by neufer » Tue Sep 07, 2021 9:11 pm
johnnydeep wrote: ↑Tue Sep 07, 2021 7:31 pm
Huh, so the Moon
probably orbits (at about 60 Earth radii)
within the geocorona !
- The very tenuous outer limits of the geocorona, yes.
johnnydeep wrote: ↑Tue Sep 07, 2021 7:31 pmneufer wrote: ↑Tue Sep 07, 2021 6:46 pm
<<Forbidden emission lines have been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth.
In space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. Under such conditions, once an atom or molecule has been excited for any reason into a meta-stable state, then it is almost certain to decay by emitting a forbidden-line photon. Since meta-stable states are rather common, forbidden transitions account for a significant percentage of the photons emitted by the ultra-low density gas in space.>>
Still not sure what a forbidden emission line is. Is it simply caused by a electron transitioning from one shell to a lower one that doesn't usually happen, but can still happen under extreme or rare conditions?
Is it caused by a electron dropping into a low "shell" from which it can escape only by either:
- 1) slowly emitting a photon by a non-electric dipole transition (e.g., magnetic dipole, electric quadrupole...)
2) or by colliding in-elastically with another atom/molecule.
At the level airglow occurs, atomic/molecular collisions still take place tens of thousands of times a second so there is no time for a forbidden emission. In planetary nebula, however, atomic/molecular collisions require thousands of seconds such that slow forbidden emissions are the preferred mode of escape.
Delayed forbidden emissions in gases/plasmas are similar to phosphorescence in solids & liquids.
https://en.wikipedia.org/wiki/Phosphorescence wrote:
<<Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength.
Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and re-emits it for a much longer time after the radiation source is removed.
In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for glow-in-the-dark materials which are "charged" by exposure to light. Unlike the relatively swift reactions in fluorescence, such as those seen in laser mediums like the common ruby, phosphorescent materials "store" absorbed energy for a longer time, as the processes required to reemit energy occur less often.
When the stored energy becomes locked in by the spin of the atomic electrons, a triplet state can occur, slowing the emission of light, sometimes by several orders of magnitude. Because the atoms usually begin in a singlet state of spin, favoring fluorescence, these types of phosphors typically produce both types of emission during illumination, and then a dimmer afterglow of strictly phosphorescent light typically lasting less than a second after the illumination is switched off. Conversely, when the stored energy is due to persistent phosphorescence, an entirely different process occurs without a fluorescence precursor. When electrons become trapped within a defect in the atomic or molecular lattice, light is prevented from reemitting until the electron can escape. To escape, the electron needs a boost of thermal energy to help spring it out of the trap and back into orbit around the atom. Only then can the atom emit a photon. Thus, persistent phosphorescence is highly dependent on the temperature of the material.>>
[quote=johnnydeep post_id=316479 time=1631043109 user_id=132061]
Huh, so the Moon [i]probably [/i]orbits (at about 60 Earth radii) [u]within[/u] the geocorona ![/quote]
[list]The very tenuous outer limits of the geocorona, yes.[/list]
[quote=johnnydeep post_id=316479 time=1631043109 user_id=132061][quote=neufer post_id=316478 time=1631040372 user_id=124483]
<<Forbidden emission lines have been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. [b][u][color=#0000FF]In space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. Under such conditions, once an atom or molecule has been excited for any reason into a meta-stable state, then it is almost certain to decay by emitting a forbidden-line photon.[/color][/u][/b] Since meta-stable states are rather common, forbidden transitions account for a significant percentage of the photons emitted by the ultra-low density gas in space.>>[/quote]
Still not sure what a forbidden emission line is. Is it simply caused by a electron transitioning from one shell to a lower one that doesn't usually happen, but can still happen under extreme or rare conditions?[/quote]
Is it caused by a electron dropping into a low "shell" from which it can escape only by either:
[list]1) [b][u][color=#0000FF]slowly[/color][/u][/b] emitting a photon by a [b][u][color=#0000FF]non-electric dipole[/color][/u][/b] transition (e.g., magnetic dipole, electric quadrupole...)
2) or by colliding in-elastically with another atom/molecule.[/list]
At the level airglow occurs, atomic/molecular collisions still take place tens of thousands of times a second so there is no time for a forbidden emission. In planetary nebula, however, atomic/molecular collisions require thousands of seconds such that slow forbidden emissions are the preferred mode of escape.
Delayed forbidden emissions in gases/plasmas are similar to phosphorescence in solids & liquids.
[quote=https://en.wikipedia.org/wiki/Phosphorescence]
[float=left][img3=The excitation of molecule A to its singlet excited state (1A*) may, after a short time between absorption and emission (fluorescence lifetime), return immediately to ground state, giving off a photon via fluorescence (decay time). However, sustained excitation is followed by intersystem crossing to the triplet state (3A) that relaxes to the ground state by phosphorescence with much longer decay times.]https://upload.wikimedia.org/wikipedia/commons/7/71/JablonskiSimple.png[/img3][/float]
<<Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. [b][u][color=#0000FF]Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and re-emits it for a much longer time after the radiation source is removed.[/color][/u][/b]
In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for glow-in-the-dark materials which are "charged" by exposure to light. Unlike the relatively swift reactions in fluorescence, such as those seen in laser mediums like the common ruby, phosphorescent materials "store" absorbed energy for a longer time, as the processes required to reemit energy occur less often.
When the stored energy becomes locked in by the spin of the atomic electrons, a triplet state can occur, slowing the emission of light, sometimes by several orders of magnitude. Because the atoms usually begin in a singlet state of spin, favoring fluorescence, these types of phosphors typically produce both types of emission during illumination, and then a dimmer afterglow of strictly phosphorescent light typically lasting less than a second after the illumination is switched off. Conversely, when the stored energy is due to persistent phosphorescence, an entirely different process occurs without a fluorescence precursor. When electrons become trapped within a defect in the atomic or molecular lattice, light is prevented from reemitting until the electron can escape. To escape, the electron needs a boost of thermal energy to help spring it out of the trap and back into orbit around the atom. Only then can the atom emit a photon. Thus, persistent phosphorescence is highly dependent on the temperature of the material.>>[/quote]