Just saw this on a website:


[The following was paraphrased in part from Halliday & Resnick's "Fundamentals of Physics", second edition.]

Absorption. According to quantum mechanics, absorption of photons by atoms occurs only if the wavelength of the photon is just the right size (say, of wavelength lambda). If it is, the atom will "absorb" it (the photon vanishes) and go to a higher energy state. In physics, this process is called "absorption."


Ok, if this is so then does it mean there are certain wavelengths of light that cannot be absorbed by any charged particle and therefore do not interact with matter at all?? Clearly, any wavelength (within reasonable energy bounds) can and should be represented in the universe due to the cosmological redshift, forming a truly continuous spectrum. And so there would be "invisible" radiation zipping through space? This can't be right! What am I missing?

Overdoze,

Ok, if this is so then does it mean there are certain wavelengths of light that cannot be absorbed by any charged particle and therefore do not interact with matter at all??

All photons, ever created, were emitted by atoms. Therefore, all photons can be absorbed by the very same atoms that emitted them. In other words, if a photon exists, it must be able to be absorbed by something.

Tom

Overdoze,

One more thing...

According to quantum mechanics, absorption of photons by atoms occurs only if the wavelength of the photon is just the right size (say, of wavelength lambda).

The statement from your book is wrong. The energy of a photon does not have to be the same as the difference of the electron orbital's energies, in order for it to be absorbed by the electron. A photon with a higher energy will be absorbed, as well.

Tom

Hi Overdoze,

I know that there is an absorption-method called "two-photon fluorescence": two photons arrive, each with too little energy to excite an electron, but together they have the right energy, and both get absorbed. So also lower energy photons can get absorded (even though chances that this happens are about 1 in 1.000.000 photons as I once had to calculate).

With an higher energy energy, the electron can be made free (photoelectric effect), as Tom mentioned.

Bye!

Crisp

Originally posted by Prosoothus
All photons, ever created, were emitted by atoms. Therefore, all photons can be absorbed by the very same atoms that emitted them.


Which is precisely why I dragged in the cosmic redshift to complicate things. ;)


The statement from your book is wrong. The energy of a photon does not have to be the same as the difference of the electron orbital's energies, in order for it to be absorbed by the electron. A photon with a higher energy will be absorbed, as well.


That's cool, but what about any of the lower energies? And even at higher energies, isn't the electron's absorption "profile" still quantised? Generally speaking, what about all the "in-between" energies? (remember the redshift :p)

Crisp, that applies to you too. ;)

Overdoze,

That's cool, but what about any of the lower energies? And even at higher energies, isn't the electron's absorption "profile" still quantised? Generally speaking, what about all the "in-between" energies? (remember the redshift )

Well, for lower energies I would have to agree with Crisp that two or more photons can add up to a required energy for absorbtion.

As for higher energy photons that have more energy than the next orbital level, but less energy than the orbital right above the next orbital, I would assume that the photon is absorbed and a new photon is emitted which contains the excess energy.

As for quantumization, I never understood how red shifts fit into the quantum theory. Maybe someone with more knowledge in that area can explain it to me, as well. :)

Tom

Hi Overdoze,

"That's cool, but what about any of the lower energies? And even at higher energies, isn't the electron's absorption "profile" still quantised? Generally speaking, what about all the "in-between" energies?"

The absorption profile (i.e. the energy spectrum) is ofcourse the same for all possible photon situaties, but you should not forget that part of the spectrum (the positive energy E > 0, which is contineous) represents the unbound electron states, and these are also reachable by higher energies. Thus, if an electron gets an energy that is high enough to get released from the nucleus-bond, then it *can* absorb the photon (remember that we are talking quantum mechanics here and that there are only probabilities that something will happen).

For energylevels between the groundstate and E = 0 (the discrete spectrum part), there are several possibilities: when the photon has (approximately) one of the "right" energies, the electron can get excited to that energy level. If the photon energy is not right, then the photon can be absorbed and the electron enters what is called a "virtual" energylevel (these energylevels are not part of the spectrum, and are called "virtual" because the electron is only at that level for a very, very short time - some people refer to those states as "resonance" states because of their short lifes, eg. 10^-20 s). The electron almost immediatelly falls back to the next lower possible energystate, emitting the energydifference as a photon.

I am not really certain if the above holds for all situations (i.e. if it is atom dependant, or if there are other factors that determine the possibility). I only "used" those energylevels once, in a two-photon absorption experiment, and it seemed to work out well back then ;).

Bye!

Crisp

Short answer quantum is not unitary. Because energy is available in X size does NOT mean 2x 3x 4x.. are the only allowed sizes.

Take an electron around a hydrogen atom. From the Lowest energy level to the next is large. The gap from the second to the third allowed is smaller. And so and so on. By the time you get to where the electron has almost enough energy to leave the hydrogen atom the gaps have got very small.

To absorb a photon, the photon has to have at least the energy to move the electron from one level to the next (or across multiple gaps). The excess energy will be added to the atom and cause it to change is velocity.

If the electron is already exited to a high level it is free to absorb quite low energy photons.

I could go on about Virtual energy levels and Heisenberg Uncertainty so this is not the end of the story but I tend to drivel on to much anyway...

Prosoothus,

Red shift is just the Doppler effect applied to light. As it goes away from you (possibly due to the expansion of space) the wavelength appears to be longer (ever hear an ambulance speed in the other direction, and have tone of the beeping slowly get flatter and flatter?) and therefore appears redder. Red shift is a consequence of light having the properties of waves.

Well, I can't think of anything right now, but the big bang comes to mind. Red shift is mainly caused by the expanding of the universe, which is expanding due to the big bang. The moments of the birth of the universe are governed by quantum mechanics. You could make it fit that way.

But why would one contradict the other? Explain?

Zero,

But why would one contradict the other? Explain?

When an electron falls from a higher orbital to a lower one, it releases a photon which contains the difference of the energies of the two orbitals. The resulting photons frequency is directly proportional to its energy. Since the energies of the different orbitals have certain exact values, all photons emitted from atoms have quantumized energies.

However, when you redshift a photon, its frequency changes (it's wavelength is related to its frequency). Since its frequency changes, its energy changes as well. The resulting energy of a red shifted photon is no longer quantumized because now its energy falls between the energies of two non-redshifted photons.

Unless a photon's frequency shifts in steps during a redshift (it appears that it doesn't), redshifting seems to go against quantum physics.

Tom

Originally posted by Prosoothus
Zero,
Unless a photon's frequency shifts in steps during a redshift (it appears that it doesn't), redshifting seems to go against quantum physics.

Tom

Nope the photons energy and frequency do not shift in steps. Light is not quantized in the sense you refer to. There are no allowed or disallowed frequencies or energies. It is quantised in the sense that the light will be emitted or absorbed only in pieces, the energy of the pieces being dependant on the frequency of the light.

If it helps think of peas in a pipe. Peas can be any size, but you can only add or subtract whole peas.

To make the point clearer. Would light emitted say by a Sodium atom moving from a high to a low state, then red shifted be absorbed by another sodium atom in the same low state ? The answer is no.

No disallowed energies or frequencies? But energy and length are both quantized...especially length (it's called "Planck Length" I think). Then a photon's energy would shift in steps, since it's quantized.

Originally posted by Zero
No disallowed energies or frequencies? But energy and length are both quantized...especially length (it's called "Planck Length" I think). Then a photon's energy would shift in steps, since it's quantized.

Ok if your being picky, but two things to note. Firstly planks length is extreemely tiny, it would still allow many millions of steps between any two electron gaps or if you like, undetectable (as yet?) steps to the red shift. It would also mean the universe expands in fits and starts of plank length steps.

And second planks length as the quantum length if any, is still up for argument by Quantum Physicists. Not proven or needed by any theory I know of, but it does get rid of some of those pesky infinities and singularities.

Originally posted by allant
To make the point clearer. Would light emitted say by a Sodium atom moving from a high to a low state, then red shifted be absorbed by another sodium atom in the same low state ? The answer is no.


Yes, this is exactly what I was talking about. There must be photons out there with frequencies that don't fit any quantum energy gap at all (within reasonable energy/excitation boundaries.)

So I've seen Crisp say that such photons can be absorbed briefly and possibly combined with others or re-emitted. Then you said that if a photon is absorbed then any excess energy is converted to momentum (? not sure if I'm understanding you correctly.) Are you both talking about the same thing? Or is the former for photons whose energy is "too low" for a given energy gap and the latter for photons whose energy is "too high"?

Then does that mean that the original statement I quoted at the start of this thread is not entirely correct -- that really photons of all energies are capable of interacting, however briefly or subtly, with any given bit of matter? IOW absorption, as classically defined, is not the only possible way you can detect photons?

Yes, this is exactly what I was talking about. There must be photons out there with frequencies that don't fit any quantum energy gap at all (within reasonable energy/excitation boundaries.)



Yes there is possible for light not to fit any quantum energy gap at all (within reasonable energy/excitation boundaries.) (for practical purposes we are talking radio waves) If you are allowed to pick the material it is easy - for instance say green light and glass at room temp.

If not energetic enough, ie low frequency, then it will not interact with matter at reasonable energy/excitation boundaries. But can with matter in high energy/excitation states - because at high energy levels there are lots and lots of states close together.

Tom,

<i>The energy of a photon does not have to be the same as the difference of the electron orbital's energies, in order for it to be absorbed by the electron. A photon with a higher energy will be absorbed, as well.</i>

If you are really trying to say what you seem to be saying here, I'm afraid you're wrong.

James,

"The energy of a photon does not have to be the same as the difference of the electron orbital's energies, in order for it to be absorbed by the electron. A photon with a higher energy will be absorbed, as well."

If you are really trying to say what you seem to be saying here, I'm afraid you're wrong.

Feel free to elaborate.

Tom

Ok. If a photon's energy is larger than any gap between atomic energy levels it will not be absorbed by that atom.