Emission & Absorption Spectra

Here's my current understanding:

Emission spectra shows the wavelengths that have been emitted by the surface, and is used for analysing hot gases; whereas Absorption spectra shows the wavelengths absorbed by a surface.

Therefore, if you were to compare both the emission and absorption spectrum of a surface, the black lines on the absorption spectrum would correspond to the coloured lines on the emission one?

Assuming that this is correct so far ...

Why are emission spectrum's used to measure hot gases, and absorption for cold surfaces?
What difference does it make?

One last thing:

Light is given off when an excited electron descends into an underlying shell - because the energy it loses is given off as a photon, correct? ... But, what does this have to do with the temperature of a surface? As in: why does hot gas emit light?

I never really thought of this in too much detail before, and the [incomplete] solution seemed simple—the energy used in some 'crazy' process is thermal.. but what actually happens?
Surely if the electrons are excited, then they would ascend into an outer shell... if this is the case, then I'm guessing that the photon is produced when the electron falls back down to it's original shell... but what causes the electron to fall? A star is constantly bloody hot, so what would cause the electrons to lose the energy and fall ?

(Hope I'm clear [enough]).  :)

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In hot gases, electrons are in high energy states, and can drop to lower states to emit a photon.
in cold gases, electrons are in low energy states, and can jump to higher states when they absorb a photon.
electrons and photons in thermal equilibrium are always exchanging energy back and forth, the but at different temperatures the sizes of the populations at different energys are different.
When the energy states are not limited to election orbitals, you get a continuous blackbody spectrum
InteractiveMindAuthor Commented:
What can cause electrons to drop to lower energy states?
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The emission of a photon.
Or you can say that an electron dropping to lower energy states causes the emisson of a photon,
it's just another way of saying the same thing.
Low energy electron + photon
High energy electron
can switch back and forth spontaneousy.
The presence of another photon would increase the probability of a transition.
>What can cause electrons to drop to lower energy states?

There you go bringing causation into this.  :)

If you have a jar full of marbles, loosely packed, and you jiggle it a bit, some marbles are going to drop down and pack a bit tighter.

With electrons, it's a fundamentally random process.  Like really random.

InteractiveMindAuthor Commented:
Ouch. "Random" hurts my ears.

Are there any quantum phenomena that are not based on randomness in any way ?
I think the only quantum phenomenon that is based on randomness in any way is the collapse of the wave function when you make a measurement, which may not even be a real phenomenon.
Everything else is completely deterministic.
InteractiveMindAuthor Commented:
Even what causes electrons to drop to a lower energy state / photons to be emitted ?
The electron/photon wave function is in a superposition of states with a certain amplitude to  be in the Low energy electron + photon state, and a certain amplitude to be in the High energy electron state.
Like Schrödinger's cat, it is both emiting and not emiting at the same time.
Photon emission by an electron transitioning to a lower energy state is not a random process: the high and low energy states have different likelihoods.  Sure, the exact moment where the transition occurs is random.

Why does the electron drop? Because it wants to: it is a universal tendency of systems to seek their lowest energy state.
Getting back to emission vs absorption: A blob of hot matter will tend to give off its light preferentially in some wavelengths (because they correspond to state transitions, etc.), giving rise to an emission spectrum.  Now, if you have a blob of cool gas between that source and the observer, it will absorb preferentially in its own wavelengths (it is "tuned" to those), and you will observe an absorption spectrum.  This occurs even if the intervening gas re-emits at the same wavelength, simply because it will re-emit in all directions uniformly, whereas the light it subtracted was preferentially going in one direction: the observer's.
Generally there are more Low energy electron + photon states than High energy electron states, (since more particles allows more possibilites) so the total amplitide for those states tends to be larger.

> Because it wants to: it is a universal tendency of systems to seek their lowest energy state.

I would say rather that whole system of electrons + photons tends to distibute its conserved energy roughly equally among its particles, since most systems have more well distributed states available than states with the energy concentrated in a few particles.
This means that the average time to transition from states with low probabilites to states with high probabilities tends to be less than the average time to transition from states with high probabilites to states with low probabilities.
(actually, the transition only occurs when you make a measurement that collapses the wave function -- and the reallity of the collapse is questionable, alhough that may be more of a philosophical question than a physics question -- otherwise, the wave function evolves deterministically in its superposition of states)

The average distrubution of energy among states depends on the systems total energy or  temperature (temperature is a measure of how the number of states varies with energy)
First of all some definitions:

-Emission spectrum: A spectrum obtained by a solid (or liquid, or gas, or solution) that is excited by any means (heat, electromagnetic radiation etc), and then relaxes to its ground state emmiting photons of varying wavelengths.
Examples: Black body radiation (thermal excitation), or fluorescence (excitation by a laser beam or a light beam at some specific wavelength).

-Absorption spectrum: The spectrum obtained by a body that is at least partially transparent to the radiation used.
In that case, we shine some radiation that traverses the body; a part of it interacts with the molecules, excites them, and then is isotropically expelled as heat when the molecules relax. The other part does not interact at all. THIS is the absorption spectrum (the radiation that the counter of the apparatus will see).
The restriction is that the radiation source and the counter must be in a straight line with the body, and that the body must be partially transparent to that radiation.
Examples: The spectrum of the solar atmosphere (the light source here is the sun itself and the absorber is its atmosphere)
UV spectroscopy in solution
IR spectroscopy in KBr pellets or Nujol mulls.
Then, concerning transitions:

For every system with many energy states (e.g. atomic orbitals or "shells" in atomic spectroscopy) there is at least one that is the lowest in energy (ground state) and then there are all the others that are higher in energy (excited states).

When a system absorbs energy it will go to an excited state (e.g. the electron of the hydrogen atom will move to a higher-energy orbital).

After the excitation, the system will at some point relax to a lower state (or the ground state), seeking a thermodynamically more stable state. In the process it will emit the energy difference as a photon.

When will this happen?
When you talk about ONE quantum entity (e.g. ONE atom), you never know when this will happen: maybe in a microsecond, maybe in a billion years.
But when you talk about a large ensemble of entities, then it is (YES it is) statistic, and there are transition probabilities that tell you how much of your entitites and how soon will relax to a lower-lying state. These transition probabilities are explicitly calculated for specific problems.
> After the excitation, the system will at some point relax to a lower state (or the ground state), seeking a thermodynamically more stable state.
It is not so much a seeking, as that the more stable states are more numerous, so the system tends to spends more time in those states so it tends to takes less time to go from the exited state to the ground state than from the ground state to the exited state.
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