by neufer » Fri Feb 11, 2011 5:57 pm
http://en.wikipedia.org/wiki/Forbidden_mechanism wrote:
<<In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. In formal physics, this means that the process cannot proceed via the most efficient (electric dipole) route. Although the transitions are nominally "forbidden", there is a small probability of their spontaneous occurrence, should an atom or molecule be raised to an excited state. More precisely, there is a certain probability that such an excited atom will make a forbidden transition to a lower energy state per unit time; by definition this probability is much lower than that for any transition permitted by the selection rules. Therefore, if a state can de-excite via a permitted transition (or otherwise, e.g. via collisions) it will almost certainly do so rather than choosing the forbidden route. Nevertheless, "forbidden" transitions are only relatively unlikely: states that can only decay in this way (so-called meta-stable states) usually have lifetimes of order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions.
Forbidden emission lines have only been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. Even the hardest laboratory vacuum on Earth is still too dense for forbidden line emission to occur before atoms are collisionally de-excited.
However, 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.
Forbidden line transitions are noted by placing square brackets around the atomic or molecular species in question, e.g. [O III] or
.
Forbidden lines of nitrogen ([N II] at 654.8 and 658.4 nm), sulfur ( at 671.6 and 673.1 nm), and oxygen ([O II] at 372.7 nm, and [O III] at 495.9 and 500.7 nm) are commonly observed in astrophysical plasmas. These lines are extremely important to the energy balance of such things as planetary nebulae and H II regions. Also, the forbidden 21-cm hydrogen line is of the utmost importance for radio astronomy as it allows very cold neutral hydrogen gas to be seen.>>
http://www.daviddarling.info/encyclopedia/F/forbidden_line.html wrote:
<<Forbidden line: An emission line found in the spectrum of a rarefied gas under special conditions, such as those found in some nebulae, the solar corona, and parts of active galactic nuclei. Although not seen on Earth – hence their name – forbidden lines may account for 90% or more of the total visual brightness of an object such as a planetary nebula.
A forbidden line arises when an electron in an excited (energized) atom jumps from a metastable state to a lower energy level. Under normal circumstances, when particle densities are higher (greater than about 108 per cm3), such an electron would almost immediately be knocked out of its metastable state by collision and not be given time to emit a photon. But in an environment like that of a planetary nebula, the time between collisions averages 10 to 10,000 seconds. Consequently, when ions such as O+, O2+ (singly and doubly ionized oxygen), or N+ (singly ionized nitrogen) go into metastable states by allowed transitions from higher states, they remain there undisturbed until they radiate spontaneously. A large fraction of the more highly excited ions eventually drop into these states and, in a nebular environment, practically every ion goes from them to the ground state by forbidden radiation.
Forbidden lines are denoted by enclosing them in brackets. The strongest are two lines of doubly ionized oxygen [O III], in the green part of the spectrum (to which the human eye is most sensitive) at 495.9 nm and 500.7 nm. When these lines were first seen, in the spectra of planetary nebula in the 1860s, their true nature wasn't recognized and it was thought they might be due to a new element, which was dubbed "nebulium." More than half a century passed before Ira Bowen provided the right explanation. Besides forbidden lines of ionized oxygen, others of neon, nitrogen, and other relatively abundant elements are seen making up the light of nebulae, as well as the ordinary, permitted lines of hydrogen and helium.>>
[quote=" http://en.wikipedia.org/wiki/Forbidden_mechanism"]
<<In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. In formal physics, this means that the process cannot proceed via the most efficient (electric dipole) route. Although the transitions are nominally "forbidden", there is a small probability of their spontaneous occurrence, should an atom or molecule be raised to an excited state. More precisely, there is a certain probability that such an excited atom will make a forbidden transition to a lower energy state per unit time; by definition this probability is much lower than that for any transition permitted by the selection rules. Therefore, if a state can de-excite via a permitted transition (or otherwise, e.g. via collisions) it will almost certainly do so rather than choosing the forbidden route. Nevertheless, "forbidden" transitions are only relatively unlikely: states that can only decay in this way (so-called meta-stable states) usually have lifetimes of order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions.
[size=150][color=#00BF00]Forbidden emission lines have only been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. Even the hardest laboratory vacuum on Earth is still too dense for forbidden line emission to occur before atoms are collisionally de-excited.[/color][/size]
However, 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.
Forbidden line transitions are noted by placing square brackets around the atomic or molecular species in question, e.g. [O III] or [S II].
Forbidden lines of nitrogen ([N II] at 654.8 and 658.4 nm), sulfur ([S II] at 671.6 and 673.1 nm), and oxygen ([O II] at 372.7 nm, and [O III] at 495.9 and 500.7 nm) are commonly observed in astrophysical plasmas. These lines are extremely important to the energy balance of such things as planetary nebulae and H II regions. Also, the forbidden 21-cm hydrogen line is of the utmost importance for radio astronomy as it allows very cold neutral hydrogen gas to be seen.>>[/quote][quote=" http://www.daviddarling.info/encyclopedia/F/forbidden_line.html"]
<<Forbidden line: An emission line found in the spectrum of a rarefied gas under special conditions, such as those found in some nebulae, the solar corona, and parts of active galactic nuclei. Although not seen on Earth – hence their name – forbidden lines may account for 90% or more of the total visual brightness of an object such as a planetary nebula.
A forbidden line arises when an electron in an excited (energized) atom jumps from a metastable state to a lower energy level. Under normal circumstances, when particle densities are higher (greater than about 10[sup]8[/sup] per cm[sup]3[/sup]), such an electron would almost immediately be knocked out of its metastable state by collision and not be given time to emit a photon. But in an environment like that of a planetary nebula, the time between collisions averages 10 to 10,000 seconds. Consequently, when ions such as O+, O2+ (singly and doubly ionized oxygen), or N+ (singly ionized nitrogen) go into metastable states by allowed transitions from higher states, they remain there undisturbed until they radiate spontaneously. A large fraction of the more highly excited ions eventually drop into these states and, in a nebular environment, practically every ion goes from them to the ground state by forbidden radiation.
Forbidden lines are denoted by enclosing them in brackets. The strongest are two lines of doubly ionized oxygen [O III], in the green part of the spectrum (to which the human eye is most sensitive) at 495.9 nm and 500.7 nm. When these lines were first seen, in the spectra of planetary nebula in the 1860s, their true nature wasn't recognized and it was thought they might be due to a new element, which was dubbed "nebulium." More than half a century passed before Ira Bowen provided the right explanation. Besides forbidden lines of ionized oxygen, others of neon, nitrogen, and other relatively abundant elements are seen making up the light of nebulae, as well as the ordinary, permitted lines of hydrogen and helium.>>[/quote]