| Topic: |
Science > Physics |
| User: |
"Doodedski" |
| Date: |
12 Nov 2004 07:29:42 PM |
| Object: |
Photon hits an atom |
I've been reading my notes on a course on relativity I did and several
examples talk about a photon hitting an atom. The atom is said to
then have a kinetic energy equal to the energy of the photon. I just
don't see how this happens. My friend says that when a photon has an
energy less than the gap between two energy levels of an electron,
this electron can absorb the photon and transfer the energy
(instantly) to the atom. I just don't see this being possible (an
electron will only absorb energy of a photon if it matches a gap).
Who's right, and what is going on in these examples?
Thanks,
Dood
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Posted at:
www.GroupSrv.com
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| User: "Old Man" |
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| Title: Re: Photon hits an atom |
12 Nov 2004 08:39:23 PM |
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"Doodedski" <billybilly54321@hotmail-dot-com.no-spam.invalid> wrote in
message news:41956386$1_1@Usenet.com...
I've been reading my notes on a course on relativity I did and several
examples talk about a photon hitting an atom. The atom is said to
then have a kinetic energy equal to the energy of the photon. I just
don't see how this happens. My friend says that when a photon has an
energy less than the gap between two energy levels of an electron,
this electron can absorb the photon and transfer the energy
(instantly) to the atom. I just don't see this being possible (an
electron will only absorb energy of a photon if it matches a gap).
Who's right, and what is going on in these examples?
Thanks,
Dood
It's a virtual, off-shell, interaction whereof 4-vector momentum
is conserved. Another photon, having the same energy, but
lagging in phase, is subsequently emitted. Via the photon's
electromagnetic field, the electron orbital is perturbed into a
state that's not orthogonal to that of the initial and final state.
[Old Man]
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| User: "tj Frazir" |
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| Title: Re: Photon hits an atom |
14 Nov 2004 01:20:37 PM |
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Hits an atom......not a hit ...
the photon changes drection in the low pit of gravity to remain at the
same speed and orbits the atoms pit. It will exchange its energy with
the electron made by te motions of the nucleus.
The electron will have more mass ..
morre mass than the orbit will handle cant chang speed , so some energy
will change directions because it can't speed up and some eenergy will
change directions and thats the photon re-ejected.
like hitting a curve .....
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| User: "Doodedski" |
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| Title: re:Photon hits an atom |
15 Nov 2004 12:28:02 AM |
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Hi Harry,
I do understand the photoelectric effect. My question was regarding
the transfer of energy to the atom itself and André Michaud cleared
things right up (although now I have questions on the specifics of
raman scattering...).
Doodedski
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Posted at:
www.GroupSrv.com
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| User: "Doodedski" |
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| Title: re:Photon hits an atom |
14 Nov 2004 06:28:20 PM |
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Thanks for the info.
When a photon of energy less than the smallest gap allowed hits
the electron, the electron does not jump, but simply oscillates
more strongly on its orbital. That kinetic energy may be transfered
to the atom. That's how radiant heat (infrared) is absorbed.
I really didn't think that was possible. I thought the photon had to
match the gap to excite the electron. This really clears up things
and clearly, my friend was right and I was wrong.
There also are cases when only part of the energy of an incoming
photon is absorbed. Regarding this, read on Compton scattering
and Raman scattering.
I've studied the compton effect but this was with a free electron so
his energy levels were not limited. As for Raman scattering, I
remember doing an experiment on the subject. My teacher had
explained it by saying that the photon was hitting a trampoline (
crystal lattice vibrating) and the energy of the scattered photon
depended on when the photon hit the trampoline (absorbing or emitting
a phonon). This image is pretty clear but I am just not sure about
what is really going on. Isn't the photon simply being absorbed by
an electron (with limited energy levels) and than reemitted. How is
the vibration affecting this process?
Thanks,
Doodedski
*-----------------------*
Posted at:
www.GroupSrv.com
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| User: "Andr? Michaud" |
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| Title: Re: Photon hits an atom |
15 Nov 2004 11:39:42 AM |
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billybilly54321@hotmail-dot-com.no-spam.invalid (Doodedski) wrote in message news:<4197f824$1_1@Usenet.com>...
Thanks for the info.
When a photon of energy less than the smallest gap allowed hits
the electron, the electron does not jump, but simply oscillates
more strongly on its orbital. That kinetic energy may be transfered
to the atom. That's how radiant heat (infrared) is absorbed.
I really didn't think that was possible. I thought the photon had to
match the gap to excite the electron. This really clears up things
and clearly, my friend was right and I was wrong.
I will add that with radian heat, if more infrared is contributed,
there comes a point when enough energy has been accumulated to cause
electrons to jumps to the next orbital to occur. Which, being unstable,
will soon release a photon as returns to its usual rest orbital.
This is why materials eventually start to radiate when heat is kept
on.
There also are cases when only part of the energy of an incoming
photon is absorbed. Regarding this, read on Compton scattering
and Raman scattering.
I've studied the compton effect but this was with a free electron
so his energy levels were not limited.
Textbooks do not often elaborate on the real meaning of "free" in this
context. The real qualifier should be "loosely held".
Compton carried his experiments by scattering electrons loosely held
at the surface of a graphite block.
As for Raman scattering, I remember doing an experiment on the subject.
My teacher had explained it by saying that the photon was hitting a
trampoline (crystal lattice vibrating) and the energy of the scattered
photon depended on when the photon hit the trampoline (absorbing or
emitting a phonon). This image is pretty clear but I am just not sure
about what is really going on. Isn't the photon simply being absorbed
by an electron (with limited energy levels) and than reemitted. How is
the vibration affecting this process?
I am not a specialist of the Raman effect, but the trampoline comparison
seems sound to me as most photons used in such testing rebound through
elastic scattering and so lose no energy.
Some photons however do lose or gain energy during the encounters, which
means that some kinetic energy is contributed or removed from the molecules
being scattered against, thus either increasing or decreasing the vibration
state of the concerned electrons in the material.
With Raman scattering, no quantum jump to higher orbitals is typically
involved, so only the vibration state of electrons in the material seems
to be generally affected.
Each material has a different Raman spectrum signature. I recall having
read that Raman scattering is used in the industry for certain tests
of integrity in manufacturing.
André Michaud
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| User: "Harry Conover" |
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| Title: Re: Photon hits an atom |
14 Nov 2004 07:42:32 PM |
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billybilly54321@hotmail-dot-com.no-spam.invalid (Doodedski) wrote in message news:<41956386$1_1@Usenet.com>...
I've been reading my notes on a course on relativity I did and several
examples talk about a photon hitting an atom. The atom is said to
then have a kinetic energy equal to the energy of the photon. I just
don't see how this happens. My friend says that when a photon has an
energy less than the gap between two energy levels of an electron,
this electron can absorb the photon and transfer the energy
(instantly) to the atom. I just don't see this being possible (an
electron will only absorb energy of a photon if it matches a gap).
Who's right, and what is going on in these examples?
Thanks,
Dood
Hey Dood, it's called the Photo-Electric effect. Check it out in any
physics textbook. In particular reference to your question, sweep the
wavelength of the photons over a wide range, then observe the
corresponding electron emission from the photo-emissive surface and
see if you observe the quantum steps that you predict.
Harry C.
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| User: "Andr? Michaud" |
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| Title: Re: Photon hits an atom |
14 Nov 2004 10:43:19 AM |
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billybilly54321@hotmail-dot-com.no-spam.invalid (Doodedski) wrote in message news:<41956386$1_1@Usenet.com>...
I've been reading my notes on a course on relativity I did and several
examples talk about a photon hitting an atom. The atom is said to
then have a kinetic energy equal to the energy of the photon. I just
don't see how this happens. My friend says that when a photon has an
energy less than the gap between two energy levels of an electron,
this electron can absorb the photon and transfer the energy
(instantly) to the atom. I just don't see this being possible (an
electron will only absorb energy of a photon if it matches a gap).
Who's right, and what is going on in these examples?
Thanks,
Dood
Hi Dood (Dude?)
Presently, they are both right.
When hit by a photon of energy that exactly matches a gap, the
electron will absorb the energy and jump the gap (move to the
matching location further away from the nucleus), with no kinetic
energy transfer to the atom (the atom may remain stationary if it
was before absorbtion).
When a photon of energy less than the smallest gap allowed hits
the electron, the electron does not jump, but simply oscillates
more strongly on its orbital. That kinetic energy may be transfered
to the atom. That's how radiant heat (infrared) is absorbed.
There also are cases when only part of the energy of an incoming
photon is absorbed. Regarding this, read on Compton scattering
and Raman scattering.
André Michaud
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