Spectral line
A spectral line is a weaker or stronger region in an otherwise uniform and
Types of line spectra
Spectral lines are the result of interaction between a quantum system (usually atoms, but sometimes molecules or atomic nuclei) and a single photon. When a photon has about the right amount of energy (which is connected to its frequency)[3] 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 the energy will be spontaneously re-emitted, either as one photon at the same frequency as the original one or in a cascade, where the sum of the energies of the photons emitted will be equal to the energy of the one absorbed (assuming the system returns to its original state).
A spectral line may be observed either as an emission line or an absorption line. Which type of line is observed depends on the type of material and its temperature relative to another emission source. An absorption line is produced when photons from a hot, broad spectrum source pass through a cooler material. The intensity of light, over a narrow frequency range, is reduced due to absorption by the material and re-emission in random directions. By contrast, a bright emission line is produced when photons from a hot material are detected, perhaps in the presence of a broad spectrum from a cooler source. The intensity of light, over a narrow frequency range, is increased due to emission by the hot material.
Spectral lines are highly atom-specific, and can be used to identify the chemical composition of any medium. Several elements, including helium, thallium, and caesium, were discovered by spectroscopic means. Spectral lines also depend on the temperature and density of the material, so they are widely used to determine the physical conditions of stars and other celestial bodies that cannot be analyzed by other means.
Depending on the material and its physical conditions, the energy of the involved photons can vary widely, with the spectral lines observed across the electromagnetic spectrum, from radio waves to gamma rays.
Nomenclature
Strong spectral lines in the
In other cases, the lines are designated according to the level of
Cu II — copper ion with +1 charge, Cu1+
Fe III — iron ion with +2 charge, Fe2+
More detailed designations usually include the line
Line broadening and shift
There are a number of effects which control
Broadening due to local effects
Natural broadening
The lifetime of excited states results in natural broadening, also known as lifetime broadening. The uncertainty principle relates the lifetime of an excited state (due to spontaneous radiative decay or the Auger process) with the uncertainty of its energy. Some authors use the term "radiative broadening" to refer specifically to the part of natural broadening caused by the spontaneous radiative decay.[4] A short lifetime will have a large energy uncertainty and a broad emission. This broadening effect results in an unshifted
Thermal Doppler broadening
The atoms in a gas which are emitting radiation will have a distribution of velocities. 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 higher the temperature of the gas, the wider the distribution of velocities in the gas. Since the spectral line is a combination of all of the emitted radiation, the higher the temperature of the gas, the broader the spectral line emitted from that gas. This broadening effect is described by a Gaussian 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 or collisional broadening: The collision of other particles with the light emitting particle interrupts the emission process, and by shortening the characteristic time for the process, increases the uncertainty in the energy emitted (as occurs in natural broadening).Lorentzian profileand there may be an associated shift.
- Quasistatic pressure broadening: The presence of other particles shifts the energy levels in the emitting particle (see spectral band), 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 general expression for the lineshape resulting from quasistatic pressure broadening is a 4-parameter generalization of the Gaussian distribution known as a stable distribution.[7]
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 of a charged particle at a distance , causing a shift in energy that 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.
- Quadratic Stark broadening occurs via the quadratic Stark effect, which results from the interaction of an emitter with an electric field, causing a shift in energy that 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[note 1] is often useful in describing the profile. The energy shift as a function of distance between the interacting particles is given in the wings by e.g. the Lennard-Jones potential.
Inhomogeneous broadening
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.
Opacity broadening
Opacity broadening is an example of a non-local broadening mechanism. Electromagnetic radiation emitted at a particular point in space can be reabsorbed as it travels through space. This absorption depends on wavelength. The line is broadened because the photons at the line center have a greater reabsorption probability than the photons at the line wings. Indeed, the reabsorption near the 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 process is also sometimes called self-absorption.
Macroscopic Doppler broadening
Radiation emitted by a moving source is subject to
Combined effects
Each of these mechanisms can act in isolation or in combination with others. Assuming each effect is independent, the observed line profile is a convolution of the line profiles of each mechanism. For example, a combination of the thermal Doppler broadening and the impact pressure broadening yields a Voigt profile.
However, the different line broadening mechanisms are not always independent. For example, the collisional effects and the motional Doppler shifts can act in a coherent manner, resulting under some conditions even in a collisional narrowing, known as the Dicke effect.
Spectral lines of chemical elements
Bands
The phrase "spectral lines", when not qualified, usually refers to lines having wavelengths in the visible band of the full electromagnetic spectrum. Many spectral lines occur at wavelengths outside this range. At shorter wavelengths, which correspond to higher energies, ultraviolet spectral lines include the Lyman series of hydrogen. At the much shorter wavelengths of X-rays, the lines are known as characteristic X-rays because they remain largely unchanged for a given chemical element, independent of their chemical environment. Longer wavelengths correspond to lower energies, where the infrared spectral lines include the Paschen series of hydrogen. At even longer wavelengths, the radio spectrum includes the 21-cm line used to detect neutral hydrogen throughout the cosmos.
Visible light
For each element, the following table shows the spectral lines which appear in the visible spectrum at about 400-700 nm.
See also
- Absorption spectrum
- Atomic spectral line
- Bohr model
- Electron configuration
- Emission spectrum
- Fourier transform
- Fraunhofer line
- Table of emission spectra of gas discharge lamps
- Hydrogen line (21-cm line)
- Hydrogen spectral series
- Spectral band
- Spectroscopy
- Splatalogue
Notes
- ISBN 978-0-471-27663-0; and in Journal of technical physics, Volume 36, by Instytut Podstawowych Problemów Techniki (Polska Akademia Nauk), publisher: Państwowe Wydawn. Naukowe., 1995,
References
- ^ Kramida, Alexander; Ralchenko, Yuri (1999), NIST Atomic Spectra Database, NIST Standard Reference Database 78, National Institute of Standards and Technology, retrieved 2021-06-27
- ISSN 0022-4073.
- On a Heuristic Viewpoint Concerning the Production and Transformation of Light".
- ISBN 978-0-471-12533-4.
- PMID 10031682.
- ^ "Collisional Broadening". Fas.harvard.edu. Archived from the original on 2015-09-24. Retrieved 2015-09-24.
- doi:10.1080/00018738100101467. Archived from the originalon 2013-01-14. Retrieved 2005-12-09.
Further reading
- Griem, Hans R. (1997). Principles of Plasma Spectroscopy. Cambridge: University Press. ISBN 0-521-45504-9.
- Griem, Hans R. (1974). Spectral Line Broadening by Plasmas. New York: ISBN 0-12-302850-7.
- Griem, Hans R. (1964). Plasma Spectroscopy. New York: McGraw-Hill book Company.