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 with the nearby frequencies. The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ...

Spectral lines are the result of interaction between a quantum system (usually atoms, but sometimes molecules or atomic nuclei) and single photons. When a photon has exactly the right energy to allow a change in the energy state of the system (in the case of an atom this is usually an electron changing orbitals), the photon is absorbed. Then it will be spontaneously re-emitted, either in the same frequency as the original or in a cascade, where the sum of the energies of the photons emitted will be the same as the energy of the one absorbed. The direction of the new photons will not be related to the direction of travel of the original photon. Fig. ... In physics, atomic spectral lines are formed when an electron makes a transition from a particular energy level of an atom, to a lower energy state. ... A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... A stylized representation of a lithium atom. ... In physics, the photon (from Greek φως phos, meaning light) is the quantum of the electromagnetic field, for instance light. ... Properties The electron is a fundamental subatomic particle which carries a negative electric charge. ... Electron atomic and molecular orbitals In atomic physics, the electron configuration is the arrangement of electrons in an atom, molecule or other body. ...

Depending on the geometry of the gas, the photon source and the observer, an emission line or an absorption line will be produced. If the gas is between the photon source and the observer, a decrease in the intensity of light in the frequency of the incident photon will be seen, as the reemitted photons will mostly be in directions different than the original one. This will be an absorption line. If the observer sees the gas, but not the original photon source, then the observer will see only the photons reemitted in a narrow frequency range. This will be an emission line.

Absorption and emission lines are highly atom-specific, and can be used to easily identify the chemical composition of any medium capable of letting light pass through it (typically gas is used). Several elements were discovered by spectroscopic means -- helium, thallium, cerium, etc. Spectral lines also depend on the physical conditions of the gas, so they are widely used to determine the chemical composition of stars and other celestial bodies that cannot be analyzed by other means, as well as their physical conditions. A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... General Name, Symbol, Number thallium, Tl, 81 Chemical series poor metals Group, Period, Block 13, 6, p Appearance silvery white Atomic mass 204. ... General Name, Symbol, Number cerium, Ce, 58 Chemical series lanthanides Group, Period, Block ?, 6, f Appearance silvery white Atomic mass 140. ... The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ...

Isomer shift is the displacement of an absorption line due to the absorbing nuclei having different s-electron densities from that of the emitting nuclei.

Mechanisms other than atom-photon interaction can produce spectral lines. Depending on the exact physical interaction (with molecules, single particles, etc.) the frequency of the involved photons will vary widely, and lines can be observed across all the electromagnetic spectrum, from radio waves to gamma rays. Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultrahigh frequency VHF = Very high frequency HF = High frequency... Radio frequency, or RF, refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. ... This article is about electromagnetic radiation. ...

Spectral line broadening and shift

A line extends over a range of frequencies, not a single frequency. In addition its center may be shifted from its nominal central wavelength. There are several reasons for this broadening and shift:

• Natural broadening: The Uncertainty principle relates the life of an excited state with the precision of the energy, so the same excited level will have slightly different energies in different atoms. This broadening effect is described by a Lorentzian profile and there is no associated shift.

• Thermal Doppler broadening: Atoms will have different thermal velocities, so they will see the photons red or blue shifted due to the Doppler effect, absorbing photons of different energies in the frame of reference of the observer. The higher the temperature of the gas, the larger the velocity differences (and velocities), and the broader the line. This broadening effect is described by a Doppler profile and there is no associated shift.

• Pressure broadening: the presence of nearby particles will affect the radiation emitted by an individual particle. There are two limiting cases by which this occurs:

• Impact pressure broadening: The collision of other particles with the emitting particle interrupts the emission process. The duration of the collision is much shorter than the lifetime of the emission process. This effect depends on both the density and the temperature of the gas. The broadening effect is described by a Lorentzian profile and there may be an associated shift.
• Quasistatic pressure broadening: The presence of other particles shifts the energy levels in the emitting particle, thereby altering the frequency of the emitted radiation. The duration of the influence is much longer than the lifetime of the emission process. This effect depends on the density of the gas, but is rather insensitive to temperature. The form of the line profile is determined by the functional form of the perturbing force with respect to distance from the perturbing particle. There may also be a shift in the line center. The Lévy skew alpha-stable distribution has been found to be a useful generalization describing a quasistatic line profile. (Peach, 1981 § 4.5).

Pressure broadening may also be classified by the nature of the perturbing force.

• Linear Stark broadening occurs via the linear Stark effect which results from the interaction of an emitter with an electric field, which causes a shift in energy which is linear in the field strength. ()
• Resonance broadening occurs when the perturbing particle is of the same type as the emitting particle, which introduces the possibility of an energy exchange process. This broadening effect is described by a Lorentzian profile in both the impact and the quasistatic case. ()
• Quadratic Stark broadening occurs via the quadratic Stark effect which results from the interaction of an emitter with an electric field, which causes a shift in energy which is quadratic in the field strength. ()
• Van der Waals broadening occurs when the emitting particle is being perturbed by Van der Waals forces. For the quasistatic case, a Van der Waals profile is often useful in describing the profile. The energy shift as a function of distance is given in the wings by e.g. the Lennard-Jones potential ()

• Opacity broadening: Considerable reabsorption of emission line photons, an effect known as opacity, often causes line broadening. The line is broadened since photons at the line wings have a smaller reabsorption probability than photons at the line center. Indeed, the absorption near line center may be so great as to cause a self reversal in which the intensity at the center of the line is less than in the wings. This type of broadening is different from the above mentioned broadening mechanisms because it depends upon the conditions along the entire path taken by the radiation, rather than simply upon conditions that are local to the emitting particle.

These mechanisms can act in isolation or in combination. Assuming each effect is independent of the other, the combined line profile will be the convolution of the line profiles of each mechanism. For example, a combination of thermal Doppler broadening and impact pressure broadening will yield a Voigt profile. In quantum physics, the Heisenberg uncertainty principle states that one cannot assign with full precision values for certain pairs of observable variables, including the position and momentum, of a single particle at the same time even in theory. ... ... Doppler broadening is a broadening of spectral lines due to thermal agitation. ... Sound waves emanating from an ambulance moving to the right. ... The Doppler profile is a spectral line profile which results from the thermal motion of the emitting atom or molecule. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... In probability theory, a Lévy skew alpha-stable distribution or just stable distribution, developed by Paul Lévy, is a probability distribution where sums of independent identically distributed random variables have the same distribution as the original. ... ... In chemistry, the term Van der Waals force originally referred to all forms of intermolecular forces; however, in modern usage it tends to refer to intermolecular forces that deal with forces due to the polarization of molecules. ... A set of four symmetric centered Lévy distributions with scale factor c=1. ... Neutral atoms and molecules are subject to two distinct forces in the limit of large distance, and short distance. ... In spectroscopy, the Voigt profile is a spectral line profile named after Woldemar Voigt and found in all branches of spectroscopy in which a spectral line is broadened by two types of mechanisms, one of which alone would produce a Doppler profile, and the other of which would produce a...

References

• Griem, Hans R. (1974). Spectral Line Broadening by Plasmas, New York: Academic Press. ISBN 0123028507.
• Griem, Hans R. (1964). Plasma Spectroscopy, New York: McGraw-Hill book Company.
• Peach, G. (1981). Theory of the pressure broadening and shift of spectral lines. Advances in Physics 30 (3): 367-474.

See also

• Absorption spectrum
• Atomic spectral line
• Bohr model
• Electron configuration
• Emission spectrum
• Fraunhofer line
• Hydrogen line