Thermal Doppler Broadening: due to the temperature of the emitter or absorber.Spectral lines may also be broadended by: A redshift (spectral lines move to longer wavelengths) occurs for an emitter/absorber moving away from the observer, while a blueshift (spectral lines move to shorter wavelengths) corresponds to motion towards the observer. If the emitter or absorber is in motion, however, the position of the spectral lines will be Doppler shifted along the spectrum. For this reason, we are able to identify which element or molecule is causing the spectral lines. The spectral lines of a specific element or molecule at rest in a laboratory always occur at the same wavelengths. Emission lines occur when the electrons of an excited atom, element or molecule move between energy levels, returning towards the ground state. This causes an electron to be promoted into a higher energy level, and the atom, element or molecule is said to be in an excited state. Absorption lines occur when an atom, element or molecule absorbs a photon with an energy equal to the difference between two energy levels. Atoms prefer to be in their ground state, where all of the electrons are located as close to the nucleus as possible. These energy levels depend on the numbers of protons, electrons and neutrons in an atom, and the limited set of configurations in which these elemental particles can exist (the set of quantum numbers). The presence of spectral lines is explained by quantum mechanics in terms of the energy levels of atoms, ions and molecules. The Uncertainty Principle also provides a natural broadening of all spectral lines, with a natural width of Δν = ΔE/h ≈ 1/Δt where h is Planck’s constant, Δν is the width of the line, ΔE is the corresponding spread in energy, and t is the lifetime of the energy state (typically ~10 -8 seconds).Absorption lines are seen as black lines on a coloured background. Zeeman Splitting: where energy levels are split into sub-levels by a magnetic field.Collisional Broadening: due to interactions with neighbouring atoms, ions or molecules.Absorption lines are seen as black lines on a coloured background.
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