A forbidden mechanism or forbidden line is a concept in physics/chemistry. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties It is a spectral line emitted by atoms undergoing energy transitions not normally allowed by the selection rules of quantum mechanics. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny In Physics and Chemistry, especially in the context of Quantum mechanics, a selection rule is a condition constraining the physical properties of the initial Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In chemistry, "forbidden" means absolutely impossible due to natural laws, but with the assumption of an ideal symmetry. Although the transitions are technically "forbidden", there is a non-zero probability of their spontaneous occurrence, should an atom or molecule be raised to an excited state. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by The transition probability is extremely low, however, so if the atom or molecule can make an allowed transition or be otherwise de-excited to a lower energy state, it will almost certainly do so.
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. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Even the hardest laboratory vacuum on Earth is still too dense for forbidden line emission to occur before atoms are collisionally de-excited. This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. A collision is an isolated event in which two or more bodies (colliding bodies exert relatively strong forces on each other for a relatively short time However, in space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. A cubic centimetre or cubic centimeter (symbol cm3 —the abbreviation cc, though widely used is deprecated is a commonly used unit of Volume Under such conditions, forbidden line transitions may account for a significant percentage of the photons emitted.
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. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 8 and 658. 4 nm), sulfur ([S II] at 671. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a Sulfur or sulphur (ˈsʌlfɚ see spelling below) is the Chemical element that has the Atomic number 16 6 and 673. 1 nm), and oxygen ([O II] at 372. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the 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. A planetary nebula is an Emission nebula consisting of a glowing shell of Gas and plasma formed by certain types of Stars when they die An H II region (also known as Emission nebula) is a cloud of glowing Gas and plasma, sometimes several hundred Light-years across Also, the forbidden Hydrogen line is of the utmost importance for radio astronomy as very cold neutral hydrogen gas can be seen. The hydrogen line, 21 centimeter line or HI line refers to the Spectral line created by changes in the energy state of neutral Hydrogen and
References
- Osterbrock, D.E., Astrophysics of gaseous nebulae and active galactic nuclei, University Science Books, 1989, ISBN 0-935702-22-9. Donald Edward Osterbrock ( July 13, 1924 - January 11, 2007) was an American Astronomer, best known for his work on The year 1989 in literature involved some significant events and new books
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