Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Nicholas Lee on 24/05/2016 22:39:10
-
While electrons in the band gap is there no wavelength, or frequency to move electrons to the next shell level.
So no wavelength or frequency in any EM waves can make electrons move from the ground state, to next shell level, while in band gap region?
In this statement from how stuff works about energy bands, does it mean that there is not enough energy in any electromagnetic wavelength, from radio waves to gamma waves, to move electrons at all to the next shell level.
This is the statement.
"In energy bands in between these bands are regions, known as band gaps, where energy levels for electrons don't exist at all. Some materials have larger band gaps than others. Glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another and back again. "
So no wavelength or frequecy in any EM waves, can make electrons move from the ground state, to the next level while the electron is in the band gap region.
Is this correct?
I am grateful for your help, anything helps even a few words. [:D]
-
No. The band gap is between energy levels. Electrons can be below the band gap, and then moved above the band gap, but will never be in the gap (hence the word "gap"). The span of the gap determines how much energy is required, and glass has a large band gap, so therefore requires photons with high energies.
-
[;D] [;D] [;D]great thank you
)]No [;D]
. The band gap is between energy levels. Electrons can be below the band gap, and then moved above the band gap, but will never be in the gap (hence the word "gap"). The span of the gap determines how much energy is required, and glass has a large band gap, so therefore requires photons with high energies.
-
No. The band gap is between energy levels. Electrons can be below the band gap, and then moved above the band gap, but will never be in the gap (hence the word "gap"). The span of the gap determines how much energy is required, and glass has a large band gap, so therefore requires photons with high energies.
What will happen if photons with slightly higher energy than required by the band gap hit the electrons?
-
No. The band gap is between energy levels. Electrons can be below the band gap, and then moved above the band gap, but will never be in the gap (hence the word "gap"). The span of the gap determines how much energy is required, and glass has a large band gap, so therefore requires photons with high energies.
What will happen if photons with slightly higher energy than required by the band gap hit the electrons?
Usually (for hard matter) there is a fairly broad band that the electron can get excited to above the band gap, so there will be a broad absorption starting at the band gap energy, and extending to higher energies. There can be a second band gap that prevents absorption at specific range of higher energies. Eventually when the energy of the photon is high enough, it can be absorbed and cause an electron to leave the material completely (photoelectric effect).
-
Usually (for hard matter) there is a fairly broad band that the electron can get excited to above the band gap, so there will be a broad absorption starting at the band gap energy, and extending to higher energies. There can be a second band gap that prevents absorption at specific range of higher energies. Eventually when the energy of the photon is high enough, it can be absorbed and cause an electron to leave the material completely (photoelectric effect).
How do you define hard matter?
How can laser cooling work if photons with lower energy than the band gap are neglected?
-
Usually (for hard matter) there is a fairly broad band that the electron can get excited to above the band gap, so there will be a broad absorption starting at the band gap energy, and extending to higher energies. There can be a second band gap that prevents absorption at specific range of higher energies. Eventually when the energy of the photon is high enough, it can be absorbed and cause an electron to leave the material completely (photoelectric effect).
How do you define hard matter?
How can laser cooling work if photons with lower energy than the band gap are neglected?
I say "hard matter" meaning mostly metal or main group elements bonded with metallic, covalent or ionic bonds into a large lattice or amorphous glass. This is different from "soft matter" which is molecular (usually organic) or polymeric, including small molecules like benzene and formaldehyde, and large molecules like proteins, DNA, and organic polymers like nylon...
My understanding of laser cooling is that it works by bouncing light off of a sample, and the reflected light has somewhat increased frequency due to Doppler shift, thereby draining some kinetic energy (and momentum) from the sample. I don't believe that the sample has to absorb the light, just as glass and silver and water can reflect light of most colors (in fact, I think this may work best for photons with energies that are unlikely to be absorbed by the sample...)
-
http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lascool.html <- Laser cooling
Of note:
A conceptual problem is that an absorption can also speed up an atom if it catches it from behind, so it is necessary to have more absorptions from head-on photons if your goal is to slow down the atoms. This is accomplished in practice by tuning the laser slightly below the resonance absorption of a stationary sodium atom. From the atom's perspective, the headon photon is seen as Doppler shifted upward toward its resonant frequency and it therefore more strongly absorbed than a photon traveling in the opposite direction which is Doppler shifted away from the resonance. In the case of our room temperature sodium atom above, the incoming photon would be Doppler shifted up 0.97 GHz, so to get the head on photon to match the resonant frequency would require that the laser be tuned below the resonant peak by that amount.
You specifically need to use an absorption event rather than simple scattering (i.e. what happens when the photon energy doesn't coincide with an energy level difference) and you actually need to use light slightly under the energy of the edge in order to selectively excite only the atoms you want to excite (the ones coming towards you that would be slowed instead of the ones going away that would speed up). Using just simple scattering the momentum transfer is generally much much smaller than an absorption event and you lose any hope of selectively interacting with only the atoms you can cool.
-
Can you move electrons to higher shell levels, or change band gap, with negative charges, or by making the electron more energetic.?
When electrons move to higher shell levels the electron voltage requirment from light to move the electron to a higher shell level changes.
I was thinking is there another way to move electrons to higher shell levels to change the electrons electron voltage, or band gap, by these 2 techniques
1. pushing the electron away from the nucleus with negative charges somehow
2. Is there a way to make the electron more energetic, because then if its more energetic it will move further away from the nucleus.
Are these two ways possible to effect the electron to move to higher shell levels, farther than using light, in the absorption, and emission process.
Electrons have a negative electrical charge. This means they are attracted to positive electrical charges and pushed away by negative charges. Electrons cling to the nucleus because protons have a positive charge equal to the electron’s negative charge.
Electrons have so much energy that they whizz round too fast to fall into the nucleus.
Instead they circle the nucleus in shells (layers) at different distances, or energy levels, depending on how much energy they have.
The more energetic an electron, the farther from the nucleus it is.
So could it be done on these two way.
I am grateful for your help, anything helps even a few words. [:D]
[MOD EDIT - PLEASE TRY TO FORMAT YOUR THREAD TITLES AS SELF-CONTAINED SUCCINCT QUESTIONS IN FUTURE]
-
1. pushing the electron away from the nucleus with negative charges somehow
2. Is there a way to make the electron more energetic, because then if its more energetic it will move further away from the nucleus.
Making any measurable adjustment to the energy levels of electrons would cause many essential chemical reactions in the body to fail. Undesirable side-products would appear, which would kill every cell in your body.
-
Thank you for answer. [:D]
Absorption, and emission process happens in tissue all the time, with the electron changing from shell to shell all the time.
So in my mind changing the electrons electron voltage requirement, to make human tissue more translucent should be harmless.
I thought with gamma radiation, is what destroys cells, the proton geting knocked out of the nucleus, or electron leaving the nucleus through ionization, like with ultraviolet rays in sunburnt skin.
In my mind there is five ways to increase the electrons band gap to light.
1. Heat, hot, or cold temperature.
2. Electromotive force
3. MAYBE microwaves, in combination with another heat source.
4. Negative charges
5. Somehow making the electron more energetic to move away from the nucleus to a different shell level.
Do you think a combination of any of these techniques, can change the electrons electron voltage to Not get excited by light, but in solid opaque matter.
Thank you for your help.
[O8)]1. pushing the electron away from the nucleus with negative charges somehow
2. Is there a way to make the electron more energetic, because then if its more energetic it will move further away from the nucleus.
Making any measurable adjustment to the energy levels of electrons would cause many essential chemical reactions in the body to fail. Undesirable side-products would appear, which would kill every cell in your body.