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Old Nov 29th 2009, 06:49 AM   #1
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a fool's question on photoelectricity

I have thought about this for nearly 6 months but still in dark.
why in photoelectricity only one photon is absorbed per electron??
why cannot an electron absorb two photons(infact,photons are massless(ok! in rest frame ), so no bulk repulsion)?
yet in experiments , there is a definite stop potential which implies only one electron interacts with one photon. BUT WHY??

Last edited by r.samanta; Nov 29th 2009 at 06:52 AM.
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Old Nov 29th 2009, 09:53 PM   #2
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This is definitely not a fool's question. It shows some really original thinking.

Maybe it is because the particle nature of light is considered here and the photon is treated as "hitting" the electron. So the probability of two photons simultaneously hitting an electron would be low as "light speeds" are involved.

And the fact that there is a definite stopping potential can prove that only one photon is involved only if the k.e. of the emitted electron is less than that of the incident photon. I dont know if this is always the case.
If a stopping potential is found which puts the k.e. of the electron in between the energy of a single photon and two photons, what you say could be possible.

It is of interest to note that the probabilty for double quantum emission i.e that two photons of the same energy rather than one photon with twice the energy are emitted was worked out by Maria Goeppert Mayer who later on won the nobel for nuclear shell structure. She also worked out the probability for the reverse namely absorption. I have a vague idea this is used in lasers nowadays. This of course was related to atomic spectra, but i find an interesting parallel in your line of thinking.

I didnt quite get what was your query about bulk repulsion.
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Old Dec 5th 2009, 08:06 AM   #3
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"bulk repulsion " was a term that i used because i was at that time suffering from chem syndrome. - ie. steric hindrance of organic chemistry.
however, i am still working on this. will let u know my progress soon.
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Old Jan 16th 2010, 04:23 PM   #4
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Smile The atom limits the frequency of an emitted photon from that atom

Originally Posted by r.samanta View Post
I have thought about this for nearly 6 months but still in dark.
why in photoelectricity only one photon is absorbed per electron??
why cannot an electron absorb two photons(infact,photons are massless(ok! in rest frame ), so no bulk repulsion)?
yet in experiments , there is a definite stop potential which implies only one electron interacts with one photon. BUT WHY??
Typically when an incoming photon of the proper frequency (and hence energy) strikes an electron it excites it into a higher orbital level; it is unstable at this higher level and will jump back down very shortly after being struck. Upon coming back down to a stable orbit it emits a photon of its own that uniquely defines what type of atom that it is, i.e. Hydrogen, Helium, Oxygen, etc...

It is highly unlikely that two photons will strike the same electron one right after the other. As to why it should not be able to absorb two photons is because the electron is part of an atom that emits photons of a certain frequency when excited. The frequency of a photon emitted by each element (atom) is different for each element; that's how we can use spectroscopy to find the chemical makeup of say the atmosphere on Mars, we know Mars's atmosphere is mostly carbon dioxide because the spectral lines indicate so, if carbon dioxide could emit photons at a different frequency, then it would be difficult to tell what anything is composed of using spectroscopy, because if some atom were able to emit photons at a greater frequency, we may well mistake the spectral lines from the analysis as coming from a different atom altogether if the atom were allowed to emit photons of a greater frequency than usual as it may well line up with the frequency of photons emitted by a completely different element, and we wouldn't know for sure which one we were looking at.

So if an element gets struck by a photon of the right frequency such that it imparts its energy into an electron that jumps up and then back down emitting a photon of its own in the process, since it can only emit a photon of a particular frequency for that element, that means it can only emit a photon of a certain energy for that element.

If it were struck by two photons that would violate the idea that for a particular element, the electrons for that element emit a photon of a particular energy/frequency; because if it were able to absorb the energy from two photons it would be very excited and would jump up even higher in orbital levels than normally for that atom and then when it falls back to a stable orbit it would have to emit a photon with a greater amount of energy (or of a greater frequency) than that electron as part of that particular atom is "allowed" to emit photons at.

So if an electron for a particular atom can only emit photons of a certain frequency, it makes sense that they should only be able to absorb the energy needed to bring them up and then down in orbital levels such that they emit a photon of that atoms photon emitting frequency. If an electron within that atom were capable of absorbing two photons of the correct frequency, again that would cause it to jump up further and then fall back to a stable orbit emitting a photon of a frequency that is greater than that atom is "allowed" to emit photons at and the energy seen emitted from that atom may well be the same or close to the energy of a photon emitted from a completely different atom/element and therefore we wouldn't be able to distinguish between the two different atoms; and therefore the emitted photon would have more energy than that atom is "allowed" to impart into an emitted photon.

Does that make sense?

Many Smiles,
Craig
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Old Jan 17th 2010, 01:31 AM   #5
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Smile Does that make sense?

I'm actually wondering if my, informed guess, actually makes sense. I ask this because the idea of mine that stated that if an electron in orbit around a particular atom emits a photon whose energy may be great enough to be confused with a different type of element, if capable of absorbing two photons worth of energy, then spectroscopy would be useless in distinguishing different types of elements. This logically makes sense.

After reading what physicsquest wrote stating, "It is of interest to note that the probability for double quantum emission i.e. that two photons of the same energy rather than one photon with twice the energy are emitted was worked out by Maria Goeppert Mayer who later on won the Nobel for nuclear shell structure."

Now assuming that probability is non zero and that the two emitted photons came from the same electron which is implied by the statement that two photons are emitted instead of one with twice the energy, which implies a single electron can emit a photon with twice the energy and therefore it makes sense that the one electron can instead with some probability can emit two photons with half the energy of the possibly one emitted photon that can be emitted from the same electron.

The only way this could happen is if this one electron was capable of taking on more energy than the discrete quantities needed to move it up an orbital level to emit a single photon.

So if the two photons came from the same electron this could only occur if this single electron could absorb more energy than I thought possible. The idea of spectroscopy still being able to differentiate between two different elements would still be true since the two emitted photons would be of that elements natural emitting frequency (when excited to emit its natural frequency of light energy).

So if it’s possible for an electron to take on enough energy to emit two photons, the only way I see this being possible is if the electron is hit by, I would think, at least two photons at the same time. Since the discrete amounts of energy needed to raise an electron to emit a photon decrease in amount the further away each orbit is; that is it takes a little less energy to move an electron from the third orbital level to its valance band than it takes moving that same electron from its second orbital level to its third, because the pull on the electrons decreases the further they get from the nucleus.

Then you r.sanata (I believe your name is Samantha, so I will use your name, if I'm wrong do correct me) say this, "there is a definite stop potential which implies only one electron interacts with one photon.
What is this stopping potential?"

Is this stopping potential so great that an electron cannot take on enough energy to move it to an orbital level great enough for that electron to emit two photons? Or is it that the electron simply takes on the energy required for it to "jump" up and down and emit only a single photon?

If so where does the additional energy go from being struck by two photons at the same time; as I don't see how two photons striking the same electron can absorb the energy from one photon but simultaneously "ignore" the second photon allowing it to somehow pass through it not affecting the electron leaving only the one photon to excite the electron to emit a photon?

So it seems unlikely that the electron somehow simultaneously ignores one photon while accepting the other. It seems more likely that the electron can take on the energy from at least two photons at the same time. So again after reading what physicsquest wrote about two photons being emitted instead of one with twice the energy implying an electron can somehow take on enough energy to emit two photons along with what you wrote Samantha that there is this stopping potential that appears to limit the amount of energy that an electron can absorb; it seems that the discovery that physicsquest found, has somehow found a way to deliver into a single electron the energy that is great enough to violate the principle that there is this stopping potential that will limit the energy an electron can take on.

So basically my new theory based on both what physicsquest and Samantha wrote is that it should only be possible for an electron to emit two photons of the same frequency/energy instead of one with twice that energy if that electron can absorb the energy from at least two photons at the same time such that the single electron has the potential from being raised to such a higher orbital level to emit two photons at the same time as well. In other words it makes sense that if an electron can be excited great enough such that it may emit two photons of the same frequency, it seems just as possible that the single electron may be capable of absorbing the energy from two photons at the same time.

This stopping potential you speak of Samantha may be based simply on the highly unlikely probability the an electron may be struck by two incoming photons at the same exact time. So this stopping potential may not really be associated with somehow stopping an electron from absorbing the energy from two photons at once and may be based on the high unlikelihood that an electron may actually be struck by two photons at the same time; I don't know, what do you think of this theory?

Does this one make sense?

Many Smiles, Craig

Last edited by clombard1973; Jan 17th 2010 at 01:41 AM.
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Old Jan 19th 2010, 04:29 AM   #6
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sir,
i understood your first post but am finding difficult to understand the second. i am just clarifying myself a bit....
by stopping potential , i mean this....
einstein photoelectric eq tells me that the maximum kinetic energy of an electron which is released from metal surface shone by light rays, is the energy of incident photons minus the work function(min energy required to free the outermost electron from the metal surface). the potential difference applied if equals this maximum kinetic energy , then even the fastest electrons will not be able to reach the plate and so i get no photocurrent. in a graph of i versus V, will have a point on negative V axis where i=0, this V is stopping potential.
however i am not able to understand why we are bringing photon emission here? i mean, i need to think about the emitted electrons here? or i am going wrong ?
many thanks for the help .
http://galileo.phys.virginia.edu/cla...ic_effect.html

Last edited by r.samanta; Jan 19th 2010 at 04:47 AM.
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Old Jan 19th 2010, 07:13 PM   #7
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Smile I see

Originally Posted by r.samanta View Post
sir,
i understood your first post but am finding difficult to understand the second. i am just clarifying myself a bit....
by stopping potential , i mean this....
einstein photoelectric eq tells me that the maximum kinetic energy of an electron which is released from metal surface shone by light rays, is the energy of incident photons minus the work function(min energy required to free the outermost electron from the metal surface). the potential difference applied if equals this maximum kinetic energy , then even the fastest electrons will not be able to reach the plate and so i get no photocurrent. in a graph of i versus V, will have a point on negative V axis where i=0, this V is stopping potential.
however i am not able to understand why we are bringing photon emission here? i mean, i need to think about the emitted electrons here? or i am going wrong ?
many thanks for the help .
http://galileo.phys.virginia.edu/classes/252/photoelectric_effect.html

Sorry about not getting back to you sooner, I only have a minute so I don't even have time right now to address your question, but I did want to tell you why I brought up photon emission.

I thought you were considering the possibility of en electron taking on the energy from two incoming photons and were wondering why if an electron were simlmutaneously struck by two photons it would emit only a single photon of that atoms natural photon emitting frequency/energy; hence the reason why I stated that after reading what physicsquest had written I thought it may indeed be possible for an electron to take on the energy from two photons at the same time and emit two photons of that atoms frequency and not one with a greater frequency that could be confused with a different atom via spectroscopy. So again, I misunderstood your question. It's difficult sometimes to correctly interpret someones questions, particularly when I had a different idea in mind when I answered what I thought you were inquiring about.

Although it wasn't what you were inquiring about, it's a neat concept, and since I thought it was what your inquiry was about, I did put a fair amount of time into thinking about that particular event. Not that it matters as I know now it wasn't what you were inquiring about.

I didn't realize you were referring to basically the photoelectric effect. I do understand the photoelectric effect fairly well, and I promise when I get some free time I will reply with some useful information. I wish I had time now, but my schedule for the next two days is pretty much "stuffed" with exams to give (which means I have to write them up), I have a talk I'm giving on Carbon Nanotube Technology, it being a very light material with a conduction process that is ballistic in nature and a conductivity, based on how it is post processed, i.e. chemically treated, streched, condensed, etc... that is close to Aluminum and is about 10 times smaller than Copper; the idea is to keep increasing its conductivity in hopes of replacing copper as an electrical conductor. CNT is about 0.4 g/cc where copper has a desnity of about 8.9 g/cc, CNT is far lighter. Plus it can be doped into a semiconducting material and I've found it has a good Seebeck value with a decent figure of merit (the ZT of the material) and a low thermal conductivity; good for thermoelectrically inducing a voltage from a temperature gradient, but not so good as a heat dissipating material. So I have some more preparation for that talk to do.

I will get get back to this post and explain my understanding of the photoelectric effect that I have; hopefully it will be of help to you. Sorry about the misundersanding.

Lastly, please feel free to hit me up with any questions in C programming that you may have; C I know better than any other programming lanuage with the exception of perhaps Java.

Ricky, wow I was way off calling you Samantha, sorry about that as well.

Many Smiles,
Craig
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Old Jan 20th 2010, 01:58 AM   #8
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Heres some more stuff on two photon emission and absorption. The beauty of the argument by Mayer lies in the fact that there is nothing in the eqns governing Q.M. which forbids this, though the physics community at that time didnt think of it. As can be seen, the probability is very low it being a higher order process but Q.M. permits even seemingly impossible things to happen, provided there is a non-zero probability.




Two photon absorption
(TPA) 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 electronic state. The energy difference between the involved lower and upper states of the molecule is equal to the sum of the energies of the two photons. Two-photon absorption is a second-order process several orders of magnitude weaker than linear absorption and is therefore not an everyday phenomenon. It differs from linear absorption in that the strength of absorption depends on the square of the light intensity, thus it is a nonlinear optical process.Two photon absorption (TPA) 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 electronic state. The energy difference between the involved lower and upper states of the molecule is equal to the sum of the energies of the two photons. Two-photon absorption is a second-order process several orders of magnitude weaker than linear absorption and is therefore not an everyday phenomenon. It differs from linear absorption in that the strength of absorption depends on the square of the light intensity, thus it is a nonlinear optical process.



Two-photon emission

The opposite process of TPA is two-photon emission (TPE), which is a single electron transition accompanied by the emission of a photon pair. The energy of each individual photon of the pair is not determined, while the pair as a whole conserves the transition energy. The spectrum of TPE is therefore very broad and continuous [6]. TPE is important for applications in astrophysics, contributing to the continuum radiation from planetary nebulae, and atomic physics due to the emitted spectrum dependence on the entire quantum level structure of the system [7]. TPE in condensed matter and specifically in semiconductors was only recently observed [8] , with emission rates nearly 5 orders of magnitude weaker than OPE, with potential applications in quantum information.
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Old Jan 20th 2010, 05:48 AM   #9
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Smile Thank you. (Hi)

Thank you for the additional info, that was exactly what I was wondering about. It would appear I don't need to conjecture on the possibility of double photon emission as it seems this has already been accomplished. Or perhaps more accurately, can and does occur just with a low probability.

I wonder why the energy of the double emitted photons is unknown. I would think it could be calculated if one knows the frequency of the incoming photon and the energy needed to raise the electron an extra discrete level such that it emits two photons.

Maybe I misunderstood but it does say the transition energy of the electron is conserved within the photon pair yet the energy of the photon pair, when emitted, is not determined. If it is not determined how can they be sure the transition energy is conserved within the photon pair? I'm sure it is conserved, it makes sense that it would be, but I wonder if they simply assumed the transition energy is conserved within the photon pair, or if somehow they found another way to confirm that statement with a non-determined amount of energy from the dual photons.

Anyway, thank you for the additional info; it's much appreciated.

Many Smiles, Craig.
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