Line Spectra and low density gases

[fog37]

Hello,
A low density gas, when heated at a temperature T, emits radiation having a line spectrum, i.e. having only discrete spectral lines. Each spectral line corresponds to a very specific energy transition (jump) for the electron in the atom. Some energy transitions are allowed, some are forbidden. A low density gas contains very many atoms even if low density. Statistically, are some atoms in the gas emitting radiation at certain frequencies while other atoms are emitting at other frequencies depending on which transitions are taking place for those atoms? I imagine that, among all the many possible allowed transitions, the electrons in some atom are going through certain transitions while the electrons in other atoms are facing different transitions.

a) What if we considered the spectrum from just a single gas atom/molecule? If the atom has multiple valence electrons, each different valence electron could have its own transition. Would the line spectrum be time dependent and changing from second to second depending on which transitions are taking place at that moment?

b) If a cool low density gas is placed in front of a broadband source emitting a continuous spectrum, the emission spectrum of the broadband source will show some dark absorption lines. These dark lines indicate where, spectrally, the low density gas has absorbed radiation from the broadband source. What happens to the energy absorbed by the low density gas? Why doesn't the low density gas emit its line spectrum filling up the dark lines? Is it because the quantum jumps in the low density gas correspond to nonradiative transitions? Why would these transitions be nonradiative?

thanks!

 

[Charles Link]

For a), I believe you have it correct. $\\$ For the answer to (b), that's what takes place at the outer surface of the sun so that the solar spectrum consists of absorption lines in a mostly smooth and continuous and close to blackbody of $T \approx 6000 \, K$ spectrum. Some of this absorbed energy gets re-radiated, but it doesn't all get re-radiated. Sometimes, the electrons may undergo additional transitions, so the cooler gas, in general, does not re-radiate out at the same wavelengths, as much as it absorbs. $\\$ One thing that you omitted that might be relevant is that there are often many atoms at any given time that are in the ground state and not participating in any spectral emission. $\\$ On another note, normally, to get a good line spectrum from a gas, it requires to inject electrons through the gas, as in an arc discharge lamp. Normal heating of the gas can result in spectral lines, but most of the time, the process is facilitated by running an arc of electrons through the gas, which often can result in some ionization of the atoms=chain-reaction effect. These arc lamps can often run at relatively low voltages. The electrons are normally injected by using a heated cathode along with a low voltage between the cathode and anode. Initially to get an arc lamp started, it usually requires a somewhat high voltage.

 

[tech99]

a) What if we considered the spectrum from just a single gas atom/molecule? If the atom has multiple valence electrons, each different valence electron could have its own transition. Would the line spectrum be time dependent and changing from second to second depending on which transitions are taking place at that moment?

b) If a cool low density gas is placed in front of a broadband source emitting a continuous spectrum, the emission spectrum of the broadband source will show some dark absorption lines. These dark lines indicate where, spectrally, the low density gas has absorbed radiation from the broadband source. What happens to the energy absorbed by the low density gas? Why doesn't the low density gas emit its line spectrum filling up the dark lines? Is it because the quantum jumps in the low density gas correspond to nonradiative transitions? Why would these transitions be nonradiative?

thanks!

a) I think we see every emission line but modulated with quantum noise. I suppose this smooths out if there are many atoms.
b) I found when trying to see the spectrum of Sodium that a cool gas will absorb and a hot gas will emit. I looked at white light passing through a Sodium flame, and could obtain emission or absorption by sending the light through the hot flame or through the cool, ionised gas above it.

 

[Charles Link]

a) I think we see every emission line but modulated with quantum noise. I suppose this smooths out if there are many atoms.
b) I found when trying to see the spectrum of Sodium that a cool gas will absorb and a hot gas will emit. I looked at white light passing through a Sodium flame, and could obtain emission or absorption by sending the light through the hot flame or through the cool, ionised gas above it.

By emission, I believe you are saying that when the white light went through the hottest part of the flame, you found the combined spectrum to consist of the white light plus additional energy at the wavelengths of the emission lines.

 

[tech99]

By emission, I believe you are saying that when the white light went through the hottest part of the flame, you found the combined spectrum to consist of the white light plus additional energy at the wavelengths of the emission lines.

Yes. I was actually trying to demo the absorption lines, and with my simple equipment they seem to be filled in with the Sodium emission if the hot part of the flame is used. It was actually possible to obtain a null point. I seem to remember that the absorption lines of the Sun arise when the light passes through the cooler outer gases.

 

[Charles Link]

Yes. I was actually trying to demo the absorption lines, and with my simple equipment they seem to be filled in with the Sodium emission if the hot part of the flame is used. It was actually possible to obtain a null point. I seem to remember that the absorption lines of the Sun arise when the light passes through the cooler outer gases.

This article is a good one about the solar spectral lines : https://en.wikipedia.org/wiki/Fraunhofer_lines