, electron in 2s level will spend very long time until it relaxes to 1s. In laser material metastable states serve as the upper lasing level because its long lifetime helps achieving population inversion. See https://www.olympus-lifescience.com/en/microscope-resource/primer/java/lasers/stimulatedemission/

Yep that's exactly why I asked about metastability due to its help in laser formation(population inversion).is the answer to why its metastable simply because "quantum mechanics".

 

[Somali_Physicist]

This is probably as basic as it will get, https://www.eng.famu.fsu.edu/~dommelen/quantum/style_a/ase.html

From, https://www.google.com/search?safe=....64.psy-ab..0.19.733...0j0i67k1.0.0JgKCCsqcoM

Basic enough?

If I said yes I would be lying
:P

 

[Spinnor]

If I said yes I would be lying
:P

The key part is equation 7.41. The electromagnetic field changes different parts of the excited atom wave function differently.

That probably wasn't very satisfying for you? I am glad you asked your question, I will think about it today.

 

[Somali_Physicist]

The key part is equation 7.41. The electromagnetic field changes different parts of the excited atom wave function differently.

That probably wasn't very satisfying for you? I am glad you asked your question, I will think about it today.

Background knowledge is great and challenging.I enjoy this forum as it forces me to question my views on topics.Thankyou for the articles, they will be on my long list of tabs :P

 

[blue_leaf77]

is the answer to why its metastable simply because "quantum mechanics".

The answer cannot be explained with few words or equations with which your level of familiarity with quantum mechanics, as you admitted yourself above, can assist a speed learning, in case you want to do so. It has to do with the so-called dipole selection rules. You already know that electrons can undergo transitions between states but not any pair of state can have appreciable probability of being coupled by electromagnetic field. Practically speaking, an electron sitting on a given level does not go to just another arbitrary level. For it to end up to such forbidden level, it needs very long time because of the low probability of transition between the electron's current state and those forbidden states. Which levels are forbidden to go to from a given starting level is governed by selection rules. A good amount of explanation on this topic is given in the book chapter by Griffiths I suggested in my previous post, but apparently you will need to learn the basic part of QM first, time-dependent Schroedinger perturbation theory with which stimulated emission can be explained can be a source of much head-scratches if you don't have a good grasp of the underlying theory.

 

[DrDu]

I think a key point here is that the electromagnetic field has infinite degrees of freedom and you don't even need quantum mechanics to see this. If you were to consider really only monochromatic light (1 degree of freedom) then the coupling of the electromagnetic field and the electronic motion would lead only to an oscillation of energy between the electron and the field, much like you observe an oscillation of energy between two coupled pendula. However, if there is a continuum of frequencies, the energy will spread out over the different electromagnetic modes and they will never interfere again constructively so that the energy becomes concentrated on the atom, again. Look up "Fermi's Golden Rule" for a formal derivation.

 

[Somali_Physicist]

I think a key point here is that the electromagnetic field has infinite degrees of freedom and you don't even need quantum mechanics to see this. If you were to consider really only monochromatic light (1 degree of freedom) then the coupling of the electromagnetic field and the electronic motion would lead only to an oscillation of energy between the electron and the field, much like you observe an oscillation of energy between two coupled pendula. However, if there is a continuum of frequencies, the energy will spread out over the different electromagnetic modes and they will never interfere again constructively so that the energy becomes concentrated on the atom, again. Look up "Fermi's Golden Rule" for a formal derivation.

I thought Spontaneous Emission was simply because of random anti particles popping into existence and forming photons.These photons are randomly stimulating emission In exicited atoms. Due to their randomness they stimulate emission randomly depending on their frequency.

 

[blue_leaf77]

I thought Spontaneous Emission was simply because of random anti particles popping into existence and forming photons.These photons are randomly stimulating emission In exicited atoms. Due to their randomness they stimulate emission randomly depending on their frequency.

I don't think DrDu talks specifically about spontaneous emission.

Due to their randomness they stimulate emission randomly depending on their frequency.

While it is true that spontaneous emission photons can trigger stimulated emissions (in fact some lasing initiation methods rely on spontaneous emission, see amplified spontaneous emission), the former is not the sole process that stimulates atom to emit photon. Basically any photon, regardless of where and how it came into being, present around the atom can induce stimulated emission.

 

[bhobba]

I think the answer you are looking for is the following:
http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

It's because electrons in an atom are not really in a stationary state because they are coupled to the Quantum EM Field that is everywhere. That leads to the phenomena of spontaneous and stimulated emission. It really is one of the first indications you need Quantum Field Theory.

You consider simulated emission as caused by the photon disturbing the combined electron and quantum EM Field and is handled by perturbation theory. Its an application, if I remember correctly, and mentioned by Dr Du, of Fermi's Golden Rule:
http://staff.ustc.edu.cn/~yuanzs/teaching/Fermi-Golden-Rule-No-II.pdf

Thanks
Bill

 

[bhobba]

I thought Spontaneous Emission was simply because of random anti particles popping into existence and forming photons.These photons are randomly stimulating emission In exicited atoms. Due to their randomness they stimulate emission randomly depending on their frequency.

That's wrong. Spontaneous emission is because the combined electron and EM Field it is coupled to is not in a stationary state. It must eventually decay.

Thanks
Bill

 

[Somali_Physicist]

I don't think DrDu talks specifically about spontaneous emission.

While it is true that spontaneous emission photons can trigger stimulated emissions (in fact some lasing initiation methods rely on spontaneous emission, see amplified spontaneous emission), the former is not the sole process that stimulates atom to emit photon. Basically any photon, regardless of where and how it came into being, present around the atom can induce stimulated emission.

That's wrong. Spontaneous emission is because the combined electron and EM Field it is coupled to is not in a stationary state. It must eventually decay.

Thanks
Bill

Interesting so its both but the one stated by bhobba dominates.

 

[bhobba]

Interesting so its both but the one stated by bhobba dominates.

They are the same. If a photon is emitted then that is a perturbation so you can get stimulated emission/absorbtion.

Thanks
Bill

 

[Somali_Physicist]

They are the same. If a photon is emitted then that is a perturbation so you can get stimulated emission/absorbtion.

Thanks
Bill

Thank you for the help , I have a lot to read up on.I won't lie tho, Quantum physics makes zero sense to me

 

[bhobba]

Thank you for the help , I have a lot to read up on.I won't lie tho, Quantum physics makes zero sense to me

That's normal to start with. But tends to get better with time.

However the following may help you:
https://www.scottaaronson.com/democritus/lec9.html

Eventually, after many years study, most reach at least an 'accommodation' where it still seems weird but really the only way nature can work at the small scale:
https://arxiv.org/pdf/quant-ph/0401062.pdf

Even if you change it in small ways problems occur - you can't seem to escape it, so over time you lust accept it more and more until some young person like you comes along and an old hand explains we all go through it. I have reached my accommodation:
https://arxiv.org/pdf/quant-ph/0101012.pdf

Others do it in different ways - mine is virtually the same as very well respected science adviser Vanhees, who is a professor of physics, and the equally well respected Dymystifyer, also a professor of physics, reached his with Bohmian Mechanics. The sign of maturity in QM is recognizing such choices are simply personal cruxes we all have to come to grips with the weirdness. That is not to say learning about various views/interpretations is of no value - they all shed light of the formalism of QM which everyone agrees on.

Thanks
Bill