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. 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.
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, the latter will observe a decrease in the intensity of light in the frequency of the incident photon, 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, he 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 passing through it (typically gas is used). Several elements were discovered by spectroscopic means -- helium, thallium, cerium, etc. Spectral lines also depend of 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.
Other mechanisms 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.
Spectral line broadening
A line extends over a range of frequencies, not a single frequency. The reasons for this broadening are several:
- 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.
- Resonance broadening: This broadening effect is described by a Lorentzian profile.
- Thermal Doppler broadening: Atoms will have different thermal velocities, so they will see the photons red or blue shifted, 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.
- 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 the density of the gas. The broadening effect is described by a Lorentzian profile.
- quasistatic pressure broadening: The presence of other particles shifts the energy of the different energy levels that give rise to the lines. The duration of the influence is much longer than the lifetime of the emission process. This effect also depends on the density of the gas.
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.