A spectral line is defined as a dark or bright line in an otherwise continuous and uniform spectrum, resulting from light’s absorption or emission in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used in the identification of molecules and atoms. These "fingerprints" are compared with the previously collected "fingerprints" of molecules and atoms and are therefore used to identify (which would otherwise be impossible) the molecular and atomic components of planets and stars.
Types of Line Spectrum
The association between a quantum system (usually electrons, but at times, atomic nuclei or molecules) and a single photon is the product of spectral lines. When the photon has up to the right amount of energy (connected to its frequency) to allow a change in the system's energy state (in the case of an atom, this is generally an electron changing orbitals), the photon can be absorbed.
Then it will be re-emitted spontaneously at the same frequency as the cascade or the original, where the sum of the emitted photon energies will be equal to the energy of the absorbed photon (assuming that the system returns to the original state).
A spectral line can be observed either as an absorption line or an emission spline. The type of line observed will depend on the type of material and its temperature relative to the other source of emission. An absorption line can be formed when photons from a broad and hot spectrum source pass via cold material.
The intensity of light over a narrow frequency range can be reduced because of absorption by the material and re-emission in random directions. In comparison, when photons from some of the hot material are observed in the presence of a broad spectrum from a cold source, a bright emission spectrum line is also formed. The light intensity, over a narrow frequency range, gets increased because of the emission by the material.
Spectral lines are highly atom-specific and are used to define any medium's chemical composition that can allow light to pass through it. Many elements were discovered by means of spectroscopy, including thallium, caesium, and helium. Also, the spectral lines depend on the physical conditions of the gas. Therefore, they can be widely used in the determination of the chemical composition of stars and the other celestial bodies that cannot be analyzed by other means and their physical conditions as well.
The mechanisms other than the atom-photon interaction can form spectral lines. The frequency of the involved photons will vary widely based on the exact physical interaction (with single particles, molecules, etc.), and lines are observed across the electromagnetic spectrum, ranging from radio waves to gamma rays.
Line Broadening and Shift
There are many effects which control spectral line shape. A spectral line extends over a frequency range but not a single frequency (it means it has a nonzero linewidth). Additionally, its centre can be shifted from its nominal central wavelength. There are many reasons for this shift and broadening.
These specific reasons are divided into 2 general categories. They are: broadening because of local conditions and broadening because of the extended conditions. Broadening occurs regardless of the local conditions due to the effects around the emitting element in a small area, typically enough to ensure local thermodynamic equilibrium. Broadening due to the extended conditions may result from changes to the spectral distribution of the radiation because it traverses its observer’s path. It also can result from the combining of radiation from more regions that are far from each other.
Broadening Due to Local Effects
The lifetime of the excited states will result in natural broadening, which is also called lifetime broadening. The principle of uncertainty relates the lifetime of the excited state (due to the Auger process or spontaneous radiative decay) to the uncertainty of its energy. A short lifetime will contain a large energy uncertainty and a broad emission spectrum. This specific broadening effect results in an unshifted Lorentzian profile. At the same time, the natural broadening is experimentally altered only up to the extent where decay rates are artificially enhanced or suppressed.
Broadening Due to Non-Local Effects
Some types of broadening are explained as the result of conditions over a large region of space, rather than just upon the conditions, which are local to the emitting particle.
As it journeys through space, electromagnetic radiation emitted at a particular point in space is reabsorbed. This absorption is based on wavelength. The line is broadened due to the photons at the line centre holding a greater reabsorption probability compared to the photons at the line wings. Indeed, in contrast to the wings at the middle of the line, the reabsorption at the centre of the line may be so great as to induce a self-reversal, where the amplitude is poor.