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.

Spectral lines are the result of interaction between a quantum system (usually atoms, but sometimes molecules or atomic nuclei) and single photons. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In Physics, atomic Spectral lines are of two types An emission line is formed when an electron makes a transition from a particular discrete 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 nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena 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. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J In Atomic physics and Quantum chemistry, electron configuration is the arrangement of Electrons in an Atom, Molecule, or other 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.

Depending on the geometry of the gas, the photon source and the observer, either 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 from 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). This page is about the physical properties of gas as a state of matter Several elements were discovered by spectroscopic means -- helium, thallium, cerium, etc. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Thallium (ˈθæliəm is a Chemical element with the symbol Tl and Atomic number 81 Cerium (ˈsɪəriəm is a Chemical element with the symbol Ce and Atomic number 58 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 star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth

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. The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions

Contents 1 Spectral line broadening and shift 1.1 Broadening due to local effects 1.2 Broadening due to non-local effects 1.3 Combined effects 2 References 3 See also 4 Notes

Spectral line broadening and shift

A spectral line extends over a range of frequencies, not a single frequency (i. e. , it has a nonzero linewidth). The spectral linewidth characterizes the width of a Spectral line, such as in the electromagnetic emission spectrum of an atom or the Frequency spectrum In addition its center may be shifted from its nominal central wavelength. There are several reasons for this broadening and shift. These reasons may be divided into two broad categories - broadening due to local conditions and broadening due to extended conditions. Broadening due to local conditions is due to effects which hold in a small region around the emitting element, usually small enough to assure local thermodynamic equilibrium. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and Broadening due to extended conditions may result from changes to the spectral distribution of the radiation as it traverses its path to the observer. It also may result from the combining of radiation from a number of regions which are far from each other.

Broadening due to local effects

• Natural broadening: The Energy-Time Uncertainty Principle relates the lifetime of an excited state with the precision of its energy, so the same excited level will have slightly different energies in different atoms. In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain This broadening effect is described by a Lorentzian profile and there is no associated shift. The Cauchy–Lorentz distribution, named after Augustin Cauchy and Hendrik Lorentz, is a continuous Probability distribution. Due to the uncertainty principle, natural broadening can be experimentally altered only to the extent that decay rates can be artificially suppressed or enhanced. ^[1]

• Thermal Doppler broadening: The atoms in a gas which are emitting radiation will have a distribution of velocities. In Atomic physics, Doppler broadening is the broadening of Spectral lines due to the Doppler effect in which the thermal movement of Each photon emitted will be red or blue shifted by the Doppler effect depending on the velocity of the atom relative to the observer. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for The higher the temperature of the gas, the wider the distribution of velocities in the gas. Since the spectral line is the combination of all of the emitted radiation, the higher the temperature of the gas, the broader will be the spectral line emitted from that gas. This broadening effect is described by a Doppler profile and there is no associated shift. In Atomic physics, Doppler broadening is the broadening of Spectral lines due to the Doppler effect in which the thermal movement of

• 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 density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The broadening effect is described by a Lorentzian profile and there may be an associated shift. The Cauchy–Lorentz distribution, named after Augustin Cauchy and Hendrik Lorentz, is a continuous Probability distribution.
• 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 density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater 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. In Probability theory, a Lévy skew alpha-stable distribution or just stable distribution, developed by Paul Lévy, is a four parameter family of continuous (Peach, 1981 § 4. 5).

Pressure broadening may also be classified by the nature of the perturbing force as follows:

• 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. The Stark effect is the shifting and splitting of Spectral lines of atoms and molecules due to the presence of an external static Electric field. ()
• 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. The Cauchy–Lorentz distribution, named after Augustin Cauchy and Hendrik Lorentz, is a continuous Probability distribution. ()
• 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. The Stark effect is the shifting and splitting of Spectral lines of atoms and molecules due to the presence of an external static Electric field. ()
• Van der Waals broadening occurs when the emitting particle is being perturbed by Van der Waals forces. The Van der Waals equation is an Equation of state that can be derived from a special form of the potential between a pair of molecules (hard-sphere repulsion For the quasistatic case, a Van der Waals profile is often useful in describing the profile. In Probability theory and Statistics, the Lévy distribution, named after Paul Pierre Lévy, is a Continuous probability distribution for a non-negative The energy shift as a function of distance is given in the wings by e. g. the Lennard-Jones potential ()

Broadening due to non-local effects

Certain types of broadening are the result of conditions over a large region of space rather than simply upon conditions that are local to the emitting particle. A pair of neutral atoms or molecules is subject to two distinct forces in the limit of large separation and small separation an attractive force at long ranges ( van der Waals force, or

• Opacity broadening: Electromagnetic radiation emitted at a particular point in space can be absorbed as it travels through space. This absorption depends on wavelength. The line is broadened because 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.

• Rotational broadening: Radiation emitted from a distant rotating body, such as a star, will be subject to a doppler shift due to the line-of-sight variations in velocity on opposite sides of the star. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for The greater the rate of rotation, the broader the line.

Combined effects

Any of 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 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

References

• Griem, Hans R. (1974). Spectral Line Broadening by Plasmas. New York: Academic Press. ISBN 0-12-302850-7.  
• 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
• Spectrum analysis
• Fraunhofer line
• Hydrogen line

Notes

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