Atomic spectra

[lemur]

You heat a substance up to 4000 degrees F and it emits visible light in the blue end of the spectrum. You heat a substance up to 100 degrees F and it emits infrared light which isn't visible to us. An electron and anti-electron collide and supposedly the matter of each cease to exist while two gamma rays result.

Ok, but both hydrogen and calcium emit at @480nm; but whereas calcium will also emit at 440-450, hydrogen won't emit until under 440. What causes the hydrogen to hold out for more energy (or am I understanding the spectra wrong?)?

[steevey]

Ok, but both hydrogen and calcium emit at @480nm; but whereas calcium will also emit at 440-450, hydrogen won't emit until under 440. What causes the hydrogen to hold out for more energy (or am I understanding the spectra wrong?)?

 

My guess is it has something to do with matter being quantized and having only specific values, otherwise I don't really understand what your saying. Could you be more specific? At what temperature does this happen? What circumstances? You saying when hydrogen/calcium is heated up? Your saying when hydrogen/calcium absorbs other light?

 

If you heat up hydrogen to different temperatures, it will emit different spectrum of light. However, because matter and energy are quantized, hydrogen will only absorb energy which allow the electrons to move up exact intervals of levels, as in exactly to the next energy level or exactly to the 3rd energy level, so my guess is hydrogen wouldn't emit a specific frequency because it can't absorb the right frequency for an electron to jump down to a lower energy from the specific higher one required to make the photon and emit it. Although, from my point of view, hydrogen should be able to emit the lowest possible frequency of light seeing as how it has only 1 valence electron which can havethe valence electron in the lowest possible energy state, the ground state.

Edited April 21, 2011 by steevey

[swansont]

Ok, but both hydrogen and calcium emit at @480nm; but whereas calcium will also emit at 440-450, hydrogen won't emit until under 440. What causes the hydrogen to hold out for more energy (or am I understanding the spectra wrong?)?

 

Hydrogen does emit below 440. There's a whole set of lines in the UV, known as the Lyman series. Even the Balmer series has a couple of lines below that.

http://en.wikipedia.org/wiki/Hydrogen_spectral_series

[lemur]

My guess is it has something to do with matter being quantized and having only specific values, otherwise I don't really understand what your saying. Could you be more specific? At what temperature does this happen? What circumstances? You saying when hydrogen/calcium is heated up? Your saying when hydrogen/calcium absorbs other light?

 

If you heat up hydrogen to different temperatures, it will emit different spectrum of light. However, because matter and energy are quantized, hydrogen will only absorb energy which allow the electrons to move up exact intervals of levels, as in exactly to the next energy level or exactly to the 3rd energy level, so my guess is hydrogen wouldn't emit a specific frequency because it can't absorb the right frequency for an electron to jump down to a lower energy from the specific higher one required to make the photon and emit it. Although, from my point of view, hydrogen should be able to emit the lowest possible frequency of light seeing as how it has only 1 valence electron which can havethe valence electron in the lowest possible energy state, the ground state.

I should have posted a link to the spectrum comparison of hydrogen and calcium I googled when posting before (now I can't find it). Anyway, I mentioned different frequency blues because you mentioned blues. I don't remember what hydrogen does at ground state. I could fathom, however, that it could be possible that electron transitions in heavier atoms could produce lower frequencies if they take place farther from the nucleus; but I don't really know what I'm talking about empirically nor do I have any hypothesis about the relationship between the various shapes of orbitals and the amount of energy absorbed/emitted when they transition from one to another.

 

What does seem logical to me is that the difference in spectrum lines could have to do with the fact that all substance behave according to ideal gas laws (I think) regardless of weight. The way I understand this is that an atom/molecule of calcium may weigh a lot more than one of hydrogen, but both will express the same amount of pressure at the same temperature as a gas. If I am understanding ideal gas behavior right, then, a heavier substance exerting the same amount of pressure as a lighter one requires more energy to do more work to keep the particles moving at the same average speed. If that is indeed the case (if I don't understand it wrong), then it seems like the amount of energy required to emit a certain frequency photon would occur at a lower temperature in a heavier substance because more energy is required to bring it to that temperature.

 

 

[swansont]

What does seem logical to me is that the difference in spectrum lines could have to do with the fact that all substance behave according to ideal gas laws (I think) regardless of weight. The way I understand this is that an atom/molecule of calcium may weigh a lot more than one of hydrogen, but both will express the same amount of pressure at the same temperature as a gas. If I am understanding ideal gas behavior right, then, a heavier substance exerting the same amount of pressure as a lighter one requires more energy to do more work to keep the particles moving at the same average speed. If that is indeed the case (if I don't understand it wrong), then it seems like the amount of energy required to emit a certain frequency photon would occur at a lower temperature in a heavier substance because more energy is required to bring it to that temperature.

 

These are separate effects. Blackbody radiation is not due to transitions; it is a continuum. Transitions give a discrete spectrum.

[lemur]

These are separate effects. Blackbody radiation is not due to transitions; it is a continuum. Transitions give a discrete spectrum.

How is it possible for blackbodies to absorb and emit photons without their electron-levels changing? Is it because of the motion of the electrons together with the molecules as a whole that causes emissions?

[swansont]

How is it possible for blackbodies to absorb and emit photons without their electron-levels changing? Is it because of the motion of the electrons together with the molecules as a whole that causes emissions?

 

It's motion of electrons independent of atoms or molecules; the closest behavior to a blackbody are metals, and you have conduction-band electrons that are colliding owing to their thermal motion. Collisions cause accelerations, and that causes radiation.

[lemur]

It's motion of electrons independent of atoms or molecules; the closest behavior to a blackbody are metals, and you have conduction-band electrons that are colliding owing to their thermal motion. Collisions cause accelerations, and that causes radiation.

 

I see, so electrons have different levels of free-play that doesn't involve level-change. This explains conduction in conductive materials because such materials have more "electron flexibility" than a material for which energy gets (more) immediately expressed as level-change. Then BB radiation is an effect of kinetic energy among more conductive electrons.

[John Cuthber]

"I see, so electrons have different levels of free-play that doesn't involve level-change. "

 

There are changes in levels, but there are lots of levels very close together so there seems to be an arbitrary choice of energy they can gain or lose

 

 

If you put two atoms next to eachother they disturb the energy levels of eachother. If you start with a simple case where each atom has one (accessible) energy level then the pair of atoms will collectively have a pair of "molecular" orbitals" One will be slightly higher energy than the original, and the other will be slightly lower.

If you add a 3rd atom got get another orbital with some slightly different energy level.

Carry this on until you have zillions of atoms, like a bit of metal, and there is a huge spread of possible energy levels permitted. Typically, for a metal, not all of these energy levels are full. So you can move electrons from one level to another.

Since there are lots of energies that could start from and lots of energies they could end up with, there are lots of possible differences.

That means lots of different photon energies could be absorbed.

 

There's another factor which makes metals shiny and therefore not generally good black bodies, but that's another issue.

It means that, while it's true that "the closest behavior to a blackbody are metals" it's also true that metals are also about the worst approximations to black bodies you can get. Silver isn't normally thought of as black (unless you are a B+W photographer)

[Klaynos]

Have a look at the free electron gas model for metals.

[lemur]

"I see, so electrons have different levels of free-play that doesn't involve level-change. "

 

There are changes in levels, but there are lots of levels very close together so there seems to be an arbitrary choice of energy they can gain or lose

 

 

If you put two atoms next to eachother they disturb the energy levels of eachother. If you start with a simple case where each atom has one (accessible) energy level then the pair of atoms will collectively have a pair of "molecular" orbitals" One will be slightly higher energy than the original, and the other will be slightly lower.

If you add a 3rd atom got get another orbital with some slightly different energy level.

Carry this on until you have zillions of atoms, like a bit of metal, and there is a huge spread of possible energy levels permitted. Typically, for a metal, not all of these energy levels are full. So you can move electrons from one level to another.

Since there are lots of energies that could start from and lots of energies they could end up with, there are lots of possible differences.

That means lots of different photon energies could be absorbed.

 

There's another factor which makes metals shiny and therefore not generally good black bodies, but that's another issue.

It means that, while it's true that "the closest behavior to a blackbody are metals" it's also true that metals are also about the worst approximations to black bodies you can get. Silver isn't normally thought of as black (unless you are a B+W photographer)

Cast iron is black.

 

I guess what you are saying also explains conductivity. If there are slighter level-changes possible in the metals, they would be able to receive and emit energy in a more continuous way, which would also make their spectra more like a perfect black body.

 

I see then how you could say that smaller amount of energy could be absorbed and thus render lower frequency photons in a metal. Still, that seems different from what I was thinking, which was that two gases at extremely high temperatures could have different masses at the same level of temperature/volume/pressure and thus the heavier one would have higher-momentum collisions between the molecules although they were moving at the same average speed. This would logically cause the heavier gas to emit higher frequency photons at the same temperature, no?

 

 

[mississippichem]

Cast iron is black.

 

I guess what you are saying also explains conductivity. If there are slighter level-changes possible in the metals, they would be able to receive and emit energy in a more continuous way, which would also make their spectra more like a perfect black body.

 

I see then how you could say that smaller amount of energy could be absorbed and thus render lower frequency photons in a metal. Still, that seems different from what I was thinking, which was that two gases at extremely high temperatures could have different masses at the same level of temperature/volume/pressure and thus the heavier one would have higher-momentum collisions between the molecules although they were moving at the same average speed. This would logically cause the heavier gas to emit higher frequency photons at the same temperature, no?

 

Take care not to confuse black body radiation with stimulated emission. I could heat a piece of iron to high temperatures without any photons being added and could still cause the metal to glow because I'm thermally exciting electrons to higher energy levels that will have to later decay and emit a photon.

 

Stimulated emission is a "1 photon in, 1 photon out process". Shoot a 450 nm photon at a molecule and get a photon back at 450-n nm.

 

You should do some reading about Atomic Emission Spectroscopy or UV-vis spectroscopy.

[swansont]

Stimulated emission is a "1 photon in, 1 photon out process". Shoot a 450 nm photon at a molecule and get a photon back at 450-n nm.

 

The outgoing photon is due to spontaneous emission, (one in/one out being fluorescence), at least in atomic physics/laser parlance. Stimulated emission is one photon in, two identical photons out, because the atom is already in an excited state.

[mississippichem]

The outgoing photon is due to spontaneous emission, (one in/one out being fluorescence), at least in atomic physics/laser parlance. Stimulated emission is one photon in, two identical photons out, because the atom is already in an excited state.

 

Woops, sorry. I meant absorption/emission as in florescence. The "-n" referred to a Stoke's shift.

 

I've had lasers on the brain the last few days. We just acquired a new high-res laser Raman Spec. machine and I've been charged with conducting experiments on this new machine that I know little about . Now that I fully understand IR vibrational modes, I've gotta learn the Raman modes. Argh!

 

good catch!

Edited April 22, 2011 by mississippichem